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The Pingos of Prince Patrick Island (76 degrees N - 120 degrees W)
Pissart, A.; National Research Council of Canada. Division of Building
Research
P R E F A C E
T h i s t r a n s l a t i o n of a study of pingos in the Canadian A r c t i c A r c h i p e l a g o i s of p a r t i c u l a r i n t e r e s t t o the Division of Building R e s e a r c h in i t s i n v e s t i g a t i o n s of the fundamental and engineering a s p e c t s of p e r m a f r o s t and the n a t u r a l f e a t u r e s a s s o c i a t e d with t h i s phenomenon. P i n g o s a r e the m o s t s t r i k i n g l a n d f o r m s in the p e r m a f r o s t r e g i o n and t h e i r o r i g i n h a s long been the s u b j e c t of m u c h s p e c u l a t i o n . Two t y p e s of pingos w e r e d e s c r i b e d in the book by F. MGller, "Observations on P i n g o s , " which w a s i s s u e d in the National R e s e a r c h Council T e c h n i c a l T r a n s l a t i o n s s e r i e s i n M a y 1963
( T T 1073). T h i s p r e s e n t t r a n s l a t i o n d e s c r i b e s another type of pingo in v e r y s e v e r e p e r m a f r o s t conditions in the F a r North which, a c c o r d i n g t o the a u t h o r , h a s not been e n c o u n t e r e d e l s e w h e r e
F i e l d s t u d i e s in North A m e r i c a of t h e s e unusual f e a t u r e s a r e l i m i t e d and s c a t t e r e d . O b s e r v a t i o n s have been m a d e i n S i b e r i a f o r s o m e y e a r s but the r e s u l t i n g R u s s i a n publications have not been g e n e r a l l y a v a i l a b l e . D r . P i s s a r t ' s o b s e r v a t i o n s have enabled h i m t o p r o p o s e s u g g e s t i o n s a s t o t h e i r o r i g i n and h i s t o r y which c o n t r i b u t e t o a b e t t e r understanding of s o m e of the geological p r o c e s s e s a s s o c i a t e d with p e r m a - f r o s t i n the High A r c t i c .
The Division is g r a t e f u l t o P r o f e s s o r W i l l i a m B a r r , D e p a r t m e n t of G e o g r a p h y , U n i v e r s i t y of Saskatchewan, f o r t r a n s l a t i n g t h i s document and t o D r . R . J . E . Brown of t h i s Division who checked the t r a n s l a t i o n .
Ottawa
J a n u a r y 1970
N . B. Hutcheon, D i r e c t o r .
NATIONAL RESEARCH COUNCIL OF C A N A D A T e c h n i c a l T r a n s l a t j.on 1 4 0 1 T i t l e : T h e p i n g o s o f P r i n c e P a t r i c k I s l a n d ( 7 6 ' ~
-
1 2 0 ~ ~ ) (JLes p i n g o s d e l l ' l e P r i n c e P a t r i c k ( 7 6 O ~-
1 2 0 ~ ~ ) A u t kior : A . I ' i s s a r t , R e f e r e n c e : G e o g r a p h i c a l k 3 u l l e t i n ,9
( 3 )
: 1 8 9 - 2 1 7 ,1967
T ' r n n s l a t o r : W i l l i a m B a r r , D e p a r t m e n t o f G e o g r a p h y , U n i v e r s i t y o f S a s k a t c h e w a n , S a s k a t o o n , S a s k n t c l i e w a r iTHE PINGOS O F PRINCE PATRICK ISLAND (76"N, 120°W)
A. P i s s a r t *
ABSTRACT
M o r e than 150 pingos have been o b s e r v e d on P r i n c e P a t r i c k Island. T h e m a j o r i t y of t h e s e d o m e s of injected i c e m a y be divided into two d i s t i n c t groups:
The f i r s t g r o u p i s m a d e up of m o r e than 100 pingos located on the s u t f a c e of the i s l a n d ' s s u m m i t a t an altitude of about 100 m e t r e s . They a r e aligned i n two p a r a l l e l r o w s of d o m e s , c i r c u l a r i n ground plan, whose height and d i a m e t e r do not exceed 1 3 m e t r e s and 250 m e t r e s r e s p e c t i v e l y . P r e s e n t i n g n o positive t r a c e of p r e s e n t d a y evolution and showing no r e l a t i o n s h i p with the topography, t h e s e f o r m s e x i s t i n t h e v a l l e y s a s well a s on the s u m m i t s . They a r e p o s s i b l y r e l a t e d t o s o m e d e e p geological s t r u c t u r e .
T h e second g r o u p of pingos i s m a d e up of mounds a t a n altitude c l o s e t o s e a l e v e l and situated a t the head of two bays m o r e than 90
k m
d i s t a n t f r o m e a c h o t h e r . In t h i sg r o u p p a r t i c u l a r l y a r e elongated pingos r e s e m b l i n g e s k e r s a s well a s those with the c h a r a c t e r i s t i c c i r c u l a r shape. O b s e r v e d c u t s have shown the a r r a n g e m e n t of l a y e r s in the s i d e s of the pingos a s well a s the c o r e of i n j e c t e d i c e . They a p p e a r t o have developed following a definite change of s e a l e v e l which drowned the bay and r e s u l t e d i n the melting of the u p p e r p e r m a f r o s t . The l a t e r r e - e s t a b l i s h m e n t of the p e r m a f r o s t m a y have begun with the a p p e a r a n c e of t h i s injected ice.
O t h e r pingos of a d i f f e r e n t type have a l s o been recognized on the i s l a n d . MS submitted M a y 1966
*
M . A l b e r t P i s s a r t i s C h a r g e d e C o u r s AssociC a t the L a b o r a t o i r e de geologic e t geographic physique a t the Universit6 de Likge, w h e r e P r o f e s s o r P a u l M a c a r i s D i r e c t o r . During h i s r e s e a r c h in the A r c t i c , M. P i s s a r t w a s employed on a t e m p o - r a r y b a s i s by the G e o g r a p h i c a l B r a n c h , D e p a r t m e n t of E n e r g y , M i n e s and R e s o u r c e s , Ottawa, Canada.p h o t o g r a p h s the p r e s e n c e of s t r a i g h t o r s l i g h t l y c u r v e d l i n e s , o v e r SO k m i n l e n g t h , which have n o t o p o g r a p h i c e x p r e s s i o n , but which s e p a r a t e a r e a s of s l i g h t l y d i f f e r i n g vegetation. T o z e r and T h o r s t e i n s s o n s e e i n t h e s e l i n e s t h e s u r f a c e e x p r e s s i o n of t h e r a d i a l f a u l t s identified i n t h e u n d e r l y i n g b e d s . T h e g e o m o r p h o l o g i c a l h i s t o r y of t h e a r e a is n o t w e l l e s t a b l i s h e d . At l e a s t one m a j o r g l a c i a t i o n h a s c o v e r e d t h e e n t i r e i s l a n d , a s w i t n e s s e d by n u m e r o u s e r r a t i c s . A p a r t f r o m s o m e v e r y i n f r e q u e n t s t r i a t e d r o c k s , and t r a c e s of n u m e r o u s g l a c i a l m e l t w a t e r d r a i n a g e c h a n n e l s , t h e r e a r e few o t h e r s i g n s of g l a c i a t i o n . F y l e s (1965, p. 4 ) e x p r e s s e s t h e s a m e opinion, when he w r i t e s t h a t t h e d i s t r i b u t i o n of e r r a t i c s i n the e a s t e r n p a r t of t h e i s l a n d s u g g e s t s t h a t the m a j o r g l a c i a t i o n w h i c h t r a n s p o r t e d t h e m t h e r e w a s p r i o r t o t h e l a s t cold p e r i o d .
T h e i s l a n d d o e s not a p p e a r t o h a v e e x p e r i e n c e d a n y s i g n i f i c a n t g l a c i a t i o n d u r i n g t h e Wisconsin. I t i s , h o w e v e r , l i k e l y t h a t a thin l o c a l i c e c a p c o v e r e d t h e i s l a n d , but
without l e a v i n g a n y c l e a r t r a c e s in the topography.
T h e g e o m o r p h o l o g y of P r i n c e P a t r i c k I s l a n d i s t h u s l a r g e l y p e r i g l a c i a l . T h e p e r m a f r o s t i s c e r t a i n l y v e r y t h i c k , but t h u s f a r n o b o r i n g h a s b e e n c a r r i e d out, a n d h e n c e i t s a c t u a l t h i c k n e s s is unknown. It should be b o r n e in m i n d , h o w e v e r , t h a t a b o r e h o l e , 300 krn t o t h e s o u t h e a s t , on M e l v i l l e I s l a n d , showed the g r o u n d t o be f r o z e n t o a depth of 51 5 m ( H a m e l i n and J a c o b s e n , 1964, p. 15). T h e r e a p p e a r t o b e i n d i c a t i o n s of the p r e s e n c e of r a i s e d b e a c h e s on t h e a e r i a l p h o t o g r a p h s . No s h e l l s w e r e found on t h e g r o u n d , h o w e v e r ; t h e f o r m s of m a r i n e e r o s i o n a r e i n d i s t i n c t ; and on t h i s s u b j e c t too, knowledge i s l i m i t e d . T h e p r e s e n t c l i m a t e of P r i n c e P a t r i c k I s l a n d i s e x t r e m e l y s e v e r e . M e t e o r o l o g - i c a l o b s e r v a t i o n s m a d e at Mould B a y s i n c e 1948 i n d i c a t e a m e a n a n n u a l t e m p e r a t u r e of -1 8. 5" C ( - 1. 5 ° F ) . T h e t a b l e of m e a n m o n t h l y t e m p e r a t u r e s given i n t h e appendix s h o w s t h a t t h e motithly m e a n s e x c e e d 0 ° C only i n J u l y and August, and t h a t f r e e z i n g i s i n t e n s e d u r i n g nine m o n t h s of the y e a r . F r o m the v e r y low p r e c i p i t a t i o n ( a n n u a l m e a n 81 m m = 3. 2 3 in. ) i t i s u n d e r s t a n d a b l e t h a t n o i c e c a p c o v e r s t h e whole i s l a n d a t p r e s e n t .
T h e t a b l e of m o n t h l y m e a n s combining c o m p a r a b l e c l i m a t i c p a r a m e t e r s f o r both P r i n c e P a t r i c k I s l a n d and t h e a r e a s of G r e e n l a n d a n d t h e M a c k e n z i e D e l t a w h e r e pingos h a v e been o b s e r v e d , c l e a r l y i n d i c a t e s t h a t the c l i m a t e of the a r e a s t u d i e d h e r e i s by f a r the m o s t s e v e r e .
THE PINGOS: DESCRIPTION AND GENETIC HYPOTHESES
S y s t e m a t i c study of the a e r i a l photographs of P r i n c e P a t r i c k I s l a n d r e v e a l e d t h e p r e s e n c e a t s e v e r a l points on the i s l a n d of m o r e o r l e s s c i r c u l a r mounds, which a p p e a r e d a t f i r s t sight t o be pingos. It w a s o f t e n i m p o s s i b l e t o d e c i d e w h e t h e r t h e s e m o u n d s w e r e due, in f a c t , t o the p r e s e n c e of intruded i c e , o r w h e t h e r t h e y w e r e s i m p l y h i l l s i s o l a t e d by chance e r o s i o n . I s h a l l not d e l a y o v e r a d e s c r i p t i o n of hypothetical f o r m s but will d e s c r i b e only indisputable pingo f o r m s . T h e m a j o r i t y of t h e s e c a n be divided i n t o two groupings, totally d i s t i n c t f r o m the point of view of g e n e s i s , a g e and location f a c t o r s . F o r t h e s e r e a s o n s , I s h a l l c o n s i d e r t h e m s e p a r a t e l y . The f i r s t of t h e s e g r o u p s of pingos is located 1 8 m i l e s (29
km)
n o r t h w e s t of Mould Bay, on thes u m m i t of the i s l a n d ( P o i n t 1 on Fig. l ) , while the second c o m p r i s e s f o r m s developed a t the s e a c o a s t , and which I o b s e r v e d n e a r Satellite Bay and I n t r e p i d Inlet ( P o i n t s 2 and 3 on Fig. 1 ) .
T h e P i n g o s Located on the S u m m i t of the Island, Northwest of Mould Bay
T h e s e pingos w e r e f i r s t recognized during the surnrner of 1964 by J . G. F y l e s , Head of t h e P l e i s t o c e n e Geology Section of the D e p a r t m e n t of E n e r g y , M i n e s and R e s o u r c e s . He v e r y kindly a u t h o r i z e d m e to study t h e s e f o r m s , and w a s good enough t o supply m e with the photographs he had taken of them, p r i o r t o m y d e p a r t u r e f o r the field. I wish t o thank h i m v e r y c o r d i a l l y f o r h i s kindness.
D e s c r i p t i o n of the Topography
T h e plateau on which the o b s e r v e d pingos a r e l o c a t e d l i e s 18 m i l e s (29
km)
n o r t h w e s t of t h e w e a t h e r s t a t i o n a t Mould Bay. It f o r m s p a r t of the s u m m i t s u r f a c e of the i s l a n d , and the r i v e r s r i s i n g t h e r e diverge: s o m e flow w e s t w a r d t o the A r c t i c Ocean; o t h e r s h e a d t o w a r d s t h e southwest i n the d i r e c t i o n of W a l k e r Inlet; while still o t h e r s m a k e f o r the s e a in the s o u t h e a s t , in Mould Bay.T h e plateau l i e s at an altitude of about 100 m , and i s cut by shallow, but v e r y wide, valleys. T h e l a t t e r d i s p l a y s o m e r e m a r k a b l e p e c u l i a r i t i e s ; often they continue f r o m one d r a i n a g e b a s i n t o a n o t h e r without t h e r e being any t r u e valley head. B e c a u s e of t h i s , i t i s completely i m p o s s i b l e t o t r a c e the l i m i t s of t h e d i f f e r e n t b a s i n s on the a e r i a l photographs. They a r e , m o r e o v e r , m i s f i t v a l l e y s
-
m u c h too wide f o r thes t r e a m s m a k i n g u s e of t h e m a t the p r e s e n t day; finally, they o c c a s i o n a l l y cut e a c h o t h e r a t r i g h t angles. In fact, they a r e inexplicable i n t e r m s of s i m p l e n o r m a l e r o s i o n , and one c a n explain t h e m only a s g l a c i a l m e l t w a t e r d r a i n a g e channels. Along with a few
e r r a t i c s , which a r e i n s o m e c a s e s quite m a s s i v e , t h e s e c h a n n e l s a r e t h e m o s t obvious t r a c e s of a f o r m e r g l a c i a t i o n , whose g r e a t a g e i s a t t e s t e d by t h e f a c t t h a t the fluvial f o r m s a r e b l u r r e d by solifluction. F y l e s (1965, p. 4) a l s o o b s e r v e d on P r i n c e P a t r i c k Island a topography f o r which a m o r a i n i c o r i g i n i s a p o s s i b l e explanation, and a l s o h i l l s r e s e m b l i n g k a m e s , and two g r a v e l r i d g e s which s e e m t o be e s k e r s .
N u m e r o u s ponds dot t h e u p p e r p a r t s of the v a l l e y s . Some a r e due t o solifluction which c a r r i e s down i n t o t h e b o t t o m s of t h e v a l l e y s g r e a t q u a n t i t i e s of p e b b l e s , which a r e too l a r g e to be washed away by the s t r e a m . Since they can not be c a r r i e d away by the running w a t e r , t h e y a c c u m u l a t e , and e v e n t u a l l y p r o d u c e l o c a l r e v e r s e s l o p e s . O t h e r s r e s u l t f r o m t h e blocking of t h e v a l l e y s by pingos; and s t i l l o t h e r s , in t h i s c a s e of a t e m p o r a r y n a t u r e , a r e o c c a s i o n e d by snowbanks. When the outflow f r o m a s m a l l l a k e p r o d u c e d by the blocking of a valley by a snowbank o c c u r s by way of an outlet not l o c a t e d a c r o s s the snowbank, the l a t t e r obviously w i l l not be i n c i s e d m a r k e d l y by t h e m e l t w a t e r , and a s long a s t h e snow p e r s i s t s , s u c h a s m a l l l a k e m a y n o doubt p e r s i s t f o r s e v e r a l y e a r s .
T o c o m p l e t e t h e d e s c r i p t i o n , one should add that t h e p l a t e a u i s developed on t h e s a n d s and g r a v e l s of t h e B e a u f o r t F o r m a t i o n . F r e q u e n t l y , a c e r t a i n p e r c e n t a g e of f i n e r s e d i m e n t s i s p r e s e n t in t h e s e s a n d s , and i t i s t h i s f i n e f r a c t i o n which p e r m i t s t h e development of t h e solifluction phenomena d i s c u s s e d above.
F i g u r e 2 i s d r a w n f r o m aerial p h o t o g r a p h s , and shows the d i s t r i b u t i o n of pingos
I have identified i n t h i s a r e a . F r o m i t , one c a n s e e that t h e m o u n d s a r e v e r y n u m e r o u s , totalling o v e r 130, and t h a t they a r e g r o u p e d i n two WSW -ENE a l i g n m e n t s , o v e r 1 5
km
i n length. T h e s e v e r y c l e a r a l i g n m e n t s , s e p a r a t e d by a zone o v e r 2km
in widthwithout a s i n g l e pingo, a r e i n no way r e l a t e d t o t h e topography. T h e y extend in a d i r e c t i o n p e c u l i a r t o t h e m alone, without r e g a r d t o t h e r e l i e f , and t h e pingos which c o n s t i t u t e t h e m a r e l o c a t e d r a n d o m l y on the s u m m i t s , i n t h e v a l l e y b o t t o m s , and on t h e s l o p e s .
In 1965 I t r a v e r s e d t h i s a r e a f o r a week with W.S.B. P a t t e r s o n of t h e P o l a r Continental Shelf P r o j e c t ; he a s s i s t e d m e e n o r m o u s l y i n c a r r y i n g out t h e s e m e a s u r e - m e n t s , and t o g e t h e r we sought a t l e n g t h f o r c l u e s a s t o t h e o r i g i n of the f o r m s . The a v e r a g e d i a m e t e r of t h e s e v e n t y pingos we m e a s u r e d i n t h e f i e l d i s 59 m , and t h e i r a v e r a g e height i s 3 . 3 m . T h e s e , t h e n , a r e g e n e r a l l y f e a t u r e s of quite s m a l l
d i m e n s i o n s ; the s m a l l e s t mounds studied a r e only one m e t r e high and 20 m i n d i a m e t e r ; t h e l a r g e s t a t t a i n s a d i a m e t e r of 250 m , a n d a height of 1 3 m .
G e n e r a l l y , t h e s e mounds have an a p p r o x i m a t e l y c i r c u l a r plan view (Fig. 3); the few pingos whose outlines d i f f e r m a r k e d l y f r o m a c i r c u l a r shape a r e due t o the c o a l e s c e n c e of s e v e r a l distinguishable c i r c u l a r mounds.
T h e s m a l l e s t f o r m a t i o n s frequently have a l e n s -shaped a p p e a r a n c e , o r that of a flattened dome (Fig. 4a). S o m e t i m e s a m o r e pointed s u m m i t o c c u r s a t the t o p of t h i s dome. T h e m e d i u m - s i z e f o r m s often have the a p p e a r a n c e of a t r u n c a t e d cone (Fig. 4c and 4d). As f o r the l a r g e s t mounds (Fig. 5 &
6),
t h e y g e n e r a l l y p r e s e n t a n i r r e g u l a r l y marnmillated a p p e a r a n c e , f o r they a r e dotted with s u m m i t s , whose a r r a n g e - m e n t , s e e n i n profile, s e e m s to indicate the p r e s e n c e of a c e n t r a l c r a t e r ( F i g . 4e & 4f). F i n a l l y , the mounds produced by the juxtaposition of two, t h r e e o r f o u r individualc i r c u l a r f o r m s , have a v a r i a b l e f o r m , a s a function of the a r r a n g e m e n t of the units composing them (Fig. 4g).
On the ground, t h e s e d i f f e r e n t t y p e s of pingos a r e not g e n e r a l l y s c a t t e r e d a t random. On the c o n t r a r y , they a r e m o s t f r e q u e n t l y grouped i n c a t e g o r i e s , depending on t h e i r complexity.
N u m e r o u s pingos w e r e examined i n detail, with a view t o studying s i g n s of p r e s e n t - d a y evolution. T h e y a r e always c o v e r e d with s a n d s and g r a v e l s . F r e q u e n t l y , pebbles f o r m a type of s u r f i c i a l pavement, which, however, d o e s not e n t i r e l y o b s c u r e the underlying sands. T h e pebbles situated on t h e s e m o u n d s a r e often c o v e r e d by black l i c h e n s , an indication of a c e r t a i n age, and of a c e r t a i n stability. E l e m e n t s displaying an eolian polish a r e r a t h e r r a r e ; wind-facetted pebbles a r e completely absent. T h i s a p p e a r s to be g e n e r a l l y the c a s e on P r i n c e P a t r i c k Island, even w h e r e s a n d s a r e abundant. Eolian e f f e c t s a p p e a r to be weak because of the infrequency of s t r o n g winds ( P i s s a r t , 1966).
Slight t r e n c h - l i k e f o r m s with a depth of 10 t o 20 c m f o r m a network of polygons on t h e s e pingos; t h e i r d i a m e t e r frequently r e a c h e s 20 m . T h e pebbles a r e m o s t
n u m e r o u s i n t h e s e t r e n c h e s , piled on t o p of e a c h o t h e r , a s i f t h e i r accompanying sand had d i s a p p e a r e d downwards. Occasionally, s m a l l e r polygons of about 5 m in d i a m e t e r f i t within the previously-mentioned o n e s , and in t h e i r c a s e , the d e m a r c a t i n g f i s s u r e s a r e shallow and l e s s distinct. The p e r s i s t e n c e of t h e s e f e a t u r e s on s t e e p
slopes, which o c c a s i o n a l l y r e a c h 20°, and the a b s e n c e of l i c h e n s on the e d g e s of t h e s e polygons, w h e r e t h e r e a p p e a r s t o be a c e r t a i n downwards m o v e m e n t t o w a r d s the c e n t r e s of the t r e n c h e s , indicate that t h e s e s t r u c t u r e s a r e evolving a t the p r e s e n t day. We shall s e e l a t e r , in the d e s c r i p t i o n of s e c t i o n s which w e r e o b s e r v e d , that t h e s e a r e
i c e - f i s s u r e polygons, p a r t l y filled with i c e , and p a r t l y with an e n t i r e l y m i n e r a l fill. A s m e n t i o n e d e a r l i e r , t h e f o r m of the l a r g e s t pingos i s f r e q u e n t l y i r r e g u l a r , s i n c e r e l a t i v e l y s h a r p s u m m i t s give t h e m a m a m m i l l a t e d a p p e a r a n c e . Often, t h e s e f o r m s a r e a s s o c i a t e d with s m a l l d e p r e s s i o n s , one t o t h r e e m e t r e s in d i a m e t e r , and r e a c h i n g a depth of 50 c m . It a p p e a r s that t h i s a s s o c i a t i o n of f o r m s i s t h e r e s u l t of t h e o n s e t of t h e m e l t i n g of a n underlying i c e m a s s , the c e n t r a l c o r e of t h e pingo. While t h i s i n t e r p r e t a t i o n a p p e a r s s e l f - e v i d e n t in the c a s e of the c l o s e d d e p r e s s i o n s , it
r e q u i r e s s o m e e l a b o r a t i o n with r e g a r d t o t h e s h a r p s u m m i t s . T h r e e a r g u m e n t s m a y b e advanced in f a v o u r of t h i s point of view:
( a ) As a l r e a d y pointed out, t h e s e s e c o n d a r y s u m m i t s a r e a s s o c i a t e d with t h e c l o s e d d e p r e s s i o n s .
( b ) I d e n t i c a l m o u n d s have been o b s e r v e d n e a r S a t e l l i t e Bay, on pingos which a r e indisputably undergoing m e l t i n g ( s e e Fig. 1 2 ) .
( c ) T h e few exposed s e c t i o n s a p p e a r t o i n d i c a t e t h a t while a r e g u l a r s t r a t i f i c a t i o n p a r a l l e l t o t h e s u r f a c e m a y b e o b s e r v e d on t h e o u t e r s l o p e s of t h e pingo, t h i s i s f r e q u e n t l y d i s r u p t e d on the s i t e of t h e i r r e g u l a r i t i e s p r e s e n t i n i t s c e n t r e , a s if i t w e r e the r e s u l t of s u b s i d e n c e of t h i s a r e a . T h e s l o p e s of t h e s e pingos a r e g e n e r a l l y l e s s than 2 0 ° , and a r e t h u s i n e v e r y c a s e m a r k e d l y l e s s than t h e angle of r e p o s e f o r t h e s e m a t e r i a l s . T h e g r a d i e n t s a r e s t e e p e s t w h e r e l a t e - l y i n g snowbanks p e r s i s t , and the s t e e p n e s s of t h e s l o p e s h e r e i s c l e a r l y the r e s u l t of nivation.
In conclusion, t h e a p p e a r a n c e of t h e s l o p e s of t h e s e pingos d o e s not d i f f e r f r o m t h a t of t h e s l o p e s of the neighbouring v a l l e y s ; t h e s a m e t u n d r a polygons o c c u r on t h e m , and t h e s e s l o p e s g e n e r a l l y have a n angle of l e s s than 20". T h e s t e e p e r s l o p e s a r e t h e r e s u l t of the e f f e c t s of nivation. The m e l t i n g , which h a s l e f t t r a c e s i n t h e l a n d s c a p e , d o e s not a p p e a r t o be o c c u r r i n g a t p r e s e n t , o r , a t l e a s t , i f i t i s , i t i s s o slow that i t i s not a p p a r e n t on the s u r f a c e . In s h o r t , t h e c u r r e n t o b s e r v a t i o n s p r o d u c e d no c l e a r
s i g n s of p r e s e n t -day evolution.
However, F y l e s h a s indicated one pingo (No. 1 on Fig. 2 ) , which a p p e a r s t o show t r a c e s of p r e s e n t - d a y development. Belonging t o type B in F i g u r e 4, it d i s p l a y s a f a i r l y s h a r p s u m m i t above a g e n e r a l l e n s - s h a p e d f o r m . T h i s s u m m i t i s devoid of l i c h e n s , and t h i s f a c t would s u g g e s t a r e c e n t evolution. T h e o b s e r v a t i o n i s not, h o w e v e r , e n t i r e l y conclusive. T h e g r e a t e s t slope on t h i s pingo d o e s not exceed 20°,
and t h u s d o e s not s u b s t a n t i a t e a m o r e r a p i d evolution. The p r e s e n c e of a s h a r p e r s u m m i t , w h e r e snow c o v e r i s e n t i r e l y a b s e n t d u r i n g w i n t e r d o e s not e n c o u r a g e the p r e s e r v a t i o n of l i c h e n s . F i n a l l y , t h i s pingo i s i s o l a t e d i n the c e n t r e of a v e r y l e v e l a r e a , and m a k e s a n e x c e l l e n t vantage point. It w a s c o v e r e d with n u m e r o u s c a r i b o u t r a c k s . T h e s e a n i m a l s , by t u r n i n g o v e r p e b b l e s with t h e i r hooves, c e r t a i n l y help t o explain the s c a r c i t y of l i c h e n s on t h i s s u m m i t . A l t o g e t h e r , t h e s e o b s e r v a t i o n s d o not supply sufficient proof of p r e s e n t -day growth i n t h e s e pingos.
S e c t i o n T h r o u g h a P i n g o ( F i g u r e 7)
Between 16 and 2 3 July 1966, a s e c t i o n i n pingo 11 ( F i g u r e 2), w a s e x c a v a t e d by m e a n s of a pump. T h i s pingo, located on the s u m m i t of a hill, w a s chosen b e c a u s e i t offered conditions f a v o u r a b l e to the u s e of a pump, i . e . a sufficient g r a d i e n t , and p r o x i m i t y t o a s m a l l l a k e t e m p o r a r i l y d a m m e d by a snowbank.
T h e p r i n c i p a l f e a t u r e i s t h e p r e s e n c e of a continuous m a s s of i c e a t a d e p t h of between 1 . 2 and 3.0 m . T h i s i s g e n e r a l l y p u r e t r a n s p a r e n t i c e , c o m p o s e d of c r y s t a l s 0. 5 t o 2. 0 c m i n length, without any p r e f e r r e d o r i e n t a t i o n . I b e l i e v e t h i s t o be t h e a c t u a l i c e c o r e of t h e pingo.
T h i s i c e c o r e h a s r a i s e d the s a n d - a n d - g r a v e l b e d s of the B e a u f o r t F o r m a t i o n , with s l i g h t accompanying d e f o r m a t i o n . T h e s e d e p o s i t s , which a r e well s t r a t i f i e d and which include m u c h wood d e b r i s , a p p e a r t o be i n situ. At f i r s t s i g h t , t h e i c e m a s s a p p e a r s t o follow a bedding-plane. A m o r e d e t a i l e d e x a m i n a t i o n shows that t h i s i s not th'e c a s e ; i t t r u n c a t e s t h e o v e r l y i n g beds. In only two s p o t s , and i n e a c h c a s e f o r a l e n g t h of not m o r e than one m e t r e , thin b e d s of c l a y , about 0.5 m t h i c k , o c c u r n e a r t h e i c e . E l s e w h e r e , the sands and g r a v e l s l i e d i r e c t l y upon the ice.
A t t h e point m a r k e d 5 on F i g u r e 7 , the i c e shows different c h a r a c t e r i s t i c s . I t p r e s e n t s t h e a p p e a r a n c e of v e r t i c a l s t r a t i f i c a t i o n ; i n c l u s i o n s a r e m u c h m o r e n u m e r o u s , whence i t s white c o l o u r , and i t i s p e n e t r a t e d i n i t s u p p e r p a r t by fine v e r t i c a l l i n e s of sand. T h i s i s an example of a n i c e wedge, which c o r r e s p o n d s a t the s u r f a c e t o one of t h e s l i g h t d e p r e s s i o n s which f o r m t h e polygonal n e t w o r k d e s c r i b e d e a r l i e r . Next t o t h i s i c e wedge, the beds a r e badly d e f o r m e d and f o r m a pinched-out s y n c l i n e , no doubt r e l a t e d t o the growth of t h i s s t r u c t u r e .
T h e o u t e r s l o p e , which i n f a c t c o m p r i s e s the s i d e slope of a valley, i s c o m p o s e d of r e w o r k e d s a n d s and g r a v e l s which have m o v e d down the slope. At point 3, a l a r g e block which I believe t o b e an e r r a t i c s i n c e i t a p p e a r s too l a r g e t o be c a r r i e d by the
t h i s s l o p e d e p o s i t h a s a t h i c k n e s s of a l m o s t 1. 5 m.
In the u p p e r p a r t of t h e s e c t i o n , s e v e r a l sand and g r a v e l wedges o c c u r . They d o not e x c e e d 1.2 m i n depth, while t h e i r m a x i m u m width i s 0.6 m . T h e s e s t r u c t u r e s a r e developed i n the "active l a y e r n of the s o i l , and do not include i c e . T h e s e a p p e a r t o be f o r m s s i m i l a r t o PCwC's sand wedges (1 959) o r t o t h e ground veins of t h e R u s s i a n a u t h o r s (Dylik, 1966). T h e r e i s v e r y d i s t i n c t s o r t i n g within t h e s e wedges. T h e
s m a l l e s t e l e m e n t s d e s c e r d m o r e r a p i d l y into t h e f i s s u r e s than t h e l a r g e s t e l e m e n t s , the f i s s u r e s a p p e a r i n g e a c h y e a r due t o t h e t h e r m a l c o n t r a c t i o n of t h e soil. The p r e s e n c e and s i m u l t a n e o u s development of both i c e wedges i n the p e r m a f r o s t , and sand and g r a v e l wedges i n the a c t i v e l a y e r , a p p e a r c a p a b l e of explanation with
r e f e r e n c e t o t h e d i f f e r e n c e in i c e content, which would p e r m i t the overlying l a y e r , a s a r e s u l t of i t s l e s s e r cohesion, and l e s s e r t h i c k n e s s , t o open into a network of
polygons of a s m a l l e r m e s h than t h o s e s e p a r a t i n g t h e i c e wedges ( P i s s a r t , 1967). In a n o t h e r pingo
(III
in F i g u r e 2), a n o t h e r s e c t i o n w a s excavated. It did not e x c e e d a depth of 2 m , and did not r e a c h t h e i c e c o r e of t h e pingo. H e r e too, wedges of sand and g r a v e l t r a n s e c t e d t h e "active zone" i n a s i m i l a r fashion.I n t e r p r e t a t i o n , G e n e s i s and Age of t h e P i n g o s
T h e following i s a b r i e f rCsurnC of t h e p r i n c i p a l points f o r which any explanation of t h e g e n e s i s and a g e of t h e s e pingos m u s t account:
( a ) T h e i r ' a r r a n g e m e n t i n two p a r a l l e l l i n e s , r e l a t e d t o n e i t h e r topography, n o r t o h y d r o g r a p h y , n o r e v e n t o a n y s t r u c t u r e v i s i b l e on t h e s u r f a c e .
( b ) T h e i r a p p a r e n t p r e s e n t s t a b i l i t y ; a p a r t f r o m one dubious c a s e , t h e r e a p p e a r s t o be n o m o v e m e n t a t p r e s e n t .
( c ) T h e i r often c o n s i d e r a b l e volume: t h e l a r g e s t of t h e s e pingos h a s 3
r a i s e d a m a s s of m o r e than 150,000 cubic m e t r e s ( m ) above the ground s u r f a c e . If t h i s m a s s i s d e r i v e d f r o m the i n c r e a s e i n volume r e s u l t i n g f r o m the f r e e z i n g of w a t e r i n a c l o s e d s y s t e m , t h e quantity of w a t e r which produced i t , would have t o be nine t i m e s g r e a t e r , i. e. would have t o a p p r o a c h 1 , 3 5 0 , 0 0 0
3
cubic m e t r e s ( m ).
( d ) An age younger than t h e m a j o r g l a c i a t i o n which c o v e r e d t h e i s l a n d , f o r t h e y a r e l o c a t e d in s o m e c a s e s , i n t h e c h a n n e l s c a r v e d out by the m e l t - w a t e r s f r o m t h a t i c e m a s s .
Quite indisputably, t h e s e pingos a r e not f o r m s r e s u l t i n g f r o m phenomena o c c u r r i n g wholly i n the v e r y u p p e r m o s t p a r t of the g r o u n d , The t o t a l absence of any r e l a t i o n s h i p with s u r f a c e f e a t u r e s definitely e s t a b l i s h e s t h i s . It i s c e r t a i n , m o r e o v e r , t h a t t h e i r a l i g n m e n t s a r e due t o s o m e geological s t r u c t u r e , but the l a t t e r m u s t be a t depth. It m a y involve e i t h e r c l a y b e d s within t h e B e a u f o r t F o r m a t i o n , o r a d e e p geological s t r u c t u r e l o c a t e d beneath t h a t f o r m a t i o n . The f i r s t h y p o t h e s i s i s
u n s u p p o r t e d , while, i n c o n t r a s t , the second one f i n d s c e r t a i n s u p p o r t i n T o z e r and T h o r s t e i n s s o n ' s geological m a p . Indeed t h i s m a p , which I have reproduced (Fig. l ) , shows t h a t the s u b s t r a t u m h a s been affected by faulting. While a l m o s t a l l of t h e s e f a u l t s have a n o r t h - s o u t h a l i g n m e n t , which d o e s not c o r r e s p o n d t o t h e o r i e n t a t i o n of the l i n e s of identified pingos, t h e r e i s , however, a f u r t h e r l o c a l i z e d o r i e n t a t i o n of pingos n e a r Landing L a k e , which s e e m s t o be a p p a r e n t in t h e continuation of t h e
s o u t h e r n s e r i e s of mounds. I t would a p p e a r p o s s i b l e f r o m t h i s , t h a t t h e two o b s e r v e d a l i g n m e n t s d e l i n e a t e the l i n e s of f a u l t s c o n c e a l e d beneath a t h i c k n e s s of a l m o s t 50 m of t h e s a n d s and g r a v e l s of the B e a u f o r t F o r m a t i o n .
If i t i s a d m i t t e d t h a t t h e pingos a r e r e l a t e d t o f a u l t s affecting the b e d r o c k , two h y p o t h e m m a y be invoked t o explain t h e i r evolution. The f i r s t involves the p o s s i b i l i t y t h a t the f a u l t s have p e r m i t t e d a c c e s s of w a t e r f r o m depth; the second t h a t t h e f a u l t s d e t e r m i n e d the p r e s e n c e of g e o l o g i c a l t r a p s , holding captive i n the s a n d s of the B e a u f o r t F o r m a t i o n a w a t e r m a s s , which, on f r e e z i n g i n a c l o s e d s y s t e m , produced the o b s e r v e d i n t r u s i o n s .
T h e f i r s t of t h e s e h y p o t h e s e s , the m o v e m e n t of w a t e r f r o m d e p t h t h r o u g h t h e p e r m a f r o s t by way of t h e s e f a u l t s , i s a p r i o r i v e r y a c c e p t a b l e . T h e t h i c k n e s s of the overlying B e a u f o r t F o r m a t i o n m a k e s i t u n d e r s t a n d a b l e why the a l i g n m e n t of the pingos should not be p e r f e c t .
M o r e o v e r t h e d i m e n s i o n s of the pingos 1 o b s e r v e d c o r r e s p o n d v e r y w e l l t o t h o s e of pingos r e s u l t i n g f r o m the u p w a r d s m o v e m e n t of w a t e r f r o m the l o w e r p a r t of t h e p e r m a f r o s t ; a c c o r d i n g t o S h u m s k i i (1959) t h i s type of f o r m r e a c h e s 10 t o 12 m and o c c a s i o n a l l y even 1 5 - 1 7
rn
i n height. However, he e m p h a s i z e s t h a t t h i s type of i c e injection mound o c c u r s only on t h e m a r g i n s of the p e r m a f r o s t zone, w h e r e the p e r m a f r o s t i s r e l a t i v e l y thin. Now, when i t i s r e m e m b e r e d t h a t on n e a r b yM e l v i l l e I s l a n d , the p e r m a f r o s t i s o v e r 500 m thick, and t h a t i t i s probably of equal t h i c k n e s s h e r e , t h i s h y p o t h e s i s l o s e s i t s appeal. It i s e x t r e m e l y unlikely t h a t the w a t e r could have m a d e i t s way t h r o u g h s u c h a t h i c k n e s s of f r o z e n g r o u n d . I t would, m o r e o v e r , be c o m p l e t e l y unfounded t o postulate t h a t the p e r m a f r o s t m i g h t be t h i n n e r
h e r e , b e c a u s e t h e r e i s nothing t o l e a d one t o think t h a t the g e o t h e r m a l g r a d i e n t m i g h t be s t e e p e r locally.
I£
i t i s a c c e p t e d that the p r e s e n t t h i c k p e r m a f r o s t indeed c o n s t i t u t e s a b a r r i e r t o the m o v e m e n t of w a t e r f r o m depth, one i s f o r c e d t o the conclusion that t h e s e f o r m s developed p r i o r t o the a p p e a r a n c e of t h a t b a r r i e r , p e r h a p s a t t h e beginning of t h e Wisconsin. T h e r e i s indeed nothing to c o n t r a d i c t the view t h a t t h e s e pingos m a y be v e r y ancient. It i s p o s s i b l e t h a t i t m a y have been the d e v e l o p m e n t of t h e p e r m a f r o s t i t s e l f , a s i n t h e M a c k e n z i e type of pingo, which c o n t r o l l e d the a p p e a r a n c e of c l o s e d c e l l s , and which consequently provided t h e n e c e s s a r y p r e s s u r e to i m p e l t h e w a t e r t o w a r d s the s u r f a c e . In t h i s c a s e , t h e a l i g n m e n t s would c o r r e s p o n d t o ' t r a p s ' i n a q u i f e r o u s b e d s , which m i g h t a l s o be r e l a t e d to t h e p r e s e n c e of t h e f a u l t s d i s c u s s e d e a r l i e r .Only g e o p h y s i c a l r e s e a r c h , a i m e d a t e s t a b l i s h i n g a n y r e l a t i o n s h i p between t h e s t r u c t u r e s a t depth and t h e o b s e r v e d a l i g n m e n t s , a p p e a r s l i k e l y t o d i s c l o s e new
c l u e s which m i g h t r e s u l t i n f u r t h e r i n g knowledge of t h e g e n e s i s of t h e s e pingos. I t m u s t be acknowledged, m o r e o v e r , t h a t an explanation involving t h e development of s u c h f o r m s a s t h e r e s u l t of t h e i n i t i a l cooling of t h e c l i m a t e , h a s a l r e a d y been p r o p o s e d by Leffingwell (1919, p. 154) f o r pingos i n Alaska.
I n a l l r e s p e c t s , t h e l o c a t i o n of t h e s e pingos on t h e s u m m i t of t h e i s l a n d , f a r f r o m a n y l a k e , m e a n s t h a t quite indisputably, t h e y belong n e i t h e r t o t h e G r e e n l a n d , n o r t o the M a c k e n z i e type.
THE PINGOS SITUATED CLOSE T O SEA L E V E L NEAR S A T E L L I T E BAY AND INTREPID INLET
P i n g o s of an e n t i r e l y d i f f e r e n t type w e r e o b s e r v e d n e a r two b a y s , l o c a t e d a t points 2 and 3 on F i g u r e I , and s i t u a t e d one i n t h e n o r t h w e s t of t h e i s l a n d a t the head of S a t e l l i t e Bay, t h e o t h e r i n a s e c o n d a r y i n l e t off I n t r e p i d Inlet. In both c a s e s , t h e s e pingos have developed a t an a l t i t u d e v e r y c l o s e t o t h e p r e s e n t s e a l e v e l , and
s u r r o u n d e d by a l l u v i a l d e p o s i t s . T h e i r g e n e s i s i s r e l a t e d t o t h e i r position. It i s t h u s n e c e s s a r y t o d e s c r i b e the g e o m o r p h o l o g i c a l c o m p l e x e s i n which they a r e located.
S a t e l l i t e Bay
A s c a n be s e e n f r o m F i g u r e 8, which i l l u s t r a t e s p a r t of the f e a t u r e , an e x t e n s i v e "glacis" s t r e t c h e s e a s t w a r d f r o m S a t e l l i t e Bay f o r an a v e r a g e d i s t a n c e of
two k i l o m e t r e s . I t s length h a s been t r a c e d f r o m the n o r t h e r n e x t r e m i t y of the b a y h r a distance of m o r e than 10 km; it thus continues f o r a l m o s t a f u r t h e r 5
km
beyond the l i m i t s of F i g u r e 8. We a r e thus dealing with a n outwash plain, slightly concave in p r o f i l e , which i s continued uphill by wide d r y valleys. I t s g r a d i e n t g e n e r a l l y v a r i e s f r o m 0.6" a t the top t o 0.3" a t the bottom. T h i s d e c r e a s e in g r a d i e n t i s accompanied by a d e c r e a s e in the dimensions of the pebbles covering the surface. The m a x i m u m length of the c o a r s e e l e m e n t s d e c r e a s e s p r o g r e s s i v e l y f r o m 20 c m a t Y on F i g u r e 8, t o only 8 c m a t X on the s a m e figure, only 1. 5 km downhill. S i m i l a r l y between V and W, the m a x i m u m d i m e n s i o n s of pebbles d e c r e a s e s p r o g r e s s i v e l y , and the median of five m e a s u r e m e n t s of centile undertaken on the s u r f a c e e v e r y 200 m , produced the following r e s u l t s : 12.5 c m , 12.5 c m , 11.5 c m , 11 c m , 10 c m , 8 . 5 c m , 6 c m , 7 . 5 c m , 4. 5 cm. E l s e w h e r e on t h i s g l a c i a l outwash plain, too, t h i s g e n e r a l d e c r e a s e i n the s i z e of the c o a r s e s t e l e m e n t s o c c u r s , but i t i s m u c h l e s s r e g u l a r , i n that the contributing effects of the s l o p e s r e s u l t i n the disruption of the pattern. M o r e o v e r , l o c a l i n t e r l o c k i n g s of different s u r f a c e l e v e l s o c c u r , a s f o r example, in the c a s e of a r e a T , which l i e s a t a lower l e v e l , with r e s p e c t t o the s u r f a c e s located on e i t h e r side.M o r e o v e r , the"glacis>ssumes the g e n e r a l a s p e c t of a low-angle cone a t the mouths of the u p s t r e a m valleys; this appearance i s emphasized by the t u n d r a polygons. The l a t t e r produce a quadrangular effect, i n that t h e i r s i d e s follow two d i r e c t i o n s : one g r o u p i s aligned in accordance with the line of m a x i m u m gradient, and thus a s s u m e s a r a d i a l p a t t e r n ; the o t h e r s m o r e o r l e s s follow t h e c o n t o u r s , and thus outline s e g m e n t s of a c i r c l e , c e n t r e d on the mouth of the valley.
T h i s l a r g e "glacis" i s inexplicable in t e r m s of n o r m a l e r o s i o n . The catchment a r e a of the valleys debouching upon i t i s e x t r e m e l y s m a l l , t h e i r length u p s t r e a m
g e n e r a l l y not exceeding 2 km. M o r e o v e r , the w a t e r s h e d of the i s l a n d i s quite c l o s e , approximately 4
km
t o the e a s t , and due to t h i s f a c t , no r i v e r c a p t u r e o r drainage change can be envisaged t o explain the l a r g e volumes of w a t e r of which evidence i s found h e r e .F o r t h e s e r e a s o n s , i t i s contended that t h i s l a r g e "glacis" i s a g l a c i a l outwash plain, produced by a m a j o r glaciation f r o m the e a s t , which brought n u m e r o u s e r r a t i c s to P r i n c e P a t r i c k Island. T h i s glaciation m u s t d a t e f r o m an e a r l y period, however, f o r the outwash plain i s deeply i n c i s e d by the c o u r s e s of n u m e r o u s s t r e a m s , often of quite a s m a l l volume.
The s u r f a c e of t h i s fluvioglacial deposit continues d o w n s t r e a m right t o t h e p r e s e n t coastline. At t h i s point, i t ends a t about 12 m above s e a l e v e l in c h a r a c t e r i s t i c d e l t a i c
d e p o s i t s , i n which f o r e s e t b e d s a r e well displayed. T h e h e i g h t s of 10.7 m , 1 3 . 8 m and 12.0 m c o r r e s p o n d t o t h e c r e s t s of the s e a w a r d e x t r e m i t i e s of t h e "glacis" w h e r e f o r e s e t beds a r e d i s p l a y e d , a t points A, B , and C on F i g u r e 8. I t w a s i m p o s s i b l e to e s t a b l i s h w h e t h e r t h e s e v a r i a t i o n s w e r e t h e r e s u l t of a s e a l e v e l change d u r i n g t h e d e v e l o p m e n t of t h e fluvioglacial deposit.
T h e s e o b s e r v a t i o n s of d e l t a i c d e p o s i t s , repeated at four locations, spaced along a 4-krn s t r e t c h of t h e bay, c l e a r l y e s t a b l i s h t h a t the s e a l e v e l m u s t have stood a t a l e v e l c l o s e t o f 12m, a t t h e t i m e of g l a c i e r r e t r e a t . Two l a k e s , e a c h s e v e r a l hundred m e t r e s in l e n g t h , o c c u r on t h i s "glacis1'; one i s visible on F i g u r e 8 , and both a p p e a r t o be quite deep. T h e i r explanation i s a difficult m a t t e r . T h e y cannot be explained i n t e r m s of l a t e r e r o s i o n , and t h e y m u s t be due e i t h e r t o blocks of dead i c e , o r m o r e s i m p l y , t o a r e a s which w e r e not e n t i r e l y o v e r w h e l m e d by t h e f o r m a t i o n of t h e fluvioglacial d e p o s i t .
A l o w e r s e r i e s of d e l t a s o c c u r s a t about 3.5 m above s e a - l e v e l ; they w e r e o b s e r v e d a t s i x points, of which t h r e e a r e indicated on F i g u r e 8 , a t heights between 3 . 2 5 and 4 m . A c e r t a i n n u m b e r of low t e r r a c e s o c c u r i n the l o w e r p a r t of the valley w h e r e the pingos t o be d i s c u s s e d next a r e l o c a t e d ; t h e s e c o r r e s p o n d t o one o r s e v e r a l
s e a l e v e l s between 2.5 and 3.5 m . The s u r f a c e of t h e s e t e r r a c e s i s i r r e g u l a r ; i t i s d i s r u p t e d by i c e wedge polygons t o s u c h a n e x t e n t t h a t i t w a s difficult t o d e c i d e with any c e r t a i n t y , w h e t h e r they a l l c o r r e s p o n d e d t o t h e d e l t a s l o c a t e d a t a height of 3 . 5 m, which have j u s t b e e n mentioned. Since s u c h a r e l a t i o n s h i p i s v e r y probable, however, both t h e d e l t a s , and the t e r r a c e s a t 3. 5 m , have been r e p r e s e n t e d i n an i d e n t i c a l m a n n e r on F i g u r e 8.
D e s c r i p t i o n of the P i n g o s
T h e pingos l o c a t e d a t t h e head of S a t e l l i t e Bay a r e r e l a t i v e l y few i n n u m b e r . D i f f e r e n c e s in t h e i r a p p e a r a n c e , v i s i b l e in F i g u r e 8 , allow t h e m to be grouped i.nto 3 c a t e g o r i e s : round, oval, and v e r y long pingos m a y be d i s t i n g u i s h e d a c c o r d i n g t o t h e i r plan. T h e r e a l s o o c c u r r e m n a n t s of pingov which have c o m p l e t e l y , o r a l m o s t c o m p l e t e l y thawed; t h e i r o r i g i n a l f o r m i s s o m e t i m e s difficult t o r e c o n s t r u c t .
Only one pingo p r e s e n t s a c o m p l e t e l y c l a s s i c c o n i c a l f o r m , with a d e p r e s s i o n in i t s s u m m i t , which f r o m i t s position, r e s e m b l e s a volcanic c r a t e r . I t s d i m e n s i o n s a r e quite l i m i t e d , however,since t h i s mound i s only 50 m i n d i a m e t e r , and only 3 m high ( F i g . 9).
t h e i r s u m m i t s , a r e oval in plan. T h e y c o n s t i t u t e a t r a n s i t i o n t o t h e v e r y elongated pingos which a r e the m o s t i m p o r t a n t f o r m s which w e r e identified. The l a r g e s t of t h e s e f o r m s h a s t h e a p p e a r a n c e of a long r i d g e c o m p a r a b l e t o an e s k e r ( F i g . 10). It a t t a i n s a length of 1300 m , while i t s width i s v a r i a b l e , but g e n e r a l l y l i e s between 40 and 70 m , and i t s height d o e s not e x c e e d 8.75 m . It d i f f e r s i n e x t e r n a l a p p e a r a n c e f r o m an e s k e r , in that i t s c e n t r a l portion i s occupied by a d e p r e s s i o n , aligned along the m a j o r a x i s of t h e mound ( F i g . 11). It w a s t h i s a x i a l d e p r e s s i o n , an indisputable s i g n of t h a w - ing, which p e r m i t t e d t h e recognition of t h i s f o r m on t h e a e r i a l photos a s a type of pingo; t h i s i n t e r p r e t a t i o n w a s c o n f i r m e d , a s d e s c r i b e d l a t e r , by t h e e x p o s u r e a t two s e p a r a t e s p o t s of t h e i c e c o r e . L o c a l l y , t h i s thawing pingo d i s p l a y s not only a c e n t r a l d e p r e s s i o n , but a l s o m a m m i l l a t e d r e l i e f ( F i g . 121, which r e s e m b l e s that of t h e l a r g e pingos on t h e s u m m i t of t h e i s l a n d ( F i g . 5).
A p a r t f r o m t h e s e s i g n s of m e l t i n g , t h i s long pingo i s i n c i s e d by t r a n s v e r s e v a l l e y s which cut t h i s r i d g e a t r i g h t a n g l e s to i t s m a j o r a x i s ( F i g . 10). They a r e t h e work of w a t e r flowing down f r o m the n e a r b y t e r r a c e . T h e r e i s no evidence t o indicate w h e t h e r t h i s e r o s i o n o c c u r r e d s i m u l t a n e o u s l y with o r a f t e r t h e a p p e a r a n c e of t h e pingo.
N e a r t h e s e m e l t i n g f o r m s , and s t i l l conspicuous i n t h e s u r r o u n d i n g topography, t h e r e o c c u r pingos in a m u c h l a t e r s t a g e of evolution; only a f e w t r a c e s of t h e s e r e m a i n . T h e y a r e c o n c e n t r a t e d on t h e l e f t bank, and t h e i r c o m m o n identity with the o t h e r
s t r u c t u r e s c a n be e s t a b l i s h e d l a r g e l y f r o m the v e r y s t e e p , a l m o s t v e r t i c a l a p p e a r a n c e of t h e s t r a t i f i c a t i o n on e i t h e r s i d e of an a l m o s t i m p e r c e p t i b l e c e n t r a l hollow. T h e p a r t of t h e i c e c o r e s i t u a t e d b e n e a t h ground l e v e l i s s i t u a t e d i n t h e p e r m a f r o s t , and h a s not yet m e l t e d probably b e c a u s e of t h i s fact. F r o m t h i s i s d e r i v e d t h e d i f f e r e n c e i n
a p p e a r a n c e between t h e s e f o r m s and t h e r e l i c t pingo t r a c e s known f r o m E u r o p e .
Location of the P i n g o s i n t h e Topography
A l l t h e o b s e r v e d pingos a r e l o c a t e d beneath t h e l e v e l of the p r e s u m e d g l a c i a l outwash plain. T h e y o c c u r e i t h e r on t h e l o w e r t e r r a c e s d e s c r i b e d p r e v i o u s l y , o r e l s e p e r h a p s i n the p r e s e n t a l l u v i a l plain. The c i r c u l a r , volcano-like pingo r i s e s f r o m a t e r r a c e lying a t 3. 1 m above t h e r i v e r , while t h e v e r y long pingo, f o r t h e m a j o r p a r t of i t s length, i s i n i m m e d i a t e contact with t h e p r e s e n t a l l u v i a l plain. At s e v e r a l points, m o r e o v e r , i t i s being v i g o r o u s l y e r o d e d by t h e r i v e r .
F r o m a l l o b s e r v a t i o n s , i t i s i m p o s s i b l e t o d e t e r m i n e w h e t h e r s o m e of t h e s e f o r m s developed in t h e p r e s e n t a l l u v i a l plain, o r w h e t h e r they w e r e a l l f o r m e d on t h e low
t e r r a c e s . T h e t o p o g r a p h i c f o r m s d o not p e r m i t a s t a t e m e n t a s t o w h e t h e r t h e r e a r e two g e n e r a t i o n s of f o r m s developed at d i f f e r e n t l e v e l s , o r s i m p l y a s i n g l e g r o u p of p i n g o s , a l l of t h e s a m e a g e . T h e a s s e m b l e d f a c t s would s u g g e s t , h o w e v e r , t h a t two d i s t i n c t a s s e m b l a g e s e x i s t , one c o m p r i s i n g w e l l p r e s e r v e d f o r m s , l o c a t e d c l o s e t o the p r e s e n t r i v e r , the o t h e r c o m p r i s i n g a l m o s t c o t n p l e t e l y m e l t e d s t r u c t u r e s , which a r e d i s t r i b u t e d m a i n l y on t h e l e f t - b a n k t e r r a c e s O b s e r v e d S e c t i o n s N u m e r o u s s e c t i o n s w e r e o b s e r v e d i n t h e s e p i n g o s . M a n y of t h e s e w e r e e x p o s e d by digging ( h e r e t h e a s s i s t a n c e of Leif L u n d g a r d i n 1965, and of Raymond R i c h a r d in 1966 i s acknowledged); t h e s e showed only the u p p e r 60 c m of the s o i l , the d e p t h of the thawed l a y e r a t t h e t i m e of the v i s i t t o t h e a r e a , a t the beginning of August, 1965. T w o m o r e i m p o r t a n t c u t s have been e x c a v a t e d by the outflow f r o m l a k e s l o c a t e d on the s u m m i t s of p i n g o s . T h e flow w a s i n c r e a s e d a r t i f i c i a l l y by digging a s m a l l c h a n n e l , allowing
flow t o b e g i n . Two f u r t h e r l a r g e r c u t s w e r e p r o d u c e d by m e a n s of a pump in 1966. T h e s e s e c t i o n s e x p o s e d the i n j e c t e d i c e , a n d c o n f i r m e d t h a t the d i p of t h e b e d s i s e x t r e m e l y s t e e p on the s l o p e s of the p i n g o s . T h i s i n c l i n a t i o n of the s t r a t a on the f l a n k s i s quite w i d e s p r e a d . I t w a s v e r i f i e d a t n u m e r o u s s p o t s , b e c a u s e the s t e e p l y dipping b e d s
o u t c r o p on t h e c r e s t s bounding t h e c e n t r a l d e p r e s s i o n s p r o d u c e d by thawing. T h u s i t i s p o s s i b l e t o follow the u p r i g h t b e d s , a n d t o identify the s t r u c t u r e of t h e pingo. T h i s
s t r u c t u r e i s not a l w a y s s i m p l e . Indeed, s o m e t i m e s t h e pingo i s c o n s t i t u t e d not by a l e n s of i c e of s i m p l e f o r m , but by a m a s s of i c e d i s p l a y i n g a p o p h y s e s , a s i n d i c a t e d in F i g u r e 13.
T h e b e s t of a l l the s e c t i o n s e x a m i n e d w a s the r e s u l t of l a t e r a l e r o s i o n by the r i v e r . F i g u r e 14 i l l u s t r a t e s the p r o f i l e o b s e r v e d in 1965, and l o c a t e d on F i g u r e s 8, 10 and 1 1 by t h e l e t t e r M . T h i s s e c t i o n r e v e a l e d the s t r u c t u r e of half of t h e l o n g e s t pingo, f o r i t extended t r a n s v e r s e l y f r o m the o u t s i d e of t h e mound t o i t s m e d i a l
d e p r e s s i o n . In 1966, a s e c t i o n w a s a l s o e x c a v a t e d i n t h e o t h e r half of the pingo, and t h u s e x p o s e d the c o m p l e t e t r a n s v e r s e p r o f i l e of t h i s s t r u c t u r e , p r e s e n t e d in F i g u r e 15. T h e I c e C o r e P r e l i m i n a r y o b s e r v a t i o n s showed that t h e c e n t r a l d e p r e s s i o n c o r r e s p o n d s t o a n i c e m a s s . T h i s i s c o m p o s e d of c r y s t a l s a t t a i n i n g a l e n g t h of 2 c m ; t h e s e a r e c l e a r l y v i s i b l e t o the n a k e d e y e , by v i r t u e of t h e d i f f e r e n t o r i e n t a t i o n of g a s e o u s i n t r u s i o n s , which a r e p a r t i c u l a r l y n u m e r o u s . T h e s e c r y s t a l s p r e s e n t e x a c t l y the s a m e a p p e a r a n c e
a s the c r y s t a l s in the intruded i c e which F r i t z M k l l e r (1959, F i g . 15b) o b s e r v e d i n a pingo i n G r e e n l a n d . B e c a u s e of the multiple g a s e o u s i n c l u s i o n s , the i c e i s g e n e r a l l y whitish in c o l o u r ; l o c a l l y , h o w e v e r , the i c e c o n t a i n s f e w e r i n c l u s i o n s , and hence i s m u c h m o r e t r a n s p a r e n t . The e x t e r n a l p o r t i o n s of the i c e m a s s , i n contact with t h e
bounding v e r t i c a l b e d s , d i s p l a y s , f o r a width of 1. 5 m on one s i d e , and 3. 0 m on the o t h e r , a s t r a t i f i c a t i o n p a r a l l e l t o those beds ( F i g . 16). P r o c e e d i n g f r o m the o u t s i d e t o w a r d s the c e n t r e , t h i s s t r a t i f i c a t i o n i s d u e , f o r the f i r s t 50 c m , t o g e n e r a l l y v e r y fine sandy l a y e r s , and f u r t h e r i n , t o a r e d d i s h c o l o r a t i o n of the i c e , produced by d e p o s i t s of i r o n o x i d e s . T h e s e i r o n oxides a r e p a r t i c u l a r l y abundant i n the u p p e r p a r t of the i c e c o r e a t the contact between the i c e and the overlying d e p o s i t s , w h e r e t h e y a p p e a r t o be
a c c u m u l a t i n g a s m e l t i n g p r o c e e d s . E l s e w h e r e , they a r e e a s i l y explained by the
p r e c i p i t a t i o n of the oxides contained i n the w a t e r a t the point of f r e e z i n g ( C a i l l e u x , 1964; E k and P i s s a r t , 1965). T h i s s t r a t i f i e d a p p e a r a n c e i s a l o c a l a p p e a r a n c e and h a s not been found e l s e w h e r e . It t e s t i f i e s t o the slow g r o w t h of the i c e c o r e e a r l y in i t s f o r m a - tion by the advent of s u c c e s s i v e s m a l l q u a n t i t i e s of w a t e r .
A thin vein of m a r k e d l y d i f f e r e n t i c e , i n which t h e c r y s t a l s a r e o r i e n t e d p e r p e n - d i c u l a r l y t o t h e s t r a t i f i c a t i o n , d e t a c h e s i t s e l f f r o m the l e f t e x t r e m i t y of the m a i n i c e m a s s , and p e n e t r a t e s obliquely i n t o the s u r r o u n d i n g c l a y . T h i s i s p r o b a b l y a n e x a m p l e of i c e s e g r e g a t i o n .
On the r i g h t , the i c e m a s s i n c l u d e s two b e d s of sand d i s p o s e d p a r a l l e l t o the s t r a t i f i c a t i o n of t h e uplifted f o r m a t i o n s and a p p e a r i n g t o f o r m p a r t of t h e m .
A s noted p r e v i o u s l y , a n i c e c o r e i d e n t i c a l t o the one d e s c r i b e d above w a s o b s e r v e d r i g h t i n the c e n t r e of e a c h of t h e mounds studied, w h e r e v e r a sufficiently d e e p s e c t i o n could be e x c a v a t e d , i. e. i n five d i f f e r e n t s p o t s . T h e s e o b s e r v a t i o n s p r o v e without a shadow of doubt t h a t the c e n t r a l d e p r e s s i o n s of the m o u n d s e x a m i n e d a r e due t o the m e l t i n g of the i c e c o r e .
M a t e r i a l S u r r o u n d i n e the Ice C o r e
T h e s t r a t i f i e d m a t e r i a l pushed up by the i c e c o r e c o n s i s t s p r i n c i p a l l y of s a n d s and g r a v e l s . However, the c l o s e r t o the i c e c o r e i t s e l f , the f i n e r the sediment, t o t h e point w h e r e i t i s quite d i s t i n c t l y c l a y e y a t the a c t u a l c o n t a c t . It should be b o r n e in
m i n d that P . A . Shoumslcy (1955, p. 1 2 1 ) h a s i n d i c a t e d t h a t i n j e c t i o n i c e n o r m a l l y a p p e a r s a t the c o n t a c t between p e r m a b l e and i ~ n p e r r n e a b l e beds, a s i s t h e c a s e h e r e . V a r i a t i o n s i n c o l o u r a c c o m p a n y the g r a i n s i z e c h a n g e s , f o r with the a p p e a r a n c e of the fine f r a c t i o n , t h e b e d s b e c o m e i n c r e a s i n g l y d a r k , with a v e r y d a r k b l u i s h tinge r e p l a c i n g the yellow c o l o u r of the s a n d s .
T h e s e g e n e r a l v a r i a t i o n s i n g r a i n s i z e and c o l o u r , r e a d i l y a p p a r e n t when the e n t i r e s e c t i o n i s c o n s i d e r e d , a r e not v i s i b l e when only a p a r t of i t i s e x a m i n e d . Indeed, i n d e t a i l , t h e l i g h t e r and d a r k e r b e d s a l t e r n a t e without a n y a p p a r e n t o r d e r . I t should be added t h a t the d a r k - b l u i s h b e d s b e c o m e b r o w n on e x p o s u r e t o t h e a i r , and t h a t t h i s d a r k c o l o u r w a s t h u s only o b s e r v e d a s a r e s u l t of e x c a v a t i o n i n t o t h e p e r m a f r o s t . M i c r o s c o p i c s t u d y of a p r e p a r e d s e c t i o n f r o m t h e s e s e d i m e n t s showed t h a t the d a r k c o l o u r of t h e d e p o s i t i s d u e t o the p r e s e n c e of o r g a n i c s u b s t a n c e s which a r e f r e q u e n t l y i d e n t i f i a b l e ( p o l l e n , m o s s f r a g m e n t s , e t c ) , which s i n c e t h e t i m e o f ' d e p o s i t i o n have b e e n c o m p l e t e l y p r o t e c t e d f r o m oxidation o r a l t e r a t i o n . T h e c l a y b e d s o v e r l y i n g t h e i c e i n c l u d e o u t e r s h e l l s of m a r i n e f o r a m i n i f e r a v e r y s i m i l a r t o t h o s e e x i s t i n g a t p r e s e n t i n the A r c t i c . T h e s p e c i e s o b s e r v e d ( B u c e l l a F r i g i d a , S p i r o p l e c t a m m i n a B i f o r m i s , E l p h i d i u m I n c e r t u m , E . O r b i c u l a r e , A s t e r e l l i n a P u l c h e l l a ) a r e t y p i c a l c o a s t a l f o r r n s f r o m s h a l l o w w a t e r e n v i r o n m e n t s . T h e s t u d y of t h e f o r a m i n i f e r a c o n t a i n e d i n m y s a m p l e s w a s v e r y kindly u n d e r t a k e n by G. Vilko of the B e d f o r d O c e a n o g r a p h i c I n s t i t u t e ( D a r t m o u t h , Nova S c o t i a ) .
Wood d e b r i s a l s o often o c c u r s within the s a n d y b e d s . T h i s m a t e r i a l h a s been r e w o r k e d f r o m the B e a u f o r t F o r m a t i o n , which c o n t a i n s l a r g e a m o u n t s of wood. It p r o v i d e s no i n d i c a t i o n a s t o the a g e of the e n c l o s i n g s e d i m e n t s . T h e s e d e p o s i t s of sand, g r a v e l l y s a n d , and c l a y a r e v e r y w e l l bedded. L o c a l l y , t h e y p r e s e n t a c h a r a c t e r i s t i c i n t e r s e c t i n g s t r a t i f i c a t i o n . A s t h e s e b e d s h a v e b e e n d e f o r m e d , h o w e v e r , i t is difficult t o r e c o n s t r u c t t h e i r o r i g i n a l a p p e a r a n c e . T h e b e d s a t p r e s e n t d i s p l a y a d i p v a r y i n g f r o m 8 0 " t o 4 0 " . I c e wedge c r a c k s o c c u r a t both e x t r e m i t i e s of t h e s e c t i o n . On t h e l e f t , a n i c e wedge 50 c m in l e n g t h , g i v e s a f a l s e i m p r e s s i o n of widening i n i t s l o w e r p a r t , a s a r e s u l t of t h e w a r p i n g of t h e p r o f i l e . On t h e r i g h t , s e v e r a l i c e wedge c r a c k s f i l l e d with s a n d a n d i c e w e r e o b s e r v e d . Above t h e s e s t r u c t u r e s , t h e a p p e a r a n c e of the m a t e r i a l i s v e r y c o m p l e x . A s s o r t e d m a s s e s of s a n d s and g r a v e l s a p p e a r t o h a v e r e a c h e d t h e i r p r e s e n t p o s i t i o n t h e r e by downward m o v e m e n t of o v e r l y i n g m a t e r i a l . T h i s phenomenon no doubt w a s p r o d u c e d when, following e r o s i o n of t h e pingo, t h e u p p e r p a r t s of t h e s e i c e w e d g e s c a m e within t h e a c t i v e zone and m e l t e d . At the h e a r t of t h i s m a t e r i a l , 1. 5 mm ( s i c ) f r o m t h e p r e s e n t s u r f a c e , o r g a n i c d e b r i s w a s found, which h a s been r a d i o c a r b o n d a t e d a t t h e G e o l o g i c a l S u r v e y of C a n a d a l a b o r a t o r y ( S a m p l e No. GSC 854). I t s a g e i s 7090 f
150 y e a r s . In view of t h e i r p o s i t i o n , t h e s e o r g a n i c r e m a i n s a p p e a r t o be v e r y m u c h younger t h a n the pingo i t s e l f .
F i n a l l y , s l o p e d e p o s i t s with p o o r l y defined bedding o c c u r on the o u t e r p a r t s of the pingo; on t h e r i g h t , t h e s e c o v e r t h e confused m a s s i n which the d a t e d o r g a n i c d e b r i s w a s found.
