Data on the Principles of the Study of Frozen Zones in the Earth's Crust, Issue II

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Data on the Principles of the Study of Frozen Zones in the Earth's Crust, Issue II

V.A. Obruchev Institute of Permafrost Studies

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FOREWORD

In

1950 the Division of Building Research initiated a program of permafrost investigations as part of its research on building problems in northern Canada.

Fundamental and engineering studies are being conducted on a continuing basis to gain a better understanding of and provide solutions to permafrost problems confronting construction activities in northern Canada.

Because of the long history of investigations and construction in the permafrost region of the U.S.S.R., particular interest is being given to the large body of Russian litera,ture now available in this field. The agency in the U.S.S.R. which is conducting research on permafrost and related phenomenon is the V.A. Obruchev Instltute of Permafrost Studies (~nstitut Merzloto- vedeniya). One of its publication series is entitled " ~ a t a on the principles of the study of frozen zones in the earth's crust" and is issued about once a year.

Each issue consists of a collection of papers by several research workers in the Institute covering a variety of fields including terminology, distribution, origin,

association phenomenon, and engineering aspects of perma- frost. Issue

I

was a general introduction to the entire field of permafrost (geocryology). A brief outline of the contents of Issue I1 of which this is a translation, is contained in the Editor's Preface. Issue I11 has been translated and will be available for publication

in the near future: it provides information on studies being conducted currently in the U.S.S.R. and should be of interest and value to those engaged in permafrost investigations.

The Division Is grateful to Mr. D.A. Sinclair of the N.R.C. Translations Section for his cooperation and to Mr. V. Topchy of the same section who spent consider- able time in preparing the translation drafts for

publication. Ottawa

January 1962

R.F. Legget Director

Title:

NATIONAL RESEARCH COUNCIL OF CANADA

Technical Translation 1006

Data on the principles of the study of frozen zones In the earth's crust. Issue 11

(Materlaly k osnovam uchenlya o merzlykh zonakh zemnol kory. Vypusk 11)

Editor : L.A. Melster

Reference : Academy of Sciences of the USSR, V.A. Obruchev

Institute of Permafrost Studies. Moscow, 1355. 74p. (~kademlya Nauk SSSR Instltut Merzlotovedeniya

lmenl V. A. ~brucheva)

Translators: G. Belkov,

M.

Houson and V. Topchy, Translations Section, N.R.C. Library

TABLE OF CONTENTS

Preface....

...

A. I.

Popov. The origin and development of massive fossil

i~e..~...~...,...~...*..~~ S.P. Kachurin.

Is

the development of thermokarst always

indicative of a recession of the permafrost table... F.G. Bakul-in. Concerning M.N. Golldshteinls hypothesis

on the redistribution of moisture and ice segregation

in frozen soil

...

I.Ya.

Baranov. The southern boundary of the range of

perennially frozen soils...

...,...*

L.S. Khomichevskaya. On the idea of an "active layer" in

areas of perennially frozen soils... F.G. Bakulin. Definition of the terms "moisture contentt8

...

and "ice content" of frozen ground

P O D . Sidenko. Certain concepts in the science of

...

permafrost

L,A.

Meister. On the shortcomings in the classification

of ground water in regions of perennially frozen soil. K,F. Voitkovskii. The use of the "reduced coefficient of

thermal capacity1' in computing the depth of the

freezing and thawing of soils

...

Page

PREFACE

The present collection of articles is the second issue of the data on the principles of the study of perennially frozen soil zones.

In

his article "The origin and development of massive fossil ice",

A.I.

Popov examlnes the conditions of forma- tion and produces evidence that ice deposits (wedges) of considerable thickness are contemporaneous (syngenetic) with flood plain soil deposits containing them.

S.P. Kachurin in his article "Is the development of thermokarst always indicative of a recession of the perma- frost table" gives convincing examples of thermokarst

features developing not only where the permafrost is un- stable, but also in such areas where the physical geo- graphical conditions are conducive to a most intensive cooling of soils.

F.

G. Bakulin in his article "Concerning

M.N.

Go1 fdshtein's hypothesis on the redistribution of mois- ture and ice segregation in frozen soil." analyzes

critically the point of view according to which osmotic preasure is supposed to play an exclusive role in the process of redistribution of moisture in freezing soil.

In other articles the authors criticize the short- comings of some of the current concepts and terms in the study of permafrost (geocryology) and suggest improvements of terminology.

The ma-terials collected in this symposium deserve the attention of scientists and experts in the correspond- ing applied fields, and it is hoped that they will be

widely discussed especially since they are intended for discussion and are published in connection with the planned conference on the questions of geocryology (the study of permafrost )

.

THE O R I G I N AND DlXELOPMJ3NT OF MASSIVE FOSSIL ICE

A . I . Popov

Over v a s t a r e a s of t h e n o r t h e r n p l a i n s o f Asia. and America t h e r e a r e massive accumulations of i c e , covered only by a t h i n l a y e r of porous s i l - t y and p e a t y d e p o s i t s . A s we know, such i c e formations a r e most e v i d e n t i n n o r t h e r n S i b e r i a

-

t h e New S i b e r i a n I s l a n d s , t h e Priniorle lowlands, Taimyr Peninsula and c e n t r a l

Yakutia.

For a long time t h e o r i g i n of f o s s i l i c e w a s obscure. This q u e s t i o n always aroused i n t e r e s t i n t h e Russian Academy of S c i e n c e s , and l a t e r i n t h e Academy of S c i e n c e s of t h e USSR.

Russian i n v e s t i g a t o r s A.E. F i g u r i n ( 1 8 2 3 ) , I . A . Lopatin (1876) and A.A. Bunge (1887, 1902) a t d i f f e r e n t times came t o t h e conclu- s i o n t h a t f o s s i l i c e o r i g i n a t e d when water s e e p i n g i n from t h e s u r f a c e f r o z e i n t h e thermal f i s s u r e s and formed i c e wedges.

Another i n v e s t i g a t o r , E.V. T o l l 1 (1897), c o n s i d e r e d t h a t t h e massive- n e s s of f o s s i l i c e c o n t r a d i c t s such an e x p l a n a t i o n of i t s o r i g i n .

, I n h i s opinion, f o s s i l . i c e i s a r e l i c of t h e i c e age, a remnant of

former g l a c i e r s . I . P . Tolmachev (1903) and A.A. Grigor Iev (1932) considered t h a t f o s s i l i c e on t h e S i b e r i a n p l a i n s formed d u r i n g t h e g l a c i a l p e r i o d as a r e s u l t of accumulation of snow which subsequent- l y changed i n t o f i r n . T h i s h y p o t h e s i s proved t o be p o p u l a r and w a s dominant u n t i l r e c e n t l y .

Personal o b s e r v a t i o n s and an a n a l y s i s of t h e m a t e r i a l s c o l l e c t e d by p r e v i o u s i n v e s t i g a t o r s have l e d t h e a u t h o r t o t h e

c o n c l u s i o n t h a t t h e f i r n h y p o t h e s i s i s e n t i r e l y unacceptable. It i s n o t p o s s i b l e t o prove t h i s p o i n t h e r e , b u t t h o s e who wish t o

a c q u a i n t themselves w i t h t h e arguments a g a i n s t t h i s h y p o t h e s i s may a v a i l themselves of t h e a r t i c l e s by P . I . Koloskov ( 1 9 4 6 ) ~

S.P. Kachurin (1346) and A . 1 , Popov (1953).

A c r i t i c a l a n a l y s i s of t h e work of I . A . Lopatin, A.A. Bunge,

P.K. Khnyznikov, P . F . Shvetsov, A . N . T o l s t o v , N.F. G r i g o r P e v ,

N.A. Grave a n d . o t h e r s , made on t h e b a s i s of p e r s o n a l o b s e r v a t i o n s i n 1949 and l a t e r , prompted t h e a u t h o r t o a g r e e with A . A . Bunge t h a t t h e f o s s i l i c e i n n o r t h e r n S i b e r i a i s mainly f i s s u r e i c e .

According t o A . A . Bunge, t h e widening of t h e i c e wedges t a k e s p l a c e owing t o t h e c r a c k i n g up of t h e e n t i r e polygonal system of d e p o s i t s i n w i n t e r and t h e f r e e z i n g of w a t e r t h a t f i l l s t h e

f i s s u r e s i n s p r i n g . Every y e a r t h e f i s s u r e s o c c u r i n t h e same

p l a c e s , c l e a v i n g t h e i c e ; t h e i c e i s t h e r e f o r e v e r t i c a l l y s t r i a t e d , and t h e wedges grow, accompanied by r o l l e r - s h a p e d heaving on t h e s u r f a c e . This e x p l a n a t i o n a p p e a r s t o be c o r r e c t , b u t does n o t

r , , ' Z ' ( , ' t . i r : t .

account f o r some izapm'tfeffteftt g e n e t i c f e a t u r e s of f o s s i l i c e and, a s we s 4 a l - I s e e , needs t o be developed f u r t h e r .

That t h e p r i n c i p a l masses of f o s s i l i c e a r e of t h e same o r i g i n

i s a t t e s t e d f i r s t of a l l by t h e f a c t t h a t i t i s i n v a r i a b l y a s s o c i - a t e d with a l l u v i a l p l a i n s and always c o n t a i n s s i l t y p e a t r i v e r o r l a k e sediments, and by t h e v e r t i c a l s t r e a k s i n i t s t e x t u r e . These f e a t u r e s a r e known t o be c h a r a c t e r i s t i c of f i s s u r e o r wedge i c e , o f t e n found on p r e s e n t - d a y f l o o d p l a i n s and low-lying r i v e r t e r r a c e s * ,

Wedges of f i s s u r e i c e , as a r u l e , j o i n each o t h e r a t a r i g h t a n g l e , and form a polygonal l a t t i c e - s h a p e d d e s i g n o v e r v a s t a r e a s of r i v e r v a l l e y s . F i s s u r e i c e i s o f t e n e r r o n e o u s l y t a k e n f o r l a y e r i c e , s i n c e l a r g e wedges l a i d b a r e lengthwioe by a r i v e r o r t h e s e a a p p e a r l i k e l a y e r s ,

Thus a c c o r d i n g t o A . A . Bunge t h e p r i n c i p a l mass of f o s s i l i c e found on t h e N e w S i b e r i a n I s l a n d s and t h e P r i m o r t e lowlands i s of f i s s u r e o r i g i n . But how can one e x p l a i n t h e f a c t t h a t i t i s s o t h i c k ?

*

P.F. Shvetsov observed wedge i c e i n marine d e p o s i t s on t h e i s l a n d s i n t h e Chaun Bay. I c e wedges were found i n marine d e p o s i t s on t h e A r t i c i s l a n d s by P.A. S h u n s k i i . T h i s proves t h a t t h e f o r m a t i o n of i c e wedges t a k e s p l a c e n o t o n l y s y n g e n e t i c a l l y i n t h e c o u r s e of t h e development of f l o o d p l a i n s , b u t a l s o o c c u r s e p i g e n e t i c a l l y , a s a r e s u l t of f r e e z i n g up of s i l t t h a t i s o v e r s a t u r a t e d w i t h rrioisture, ( ~ d . )

According t o A . A . Bunge, t h e f i s s u r e s form i n f r o z e n d e p o s i t s which were porous; t h e i c e wedges g r a d u a l l y p e n e t r a t e from above downward, c u t t i n g p r o g r e s s i v e l y through d e e p e r l a y e r s of f r o z e n s o i l .

P.F. Shvetsov e x p r e s s e d a somewhat d i f f e r e n t view on t h e

formation of i c e wedges. I n h i s o p i n i o n b o t h t h e f i s s u r e s and t h e i c e wedges o r i g i n a t e d d u r i n g t h e i n i t i a l p e r i o d of t h e f r e e z i n g of unconso!.idated beds t h a t had formed under f l o o d e d c o n d i t i o n s . P.F. Shvetsov a r r i v e d a t t h i s c o n c l u s i o n on t h e b a s i s of t h e argument

t h a t t h e v e r t i c a l temperature g r a d i e n t i n t h e l a y e r s of s o i l s i s

g r e a t e s t a t t h e time when t h e y a r e b e g i n n i n g t o f r e e z e up; as t h e t h i c k n e s s of t h e f r o z e n s t r a t a of s o i l i n c r e a s e s t h e temperature g r a d l e n t d e c r e a s e s .

Thus, t h e most f a v o u r a b l e c o n d i t i o n s f o r t h e f o r m a t i o n of

f i s s u r e s and i c e wedges o c c u r d u r i n g t h e i n i t i a l p e r i o d of f r e e z i n g o f t h e s o i l s ; as t h e d e p t h of f r e e z i n g i n c r e a s e s , t h e i c e wedges grow i n depth. P.F. Shvetsov t h e r e f o r e t a k e s t h e p o i n t of view of e p i g e n e t i c growth of wedge i c e .

E.V. T o l l ' was of t h e o p i n i o n t h a t t h e p e n e t r a t i o n of i c e

f i s s u r e s i n t o t h e l a y e r s of f r o z e n s o i l could h a r d l y o c c u r on a considera-ble acal-e

.

Moreover, t h e f a c t t h a t I c e wedges p e n e t r a t e deep i n t o t h e s o i l poses t h e q u e s t i o n of where l i e s t h e s o i l

d i s p l a c c d by t h e c o n s t a n t l y growing i c e wedges.

S i l t y p e a t d e p o s i t s , which A . A . Bunge c a l l s " e a r t h " d e p o s i t s , form v e r t i c a l columns i s o l a t e d from one a n o t h e r and d i s t r i b u t e d i n checkered o r d e r i n t h e i c e , owing t o t h e polygonal system of t h e i c e d e p o s i t s . E.V. T o l l ' , o b s e r v i n g such " s o i l columns" i n t h e i c e o f t h e s h o r e o u t c r o p s , c o n s i d e r e d them as secondary m i n e r a l f i l l i n g up of f i s s u r e s t h a t c u t through a s o l i d mass of g l a c i e r i c e . Now we know t h a t t h e s e columns a r e b l o c k s d i v i d e d and i s o l a t e d by I c e , and t h e r e f o r e t h e i c e cannot be o f l.ayer formation.

I n view of t h i s i t cannot be c o n s i d e r e d t h a t t h e i c e i s o f

primary formation and t h e s o i l of secondary formation, a s E.V. T o l l ' b e l i e v e d , b u t n e i t h e r can one a g r e e w i t h A . A . Bunge t h a t t h e s o i l i s of primary formation and t h e i c e of secondary formation, which

a p p l i e s t o t h i n f i s s u r e i c e b u t cannot be advanced as an explana- t i o n of t h e o r i g i n of t h i c k f i s s u r e i c e . A new p o i n t of view, which w i l l be e x p l a i n e d belorr, breaks t h e deadlock over t h e cause of c o n t r o v e r s y between E.V. T o l l ' and A.A. Bunge.

The i n v a r i a b l e a s s o c i a t i o n of t h e i c e with s i l t y p e a t d e p o s i t s can be e x p l a i n e d as f o l l o w s . According t o o u r o b s e r v a t i o n s i n t h e North, thermal f i s s u r e s i n s i l t y and p e a t y s o i l s p e n e t r a t e very deeply, down t o and i n c l u d i n g t h e upper l a y e r s of p e r m a f r o s t , f a r beyond t h e depth of s e a s o n a l thawing i n t h e s e s o i l s . I n sandy s o i l t h e f i s s u r e s a r e n o t very deep; t h e y do n o t p e n e t r a t e i n t o perma- f r o s t , and t h e depth of t h e s e a s o n a l thaw i n sand i s g r e a t e r t h a n t h e depth of f i s s u r e p e n e t r a t i o n . I c e wedges t h e r e f o r e form and groh i n s i l t y and p e a t y s o i l s , and as a r u l e do n o t o c c u r i n sand.

These s i l t y and p e a t y f o r m a t i o n s a r e p r i m a r i l y d e p o s i t s i n l a r g e r and s m a l l e r d e p r e s s i o n s i n hulnmocky pol.ygons of f l o o d p l a i n s , u s u a l l y saucer-shaped and boggy, and c o n s i s t mostly of i n t e r b e d d e d

l a y e r s of sandy c l a y s o i l and a l l u v i a l p e a t , and a l s o indigenous p e a t sometimes c o n t a i n i n g ma~nrnoth bones, o c c a s i o n a l l y even with t h e remains of muscular t i s s u e s , s k i n and f u r .

Our r e s e a r c h h a s e s t a b l i s h e d t h a t f i s s u r e i c e forms and develops d u r i n g t h e p e r i o d when a t e r r a c e i s s t i l l a f l o o d p l a i n p e r i o d i c a l l y inundated d u r i n g thaws. l a e n t h e f l o o d i n g and t h e accumulation of d e p o s i t s c e a s e , t h e accumulation of i c e a l o o s t o p s .

The e s t a b l i s h e d connection between t h e f l o o d regime and t h e development of f i s s u r e i c e , and t h e f a c t t h a t f l o o d d e p o s i t s always accompany f o s s i l I c e on a l l u v i a l pl.ains, l e d u s t o t h e c o n c l u s i o n t h a t f l o o d d e p o s i t s and f i s s u r e i c e accumulate s i m u l t a n e o u s l y

(A.I. Popov, 1953).

D e f i n i t e proof of t h e c o r r e c t n e s s of t h i s view i s provided by t h e f o l l o w i n g f a c t s : ( a ) t h e phase c o n t e n t of t h i c k f l o o d d e p o s i t s i s uniform; ( b ) t h e c u r v a t u r e of t h e l a y e r s of s o i l a t p o i n t of c o ~ t a c t with t h e i c e wedges changes unevenly w i t h depth;

( c ) d e p o s i t s between t h e i c e wedges of d i f f e r e n t l e v e l s show h o r i z o n t a l . changes of phase.

The r e q u i s i t e c o n d i t i o n f o r a pa.ral.le1 accumulation b f i c e and s o i l d e p o s l t s i s provided by t h e p e r m a f r o s t regime t h a t accompanies t h i s p r o c e s s . I t s r o l e c o n s l a t s i n t h e g r a d u a l r i s e of t h e perma- f r o s t t a b l e as t h e s u r f a c e of t h e f l o o d p l a i n r i s e s with t h e

accumulation of d e p o s i t s . T h i s t a k e s p l a c e as a r e s u l t of deep w i n t e r f r e e z i n g of s o i l s while t h e depth of summer thawing remains t h e same o r changes o n l y v e r y s l i g h t l y .

I n 1949, 2 . e . a t t h e same time with b u t e n t i r e l y i n d e p e n d e n t l y from o u r work, G. G a l l v ~ i t z , while s t u d y i n g pseudomorphosis of s o i l on wedge i c e i n f l u v i o - g l a c i a l d e p o s i t s i n n o r t h e r n Gernlany, came t o h i s conclusion a b o u t t h e p r o b a b l e s y n g e n e t i c accumulation i n t h e p a s t o f i c e wedges and s o i l sediments.

The growth of f i s s u r e I c e i s t h e r e f o r e p a r a l l e l t o and

simultaneous with t h e accumulation of f l o o d sediments i n a l l u v i a l p l a i n s under s e v e r e c l ~ i r n a t i c c o n d i t i o n s where t h e q u a n t i t y of snow i n w i n t e r i s smal.1.

The connection between i c e formation and t h e regime of s e d i - ment accumulation i n d i c a t e s t h a t i t i s p o s s i b l e f o r t h e f i s s u r e i c e t o be of any t h i c k n e s s , depending on t h e t h i c k n e s s o f f l o o d

d e p o s i t s ; t h e r e f o r e t h e t h i c k n e s s of i c e i s i n d i c a t i v e of t h e

d i r e c t i o n and c h a r a c t e r of e p e i r o g e n i c movements, of t h e subsidence o r r l s e of a l l u v i a l p l a i n s . Thus, t h e t h i c k n e s s of f o s s i l i c e i s

a n i m p o r t a n t d i a g n o s t i c g e o t e c t o n i c f e a t u r e .

Considering massive f o s s i l i c e i n t h e p l a i n s of n o r t h e r n S i b e r i a a s f i s s u r e i c e and n o t f l r n o r g l a c i e r i c e , we a g r e e w i t h

A . A . Bunge (2902) t h a t t h i s e l i m i n a t e s t h e most c o n c r e t e argument

i n f a v o r of a n e a r l y cover g l a c i a t i o n of t h e S i b e r i a n p l a i n s . T h i s c o n c l u s i o n i s a l s o i n complete a c c o r d w i t h t h e o p i n i o n s of I . P .

Gerasimov and K.K. Pkrkov (1939) about t h e l a c k of evidence of p a s t cover g l a c i a t i o n on t h e p l a i n s of n o r t h e r n S i b e r i a .

111e do n o t i n t e n d t o deny t h e e x i s t e n c e of f o s s i l g l a c i e r s ,

f i r n i c e and v a r i o u s o t h e r t y p e s of f o s s i l i c e , b u t we a r e convinced t h a t f i s s u r e i c e i n t h e a l l u v i a l p l a i n s of t h e North

I n r e c e n t y e a r s t h e worlcs of

B.N.

Dostovalov have b e e n v e r y f r u i t f u l i n r e v e a l i n g t h e p h y s i c a l n a t u r e o f t h e o r i g i n o f t h e t e t r a g o n a l system o f low-teinperature t h e r m a l f i s s u r e s ; he gave t h e f i r s t s a t i s f a c t o r y e x p l a n a t i o n , frorn a p h y s i c i s t ' s p o i n t o f view, o f t h e n a t u r e of a simulta,neous a c c w n u l a t i o n of i c e and s e d i m e n t s .

P.A. S h u n s k i l and B . I . V t y u r i n c o l l a b o r a t e d f o r t h e p a s t few y e a r s i n t h e s t u d y of p e t r o g r a p h y of c o n t i n e n t a l f o s s i l i c e ,

p r i m a r i l y f i s s u r e i c e i n t h e n o r t h o f S i b e r i a . The d e t a i l e d and o r i g i n a l p e t r o g r a p h i c s t u d y by P.A. Shwnskii h a s confirmed from a

new a n g l e t h a t t h e principal. mass o f f o s s i l i c e i s o f f i s s u r e o r i g i n . These i n v e s t i g a t i o n s have f u r t h e r confirmed o u r view by p r o v i d i n g a d d i t i o n a l e v i d e n c e and have expanded o u r c o n c e p t on t h e

development o f f i s s u r e i c e . A f t e r a c e r t a . i n p e r i o d o f d o u b t i n g

t h e c o r r e c t n e s s of o u r b a s i c c o n c e p t s r e g a r d i n g t h c p a r a l l e l

a c c u m u l a t i o n o f i c e and s e d i m e n t s , and t h e p r e v a l e n t f o r m a t i o n o f i c e u n d e r f l o o d p l a i n c o n d i t i o n s , P . A . Shumskii l a t e r a c c e p t e d t h i s stanclpoint ( P . A . Shumskii, 1 9 5 4 ) .

Thus t h e main o b s t a c l e which h i n d e r e d E.V. T o l l t , A . A . Bunge and o t h e r i n v e s t i g a t o r s i n t h e i r a t t e m p t t o e x p l a i n t h e n a t u r e o f i c e accunlulation was t h e f a c t t h a t t h e y c o n s i d e r e d s e p a r a t e l y t h e p r o c e s s of i c e forlniation and t h e p r o c e s s o f s e d i m e n t a c c u m u l a t i o n . T h i s d i f f i c u l t y \!:as o n l y overcome a f t e r t h e s e two p r o c e s s e s were examined i n t h e i r i n t e r r e l a t i o n , as a s i n g l e p r o c e s s .

However, t h e r e c o g n i t i o n of t h e s i n l u l t a n e i t y o f t h e accumula- t i o n o f i c e and f l o o d s e d i m e n t s , which e x p l a i n e d t h e n a t u r e of t h i s p r o c e s s i n p r i n c i p l e , d i d n o t p r o v i d e a c o n v i n c i n g e x c l . a n a t i o n o f

i t s nlechanisni i n r e s p e c t t o t h e g e o l o g i c a l and m o r p h o l o g i c a l

f e a t u r e s . U n f o r t u n a t e l y , t h e v:orlcs of P.A. Shwnskii g i v e no answer t o t h i s problem.

E a r l l e r , and l a t e l y even more s o , we made o b s e r v a t i o n s of morphological f e a t u r e s of i c e wedges and t h e a s s o c i a t e d s o i l s t h a t malce t h e i n t e r r e l a t i o n between t h e a c c u m u l a t i o n o f d e p o s i t s and

t h e f o r m a t i o n o f i c e c l e a r e r and m k e p o s s l b l e a b e t t e r u n d e r -

s t a n d i n g of t h e s i ~ l l u l t a n e o u s c o u r s e o f t h e s e two r e l a t e d phenomena. Let us enumerate t h e s e f e a t u r e s and p o i n t o u t t h e i r s i g n i f i c a n c e .

I . The c h a r a c t e r i s t i c v e r t i c a l s t r e a k s o c c u r r i n g i n i c e wedges cannot always be observed, b u t one can o f t e n s e e small v e r t i c a l "elementary wedges" ( a c c o r d i n g t d t h e ternllnology of B. N.

~ o s t o v a l o v ) t h a t c u t a c r o s s t r a n s p a r e n t o r v h l t e n o n - t r a n s p a r e n t i c e . Therefore t h e i c e cannot be s a i d t o c o n s i s t e n t i r e l y of small elementary v e i n s .

2. Sorne times m i n e r a l and o r g a n i c admixtures, i n s t e a d of o c c u r r i n g i n v e r t i c a l f i s s u r e s t r e a k s , a r e d i s p e r s e d throughout

t h e i c e . Consequently t h e s e m a t e r i a l s d i d n o t p e n e t r a t e I n t o i c e through t h e narrow f r o s t f i s s u r e s o n l y .

3 .

On t h e upper s u r f a c e of i c e wedges one can sometimes observe a convex l e n s of "pure" i c e o n l y s l i g h t l y touched o r

e n t l r e l y untouched by v e r t i c a l f i s s u r e s t r e a k s . I n such c a s e s t h e l e n s of "pure" I c e i s sandwiched between t h e i c e wedge and t h e s i l t y p e a t c o v e r i n g i t ( F i g . 1 ) . The impression i s t h a t t h e

a c c r e t i o n of i c e on t h e upper s u r f a c e of a wedge occurred over I t s

e n t i r e a r e a a t t h e same time. The h y p o t h e s i s s u g g e s t s i t s e l f t h a t , f o r some p h y s i c a l r e a s o n o r o t h c r , t h e upper l a y e r of soi1.s had a t a c e r t a i n time s e p a r a t e d f r o ~ n t h e u n d e r l y i n g s u r f a c e of t h e i c e wedge with t h e resulting f o r m a t i o n of a hollow t h a t w a s t h e n f i l l e d w i t h snow and water v:hlch s u b s e q u e n t l y f r o z e up. The hollow spa.ce was t h u s f i l l e d w i t h i c e and a n i c e ].ens was formed.

4. I n most c a s e s one can observe a smooth c u r v a t u r e of t h e l a y e r s i n s i d e polygons, from t h e i r c e n t r e towards t h e i r p e r i p h e r y

( F i g . 2 a ) . This smooth curve i s d i s t u r b e d usual-ly only n e a r t h e p o i n t of c o n t a c t w i t h t h e i c e wedges, where a sudden s h a r p c u r v a t u r e

of t h e l a y e r s i n an upward d i r e c t i o n i s observed; t h e f o l d e d c o n t o r - t i o n of t h e l a y e r s t h e r e o f t e n resembles accordion p l e a t s ( ~ i . g . 2 b ) . The g e n t l y s l o p i n g curve i n s i d e t h e polygons cannot be e x p l a i n e d merely by t h e squeezing-out p r o c e s s s i n c e by i t s e l f t h e l a t t e r would never r e s u l t i n such smooth curves of t h e l a y e r s from t h e c e n t r e t o t h e p e r i p h e r y of t h e polygons o v e r t h e i r e n t i r e mass. On t h e o t h e r hand, t h e c h a r a c t e r of t h e c u r v a t u r e of l a y e r s n e a r t h e c o n t a c t p o i n t w i t h t h e i c e wedges i s d e f i n i t e l y a r e s u l t of squeezing-out of t h e s o i l by t h e widening i c e wedzes owing t o t h e p r o g r e s s i v e f o r m . t i o n of s m a l l elementary wedges.

T h i s m o r p h o l o g i c a l p i c t u r e o f t h e c u r v a t u r e o f l a y e r s i n d i c a t e s t h a t i n s i d e t h e polygons t h e c u r v a t u r e was caused by a p r i m a r y

f a c t o r , w h i l e a t t h e p o i n t o f c o n t a c t w i t h t h e i c e wedges i t was c a u s e d by a secondary f a c t o r .

5.

The hummocks bounding t h e polygons a r e m o s t l y s i t u a t e d above t h e i c e wedges ( ~ i ~ . 3 ) . The s u r f a c e of t h e hummocks,

e s p e c i a l l y i f r e c e n t f i s s u r e f o r m a t i o n i s p r e s e n t , g r a d u a l l y s l o p e s towards t h e c e n t r e of t h e polygon. T h i s s i t u a t i o n o f t h e hummocks i n r e s p e c t t o t h e i c e wedges c a n n o t be e x p l a i n e d s o l e l y by t h e

widening o f t h e i c e wedges and consequent d i s p l a c e m e n t o f t h e s o i l . A c o n n e c t i o n between t h i s m o r p h o l o g i c a l f e a t u r e of t h e s u r f a c e of t h e hwnrnocks and t h e srnooth c u r v a t u r e o f t h e l a y e r s i n s i d e t h e polygons would n a t u r a l l y s u g g e s t i t s e l f h e r e .

6.

I n a n e a r l i e r a r t i c l e ( A . P . Popov, 1953) I mentioned t h e p r e s e n c e o f p e a t l e n t i c u l e s i n s i d e t h e s o - c a l l e d " s o i l columns", which c o n s i s t o f t h e d e p o s i t s i n swampy d e p r e s s i o n s on t h e polygons and which are g r a d u a l l y d i s p l a c e d toward t h e i c e wedges. Such

l e n s e s r e g u l a r l y o c c u r i n s e v e r a l l a y e r s , one above t h e o t h e r ,

i n t e r b e d d e d w i t h m i n e r a l l a y e r s c o n s i d e r a b l y l e s s r i c h i n v e g e t a b l e m a t t e r . Each p e a t l e n t i c u l e marks t h e one-time p o s i t i o n o f a small bog w i t h i n t h e polygon on t h e s u r f a c e o f a f l o o d p l a i n t e r r a c e , which g r a d u a l l y became f i l l e d a s t h e d e p o s i t s accumulated.

P r e v i o u s l y w e t h o u g h t t h a t p e a t l e n t i c u l e s c o r r e s p o n d e d t o t h e p e r i o d s when f l o o d w a t e r s d i d n o t r e a c h t h e p o l y g o n s , o r r e a c h e d them o n l y seldom, and when t h e r a t e o f s e d i m e n t a t i o n was low b u t t h e growth o f v e g e t a t i o n i n swamps was most i n t e n s e ; t h e mineral. l a y e r s between t h e l e n s e s were c o n s i d e r e d as c h a r a c t e r i s t i c of t h e p e r i o d s o f f r e q u e n t f l o o d s , which d e p o s i t e d p r o f u s e m i n e r a l m a t e r i a l .

Now, i n t h e l i g h t of new f a c t s , we t a k e a d i f f e r e n t view, a d v a n c i n g a d i f f e r e n t e x p l a n a t i o n of t h e a l t e r n a t i o n o f p e a t y

l e n t i c u l e s w i t h m i n e r a l l a y e r s , a s c r i b i n g i t n o t t o t h e p e r i o d i c a l o c c u r r e n c e s o f h i g h e r and lower f l o o d s , b u t t o t h e n a t u r e o f

developnlent o f t h e e n t i r e complex o f i c e , o r g a n i c m a t t e r and

of sediments. More w i l l be s a i d l a t e r .

7 . The c o n t a c t s u r f a c e between i c e wedges and t h e s o i l i s

u s u a l l y uneven, jagged. This i s due t o t h e s m a l l narrow wedges which p r o t r u d e from t h e wedge i n t o t h e a d j a c e n t l a y e r s of s o i l

( I . 4 ) These l i t t l e wedges occur a l o n g t h e c o n t a c t l i n e a t more o r l e s s even i n t e r v a l s . C a r e f u l o b s e r v a t i o n shows t h a t t h e occurrence and t h e s p a c i n g of t h e s e l i t t l e wedges a r e a s r e g u l a r as t h e a l t e r n a t i o n of t h e p e a t and m i n e r a l l a y e r s i n t h e a d j a c e n t s o i l mass and, f u r t h e r m o r e , a r e i n c o n n e c t i o n w i t h i t . These L a t e r a l wedges p e n e t r a t e i n t o t h e m i n e r a l s o i l and s e p a r a t e i t s l a y e r s , u s u a l l y immediately above t h e p e a t l e n t i c u l e . These s m a l l

l . a t e r a 1 wedges a r e n o t h i n g e l s e b u t t h e s h a r p ends of t h e "pure" i c e l e n s e s mentioned b e f o r e , c u t o f f by s m a l l elementary i c e wedges w i t h i n t h e v e r t i c a l wedge. The remaining p a r t of t h e l e n s o f

o r i g i n a l l y "pure" i c e i s i n c o r p o r a t e d i n t h e main body o f t h e i c e wedge; i t i s t r a v e r s e d by t h e i n t e r n a l elementary wedges and

a p p e a r s t o be v e r t i c a l l y s t r i a t e d .

The b a s i c mechanism of p a r a l l e l accumulation of a l l u v i u m and f i s s u r e i c e w a s p r e v i o u s l y d e s c r i b e d ( A . I . Povov, 1953) as a

s u c c e s s i v e annual upward s h i f t of r e l a t i v e l y s h a l l o w f r o s t f i s s u r e s t h a t a r e s u b s e q u e n t l y f i l l e d w i t h i c e ; and t h e t o p p a r t o f t h e

f i s s u r e w a s thought t o be i n a l a y e r of s o i l s o l i d l y f r o z e n d u r i n g t h e p r e c e d i n g w i n t e r and n o t thawed o u t i n summer owing t o a n

a d d i t i o n of d e p o s i t s on t h e s u r f a c e of t h e f l o o d p l a i n . However, i n such a c a s e t h e wedge i c e would i n e v i t a b l y c o n t a i n i n c l u s i o n s of m i n e r a l s o i l from t h e h o r i z o n t a l l a y e r s which p r e v i o u s l y covered

t h e wedge, f o r i t i s d o u b t f u l t h a t each s u c c e s s i v e f i s s u r e would c u t p r e c i s e l y i n t o l a s t y e a r ' s wedge, which h a r d l y r i s e s above t h e s u r f a c e o f t h e t o t a l wedge; i t Is more p r o b a b l e t h a t t h e f i s s u r e s would form i n d i f f e r e n t p l a c e s above t h e i c e wedge and c u t o f f

l a y e r s of m i n e r a l s o i l c o v e r i n g t h e wedge. However, wedge i c e c o n t a i n s no such i n c l u s i o n s of m i n e r a l s o i l . Consequently, t h e mechanism of t h e v e r t i c a l development o f i c e wedges must be e x p l a i n e d i n some o t h e r way.

It a p p e a r s t h a t t o g e t h e r a l l t h e s e f e a t u r e s d i s c l o s e t h e g e n e r a l n a t u r e of t h e development of t h e e n t i r e complex of i c e , o r g a n i c m a t t e r and m i n e r a l s o i l . I n t h e a n a l y s i s of t h e s e morpho- l o g i c a l f e a t u r e s t h e most i m p o r t a n t c l u e t o t h e n a t u r e of t h e whole phenomenon i s provided by t h e smooth c u r v a t u r e of t h e l a y e r of

s i l t y p e a t s o i l c o v e r i n g t h e whole wedge and s e p a r a t e d by a l e n s of "pure" i c e from t h e s u r f a c e of t h e wedge i c e w i t h c h a r a c t e r i s t i c v e r t i c a l s t r e a k s .

I n o u r o p i n i o n , t h e most c o r r e c t i n t e r p r e t a t i o n of t h i s f a c t can be made by t a k i n g advantage of B.N. D o s t o v a l o v ~ s id e a t h a t v e r t i c a l f i s s u r i n g i n a s o l i d mass i s i n v a r i a b l y accompanied by h o r i z o n t a l f i s s u r e s . B.N. Dostovalov n o t e s t h a t i n such c a s e s t h e uppQr s u r f a c e s t e n d t o become concave, while t h e lower s u r f a c e s t e n d t o become convex. That t h i s i s s o , a t l e a s t i n d r y i n g c l a y e y s o i l s , i s known from o b s e r v a t i o n s . T.Ya. Gorazdovakii (1950) g i v e s r e s u l t s of experiments showing t h a t t h e s t r e s s e s of d i s t e n s i o n

o f t e n c o n s i d e r a b l y exceed the f o r c e of cohesion. T h i s f a c t a l s o p o i n t s t o a v e r y h i g h p r o b a . b i l i t y of h o r i z o n t a l f i s s u r e s forming I f n o t b e f o r e t h e n , i n any c a s e , c o n c u r r e n t l y w i t h v e r t i c a l f i s s u r e s .

S t r e s s e s o r i g i n a t i n g and t h e n g r a d u a l l y mounting as t h e f r o z e n s o i l s c o n t a i n i n g wedge i c e become c o l d e r , f i n a l l y d i s s o l v e n o t o n l y i n v e r t i c a l f i s s u r e s i n t h e s o i l and t h e i c e b u t a p p a r e n t l y a l s o i n silnul t a n e o u a l y forming h o r i z o n t a l f i s s u r e s

.

The l a t t e r a r e most 1ilcel.y t o o c c u r a t t h e c o n t a c t between t h e upper s u r f a c e of i c e wedges and t h e o v e r l y i n g s t r a t u m of s o i l

-

which i s w e l l founded p h y s i c a l - l y . Thus a hollow apace, a f i s s u r e , i s formed above t h e

i c e wedges. But a p p a r e n t l y t h e b r e a k i n g away of t h e s o i l from t h e s u r f a c e of a n i c e tredgt' i s n o t a l l . The upper s t r a t u m of t h e s o i l between t h e wedges behaves as a s i n g l e system and t h u s probably developes t h e c u r v a t u r e o v e r i t s e n t i r e a r e a . T h i s c u r v a t u r e must be a l l t h e g r e a t e r t h e t h i c k e r i s t h e s t r a t u m undergoing deforma- t i o n and t h e g r e a t e r i t s expanse. In o u r c a s e , huge concave

polygons o r i g i n a t e d , t h e s i d e s o f which sometimes r i s e s e v e r a l dozen m e t r e s . The s c a l e o f t h i s phenomenon i s a p p a r e n t l y g r e a t e s t

i n s i l t y , c l a y e y and p e a t y s o i l s , t o a h i g h d e g r e e c a p a b l e of s u c h d e f o r m a t i o n o f t h e l a y e r s i n f r o z e n c o n d i t i o n ; i t c a n n o t be as g r e a t i n sandy s o j l s , which preswnanly a r e l e s s c a p a b l e of such d e f o r m a t i o n . How, t h e n , s h o u l d we p i c t u r e t h e development o f p o w e r f u l f i s s u r e i c e i n c o n n e c t i o n w i t h sediment a c c u m u l a t i o n , on t h e b a s i s of t h e a f o r e - m e n t i o n e d f a c t s and t h e i r i n t e r p r e t a t i o n ? When a s i l t y p e a t l a y e r of s u f f i c i e n t t h i c k n e s s a c c u m u l a t e s on a f l o o d p l a i n o r any l a n d s u r f a c e p e r i o d i c a l l y s u b j e c t e d t o m o i s t u r e and s u b s e q u e n t d r y i n g , f r o s t f i s s u r e s w i l l o c c u r and t h e f i r s t small i c e wedges w i l l be formed. Subsequent f i s s u r i n g w i t h i n t h e i c e

wedges, due t o s p e c i f i c p h y s i c a l c o n d i t i o n s , i n e v i t a b l y p r o d u c e s n o t o n l y r e l a t i v e l y deep v e r t i c a l f i s o u r e s b u t a l s o h o r i z o n t a l f i s s u r e s a t r i g h t a n g l e s t o t h e v e r t i c a l ones, n e a r t h e u p p e r s u r f a c e o f t h e wedge, where t h e i c e and t h e o v e r l y i n g stratum o f s o i l cor'le i n

c o n t a c t . A t t h e same time t h e r e o c c u r s a n a t u r a l r i s e o f t h e o u t e r e d g e s of t h e p o l y g o n s , formed as a r e s u l t o f t h e t e t r a g o n a l network of f r o s t f i s s u r e s . The defornintion of t h e s i l t y i n n e r l a y e r o f a polygon r e s u l t s i n a g r a d u a l , smooth c u r v e , t h e s l o p e of which

i n c r e a s e s froni t h e c e n t r e t o t h e p e r i p h e r y o f t h e polygon. The l a y e r s a r e t h e r e f o r e b e n t most a t t h e edges of t h e polygon. Owing t o t h i s p r o c e s s , b u t i n p a r t a l s o a s a consequence o f t h e i n e v i t a b l e f o r c l n g o u t o f t h e s o i l d u r i n g t h e f o r m a t i o n o f a n i c e wedge, t h e s l o p i n g e l e v a t i o n s come i n t o b e i n g t h a t s u b s e q u e n t l y becorne t h e hummocky w a l l s a t t h e o u t s i d e edges o f a pol.ygon.

A s t h e h o r i z o n t a l f i s s u r e s , b r a n c h i n g o f f from t h e v e r t i c a l o n e s , a r e forming, t h e c o v e r i n g s o i l b r e a k s away from t h e s u r f a c e of t h e i c e wedge. I n w i n t e r , snow w i t h m i n e r a l and v e g e t a b l e

p a r t i c l e s d r i f t s i n t o t h e hollow, and i n t h e b e g i n n i n g o f s p r i n g i t

i s f i l l e d w i t h w a t e r , which h a s a t e m p e r a t u r e o f a b o u t O°C. The

water t h a t f i l l s t h i s holl-ow, s t i l l f r o z e n on a l l s i d e s and o f t e n c o n t a i n i n g snow, f r e e z e s and forms a h o r i z o n t a l l e n s above t h e o l d i c e wedge. The r a i s e d " w ~ n g s " of t h e t o r n stratum above t h e i c e wedge c o n s e q u c n t l y r e n n i n f a s t i n t h i s p o s i t i o n .

Apparently t h e p r o c e s s of f o r m a t i o n of h o r i z o n t a l f i s s u r e s between t h e i c e and t h e o v e r l y i n g s o i l , t h e growth of a n i c e l e n s and t h e consequent upward growth of t h e e n t i r e wedge goes on f o r a c e r t a i n number of y e a r s , t i l l t h e c u r v i n g of t h e l a y e r s of s o i l i n polygons has reached t h e l i m i t of deformation, and t h e r i s i n g of

t h e o u t e r edges, t h e i n i t i a l . hummocks, s t o p s . A s a r e s u l t t h e r e

w i l l be a s h a r p l y - d e f i n e d curved system ( i c e wedges r i s i n g h i g h , hummocks sharp1.y s l o p i n g , and concave polygons e n c l o s e d w i t h i n ) , which w i l l exclude a f u r t h e r formation of h o r i z o n t a l f i s s u r e s d u r i n g subsequent v e r t i c a l f i s s u r i n g and w i l l no l o n g e r p e r m i t t h e

b r e a k i n g away of t h e s o i l from t h e i c e s u r f a c e .

9t

Thus t h e upper p a r t of t h e wedge c o n s i s t s of pure" i c e b u t with an admixture o f m i n e r a l and o r g a n i c p a r t i c l e s and h a s t h e shape of a convex l e n s . T h i s new p a r t of t h e wedge can be subse- q u e n t l y s p l i t by v e r t i c a l f r o s t f i s s u r e s , and e l e m e n t a r y s m a l l i c e wedges w i l l t h e n form i n i t , s t r e a k i n g t h e p r e v i o u s l y "pure" i c e . Owing t o t h i s t h e i c e wedge widens and grows, s q u e e z i n g o u t t h e

l a y e r s of s o i l and, consequently, t h e i n n e r d e p r e s s i o n i n t h e p o l y - gon i n c r e a s e s . When t h e elementary small i c e wedges c o n t i n u e t o form f o r a l e n g t h of time, t h e a r c h e d s u r f a c e of t h e i c e l e n s g r a d u a l l y becomes almost f l a t . Thus, t h e i n i t i a l c u r v a t u r e of t h e l a y e r s i n t h e v i c i n i t y of i c e i s c o n d i t i o n e d n o t s o much by t h e squeezing-out p r o c e s s as by t h e c u r v i n g d u r i n g h o r i z o n t a l f i s s u r i n g and, secondl-y, by t h e n a t u r a l d i s t r i b u t i o n of d e p o s i t s accumulating a l o n g t h e i n i t i a l s u r f a c e . This i s a t t e s t e d t o by t h e c h a r a c t e r of t h e curve, i t s smooth climb from t h e c e n t r e of t h e polygon towards t h e p e r i p h e r y .

The secondary c o n t o r s i o n of l a y e r s a t t h e c o n t a c t p o i n t w i t h t h e i c e wedges i s caused by t h e widening of t h e i c e wedges owing t o t h e c o n t i n u i n g growth of elementary small i c e wedges. These

secondary c u r v e s a r e s h a r p e r ; t h e y a r e superi.mposed on t h e primary ones and i n c r e a s e t h e c u r v a t u r e . T h e r e f o r e , t h e c u r v a t u r e of l a y e r s i n i n n e r polygons, caused by t h e i n i t i a l f i s s u r e f o r m a t i o n , h a s a more g e n e r a l s i g n i f i c a n c e t h a n t h e c u r v e s due t o t h e s q u e e z i n g o u t

The l i m i t o f d e f o r n ~ a t i o n of t h e l a y e r s o f s o i l p r e d e t e r m i n e s t h e maximum h e i g h t o f t h e wedge, which c a n n o t d e v e l o p upwards when no more h o l l o w s p a c e s form between t h e i c e and t h e s o i l . F o r t h i s r e a s o n t h e s u r f a c e o f t h e i c e wedge ~ : i i i remain on t h e same l e v e l f o r a c e r t a i n t i m e .

I n t e r n a l p o l y g o n a l d e p r e s s i o n s , f o r m i n g as d e s c r i b e d above, become bogs i n which v i g o r o u s bog v e g e t a t i o n w i l l d e v e l o p . T h i s a l o n e i s enough f o r d e p o s i t s i n s i d e t h e p o l y g o n s t o a c c u m u l a t e f a s t e r t h a n on t h e hummocks o v e r t h e i c e . The r a p i d a c c w n u l a t i o n o f p e a t h e r e i s accompanied by a r e l a t i v e l y r a p i d a c c u m u l a t i o n o f s i l t . Owing t o c e r t a i n h y d r o l o g i c a l f a c t o r s , f l o o d d e p o s i t s on t h e hummocks w i l l be f e w e r t h a n i n s i d e t h e p o l y g o n a l d e p r e s s i o n s , which a c t l i k e s e d i m e n t a t i o n c o l l e c t o r s f o r mud and p a r t i c l e s o f v e g e t a b l e waste c a r r i e d by f l o o d w a t e r s . The humrnocks, e s p e c i a l l y i f t h e y a r e h i g h , w i l l become f r e e o f f l o o d w a t e r w h i l e t h e p o l y g o n s a r e s t i l l i n u n d a t e d and t h e w a t e r i s r u n n i n g r a p i d l y enough f o r t h e mud t o remain i n s u s p e n s i o n .

Thus t h e p o l y g o n a l d e p r e s s i o n s c o l l e c t mud a n d v e g e t a b l e s e d i - ments i n t h e p o o l s t h a t remain a f t e r t h e l e v e l o f f l o o d w a t e r d r o p s , and a l s o accumulate p e a t d e p o s i t s , a s a l r e a d y mentioned. T h i s

a c c u m u l a t i o n o f d e p o s i t s and s e d i m e n t s , more r a p i d i n deep p o l y g o n a l d e p r e s s i o n s t h a n on t h e hummocks, soon f i l l s t h e s e d e p r e s s i o n s ;

t h e r e f o r e t h e d i f f e r e n c e between t h e l e v e l o f t h e p o l y g o n a l bogs and t h e hurrunocks i s p r o g r e s s i v e l y r e d u c e d a n d t h e s u r f a c e u l - t i m a t e l y f l a t t e n s o u t .

While t h e l i m i t h e i g h t of a n i c e wedge i s d e t e r m i n e d by t h e l i m i t c u r v a t u r e of s o i l on one hand and t h e l a g o f d e p o s i t accumula- t i o n b e h i n d t h e i n c r e a s i n g s l o p e on t h e o t h e r , t h e u l t i m a t e w i d t h o f a n i c e wedge, a c c o r d i n g t o B.N. D o s t o v a l o v , depends on t h e e x t e n t t o which t h e s o i l c a n be compressed a n d s q u e e z e d o u t .

I f no f u r t h e r a c c u m u l a t i o n o f d e p o s i t s t a k e s p l a c e a f t e r t h e d e p r e s s i o n s have been f i l l e d w i t h p e a t and o t h e r m a t t e r , as i s u s u a l l y t h e c a s e i n bogs above t h e f l o o d p l a i n s , some l a k e s t h a t a r e d r y i n g o u t , e t c . , t h e n b o t h t h e l a t e r a l and t h e v e r t i c a l growth o f i c e wedges w i l l s t o p a l t o g e t h e r . T h e r e f o r e t h e c r o s s - s e c t i o n of

a norma.1 s i n g l e - c y c l e s o l 1 accumul-ation between two i c e wedges i n v a r i a b l y shows p e a t on t o p .

I f t h e a c c u m u l a t i o n o f d e p o s i t s c o n t i n u e s , t h e n t h e trhole a r e a , i n c l u d i n g t h e hummocks, which a r e now l e v e l w i t h t h e s u r f a c e o f t h e pol.ygons owing t o t h e a c c u m u l a t i o n o f p e a t and s e d i m e n t s , w i l l be covered w i t h f l o o d - p l a i n s i l t , p o o r i n v e g e t a b l e m a t t e r . A horno- geneous l a y e r o f s i l t w i l l form because t h e e a r l i e r c o n d i t i o n s f a v o u r a b l e f o r t h e accuniulation o f p e a t and uneven d i s t r i b u t i o n o f

s i l t on t h e f l o o d p l . a i n ha.ve now disappea.red.

Subsequent f r o s t f i s s u r i n g i n t h i s s t r a t u r n o f s i l t w i l l r e s u l t i n c u r v a t u r e , t h e newly accumulated s o i l w i l l . b r e a k away from t h e o l d s u r f a c e o f t h e i c e wedge, and i n p a r t p e r h a p s a l s o from t h e u n d e r l y i n g s i l t y p e a t s o i l permeated w i t h i c i n g . Snow w i t h s a n d , d u s t a n d p a r t i c l e s o f v e g e t a b l e m a t t e r w i l l a g a i n p e n e t r a t e i n t o t h i s hollow i n w i n t e r , and i n s p r i n g t h e water, which t h e n f r e e z e s i n i t , c a u s i n g a new a c c r e t i o n o f t h e i c e wedge a t t h e t o p a n d t h u s c r e a t i n g a new medium f o r f u r t h e r f r o s t f i s s u r i n g and t h e p e n e t r a - t i o n of new e l e m e n t a r y i n n e r i c e wedges.

Thus, t h e e n t i r e u p p e r s u r f a c e o f an i c e wedge grows upward owing t o t h e r e p e a t e d a c c r e t i o n o f a n i c e l e n s a t t h e t o p of t h e wedge.

T h i s way i t becomes u n d e r s t a n d a b l e P J I I ~ i t i s t h a t a t t h e con- t a c t p o i n t w i t h i c e , s o i l . l a y e r s under t h e p e a t l e n t i c u l e a r e c u r v e d most of a l l , t h e l e n t i c u l e i t s e l f i s curved l e s s and t h e c o v e r i n g m i n e r a l s o i l l e a s t o f a l l ; i t i s h e r e t h a t t h e small l a t e r a l . i c e wedges, p r o t r u d i n g from t h e v e r t i c a l wedge, p e n e t r a t e i n t o t h e s o i l . Higher up t h e s o i l i s a g a i n s h a r p l y c u r v e d , t h e p e a t l e n t i c u l e l e s s c u r v e d , and s o on upward. Thus a r i s e s t h e c h a r a c t e r i s t i c p r o f i l e of t h e wedge, t h e uneven v e r t i c a l l i n e of i t s s i d e s .

T h i s p a t t e r n i s o b s e r v e d t h r o u g h o u t t h i s f a c i e s t y p e , a l t h o u g h some r e a d i l y e x p l i c a b l e d e p a r t u r e s do o c c u r . P r o f i l e s u s u a l l y show auch i n t e r r e l a t i o n between o r g a n i c - m i n e r a l d e p o s i t s and i c e ;

e x c e p t i o n s a r e due t o e r o s i o n , t h e r m o k a r s t , s l i d i n g and o t h e r

n a t u r a l p r o c e s s e s , t h e o c c u r r e n c e o f which normal1.y c o m p l i c a t e s t h e i n t e r r l e l a t i o n s h i p between t h e i c e and t h e o r g a n i c and m i n e r a l s o i l .

However, where such p r o c e s s e s t h a t might complicate i c e accumulation a r e l i m i t e d , t h e g e n e r a l p a t t e r n i s c l e a r l y r e c o g n i z a b l e i n t h e

e n t i r e i c e and organic-mineral complex, which shows t h e normal c y c l e and t h e c h a r a c t e r i s t i c interdependence of e f f e c t s i n i t s f o r m a t i o n .

We s e e t h a t t h e accumulation of d e p o s i t s i s l a g g i n g behind t h e growth of i c e wedges upward; t h i s circumstance u l t i m a t e l y h i n d e r s t h e upward growth of wedge i c e . I n o r d e r t h a t i c e wedges c o u l d resume growing upward, accumulation of d e p o s i t s must a g a i n "over- t a k e " t h e i c e wedges. Whenever t h i s happens, i . e . when t h e l e v e l of d e p o s i t s i n t h e polygons evens o u t w i t h t h e l e v e l of t h e s o i l above t h e i c e wedges and a new l a y e r of d e p o s i t s reaches a c e r t a i n t h i c k n e s s , t h e p r o c e s s of i c e accumulation w i l l resume. The

l e n t i c u l e s of p e a t and m i n e r a l s o i l , which a l t e r n a t e r e g u l a r l y i n p r o f i l e s between i c e wedges, a r e a r e s u l t of such a " d i a l e c t i c a l "

development of t h e e n t i r e system under s u i t a b l e c o n d i t i o n s and a n e x p r e s s i o n of s p o n t a n e i t y i n t h e c o u r s e of p r o g r e s s i v e d e p o s i t accumulation. T h i s i s why t h i c k f i s s u r e i c e can o n l y develop on f l o o d p l a i n s while an abundant d e p o s i t acculnulation i s i n p r o g r e s s .

It i s a180 c l e a r why some of t h e i c e i n wedges may be t r a n s - p a r e n t , w i t h o u t v e r t i c a l s t r e a k s : i t i s t h e i c e of a l e n s n o t a s s i m i l a t e d by f r o s t f i s s u r i n g . Sometimes i t i s n o t o n l y w i t h o u t v e r t i c a l s t r e a k s b u t a l s o white; t h i s happens when i n i t i a l l y t r a n s - p a r e n t i c e becomes white due t o r e p e a t e d c o n t r a c t i o n ; and snow

converted t o i c e w i l l a l s o remain white.

P . A . Shwnskii h a s r e c o u r s e t o s u b l i m a t i o n of i c e i n o r d e r t o e x p l a i n t h e c h a o t i c d i s t r i b u t i o n of i c e c r y s t a l s i n wedges, probably assuming, a f t e r A . A . Bunge and o t h e r i n v e s t i g a t o r s , t h a t i c e o n l y forms i n small f r o s t f i s s u r e s . However, on t h e b a s i s of what h a s been s a i d i t f o l l o w s t h a t a d i f f e r e n t e x p l a n a t i o n i s p o s s i b l e f o r t h e c h a o t i c d i s t r i b u t i o n of i c e c r y s t a l s , a l r e a d y e x p r e s s e d b e f o r e by B.N. Dostovalov i n unpublished s t a t e m e n t s . N a t u r a l l y , i n i c e

t h a t h a s been o n l y p a r t i a l l y assirnil-ated by elementary i n t e r n a l . wedges t h e d i s t r i b u t i o n of c r y s t a l s w i l l be i n p a r t r e g u l a r and i n p a r t c h a o t i c .

From t h i s a n g l e i t becorr~es c l e a r why i t i s t h a t t h e p a r t i c l e s of v e g e t a b l e m a t t e r , s m a l l lumps of p e a t and m i n e r a l s o i l and

p a r t i c l e s o f sand, sometimes c o n t a i n e d i n u n s t r a t i f i e d i c e of t h e wedge, a r e s c a t t e r e d , n o t assembled i n v e r t i c a l s t r e a k s : t h e y a r e brought i n w i t h snow and w a t e r o r f a l l down from t h e v a u l t of t h e hollow t h a t forms between t h e upper s u r f a c e of t h e wedge and t h e o v e r l - y i n ~ s o i l . T h i s t i e s i n w i t h t h e view t h a t wedge i c e i s of mixed water and snow o r i g i n .

The accumulation of d e p o s i t s t h e r e f o r e promotes f i s s u r e i c e formation by p r o v i d i n g t h e very medium i n which f i s s u r e s s p r i n g up, b u t h o l d s i t back when i t l a g s behind t h e development of i c e wedges, owing t o e x c e s s i v e c u r v a t u r e of t h e o u t e r edges of polygons.

D e p o s i t s predetermine t h e p o s s i b i l i t y of t h e development o f i c e wedges; and t h e n t h e wedges check t h e i r own development as t h e y t e n d t o grow o u t of t h e d e p o s i t s . The p e a t i n t h e polygons i s a consequence of t h e development of f r o s t f i s s u r e s and wedge i c e

accumulation and i n t u r n t h e growth of i c e i s an i n d i r e c t r e s u l t of t h e accumulation of d e p o s i t s i n s i d e polygons.

Thus t h e accumulation of d e p o s i t s l i t e r a l l y dominates t h e regime of f i s s u r e i c e accumulation, cieterrnining t h e c o n d i t i o n s of i t s f o r m t i o n and i t s v e r t i c a l t h i c k n e s s . The developnlent of i c e wedges i s more r a p i d t h a n t h e accumulation of d e p o s i t s , and t h e d e p o s i t s t h e r e f o r e a r e c o l l e c t e d i n polygonal d e p r e s s i o n s , Only t h e l a y e r t h a t " o v e r t a k e s " t h e wedge p r o v i d e s ground f o r i t s f u r t h e r v e r t i c a l development. T h i s c o n s t i t u t e s t h e e s s e n c e of

paral-1-el and simultaneous accurrulation of i c e and d e p o s i t s . It is

f o r t h i s r e a s o n t h a t any s u b s t a n t i a l - accumulation o f f i s s u r e i c e can always be c o n s i d e r e d a forma.tion synchronous w i t h t h e upper s t r a t a of d e p o s i t s c o n t a i n i n g i t .

We a r e i n no way i n c l i n e d t o c o n s i d e r t h e problem of t h e o r i g i n and development of massive f i s s u r e i c e as f i n a l l y c l a r i f i e d . The s o l u t i o n p u t forward s h o u l d b e viewed as a n endeavour t o draw n e a r e r t o a clarification of t h i s q u e s t i o n by e s t a b l i s h i n g c l o s e r g e n e t i c i n t e r r e l - a t i o n s betvreen s e p a r a t e corliponents of t h i s complicated n a t u r a l phenomenon on t h e b a s i s of p r e v i o u s l y known f a c t s i n t e r - p r e t e d i n a n e w way.

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1 2(".4; ,7m

F i g . 1

A l e n s of "pure" i c e , e n c l o s e d between an I c e wedge and t h e s o i l c o n t a i n i n g It

1

-

wedge i c e ; 2

-

l e n s i c e ;

3

-

sandy-clay s o i l

zm

3m

P i g . 2

Smooth c u r v a t u r e of t h e l a y e r s of s o i l between two i c e wedges The c o n t o r t i o n of t h e l a y e r s of s o i l n e a r t h e c o n t a c t p o i n t w i t h an i c e wedge

1

-

wedge i c e ; 2

-

s o i l l a y e r s between i c e wedges;

F i g .

3

Growing i c e wedge on a f l o o d p l a i n . Layers of s i l t y - p e a t s o i l . s l o p e g r a d u a l l y i n t o t h e polygons

F i g .

4

Contact p o i n t between i c e wedge and s o i l , showing t h e v e r t i c a l d i s t r i b u t i o n of p e a t l e n t i c u l e s , t h e c h a r a c t e r i s t i c change

i n t h e s l o p e of s o i l l a y e r s and t h e r e g u l a r occurrence of m i n i a t u r e wedges p r o t r u d i n g h o r i z o n t a l l y

i n t o t h e s o i l

IS THE DEVELOPMENT OF THEFDIOKARST ALWAYS INDICATIVE OF A RECESSION OF THE PERMAFROST TABLE

S

.

P

.

Kachurin

It is generally assumed that thermokarst, i.e. settling or caving of the ground due to melting of ice contained in frozen sub- soils, is an unquestionable sign of permafrost recession in the regions where thermokarst occurs. This is almost invariably asser- ted in general descriptions of areas with thermokarst activity.

This is generally correct, the development of thermokarsts usually is an outcome of permafrost recession.

But whether thermokarst is necessarily a sign of permafrost re- cession is a different question. Can the existence of therrnokarst in itself be sufficient evidence of permafrost recession?

The question raised is a very important one, especially since it is only the positive answer that keeps recurring in literature. While it is correct in a number of permafrost areas, there are never- theless many instances when other facts contradict this assumption. It is therefore important to shed some light on this problem.

We must bear in mind that "permafrost recession" in a general sense means a decrease in the "cold reserves" of the perennially frozen subsoil

(M.I.

Sumgin, 1937)

.

This decrease in the "cold reserves" is understood as a warming of a stratum of subsoil over a considerable territory as a result of a change in the physical geographical conditions that leads to a rise in the temperature of the subsoil and to its complete thawing. This rise in temperature and the resulting recession of permafrost is accompanied by other effects, for instance the thawing-out of ice lenses and layers of ground ice and the consequent caving of the ground, i.e. to the de- velopment of therrnokarsts

.

Thus, permafrost recession due to natural conditions is usually connected with a change in the heat regime in the perennially frozen layer, over a large area. A change in the thermal conditions over