by
I. P. K A R T A S H O V
According to th e classic concept of Davis, m ountainous regions are reduced to d enudational plains b y the g ra d u a l llattening of slopes and low ering of 'Svorn- d ow n” interfluves. An alte rn a tiv e to this concept is th e idea of slopes re treatin g
“ parallel to them selves” and so m aintaining th e ir steepness. This re tre a t results in th e form ation of gently sloping d enudational surfaces, th e pedim ents. The gro w th of pedim ents and th e ir coalescence lead to the fo rm ation of pediplains.
N ow adays th e p o p u larity of this idea is certain ly increasing. T he m ost vig o r
ous supp o rter of th e p ediplanation concept seems to he L. C. K ing (1953, 1962), who believes all den u d atio n al plains in all clim atic zones to be pediplains.
There are also some supporters of th e p ediplanation concept am ong S oviet geom orphologists.
M any au th o rs believe th e relics situ ated a t various altitu d e s of planation surfaces to be of different ages. According to th e D avisian concept, a zone of dissected to p o g rap h y has to be situ a te d betw een peneplains of different ages, joining them b y gradual tran sitio n s. Hence, geom orphologists proposing th a t den u d atio n al plains originate b y th e g radual llatte n in g of slopes are forced to consider ad jac en t plains of different altitu d es as th e relics of a single plain uplifted to different heights b y tectonic m ovem ents. G eom orphologists con
vinced th a t such adjacent plains a t different altitu d e s were n o t form ed con
tem poraneously arc forced to accep t the p ediplanation concept, even if th e y c a n n o t explain th e p ediplanation m echanism . I t should he n o te d th a t so fa r nobody has explained the m echanism of the “ parallel” re tre a t of slopes in any environm ent.
E. V. S h an tser (1965) has recen tly discussed th e evolution of slopes under the effect of d en u d a tio n processes (chiefly of sheetw ash) and concluded th a t th e llatten in g of slopes is a general law , whereas th e m aintenance of steepness and
“ parallel” re tre a t are restricted to certain slopes and result from local geo
logical stru c tu re or from peculiar processes of d estru ctio n .
As has been n o ted by m any au th o rs, th e re tre a t of slopes at c o n sta n t or even increasing slope angle occurs w hen some laterally acting process of d estruction affects th e base of a slope. Such processes include th e u n d e rc u ttin g of slopes by stream s, wave action, and frost w eathering (nivation) n ear the sum m er snow lim it. Som ew hat less typical is th e abrasional re tre a t of slopes u n d e r th e effect of m oving glaciers and aeolian processes (deflation). These processes can all form g ently sloping surfaces, resem bling pedim ents in shape; w ave action can also create extensive plains, resem bling peneplains or pediplains. In some cases, these land forms can be m istaken for features due to slope d en u d a tio n , but for th e m ost p a r t th e ir origin can be established w ithout difficulty.
As is well known, slope d en u d a tio n can create steep slopes re tre a tin g of a series of river terrace s. R em ains of bedrock scarps form ed b y river abrasion and of low er horizons of te rra c e alluvia containing placer gold show t h a t there
accum ulation of colluvia and alluvia m oved by slope denudation. In c id e n tally , such an alluvium should be defined ra th e r as a heterogeneous alluvial-collu
vial facies of loose deposits.
A w ealth of d a ta accum ulated w hile prospecting num erous te rraso u v als perm its us to affirm t h a t all terrasouvals are fluvial-denudational lan d form s, river terraces rew orked b y slope den u d atio n , having a tta in e d various stages of
F ig . 1. C ro ss-sectio n of a ty p ic a l te rra s o u v a l in th e n o rth e a s te rn U S S R (d rain a g e a re a o f th e K o ly m a R iv er)
(1) Peat, (2) Clay, (3) Silty sand, (4) Gravel, (5) Gravel with angular debris, (6) Bedrock, (7) Concentrations of placer gold, (8) (a) Proved and (b) Hypothetical terrace socles and their heights in meters above the bedrock bottom of the floodplain
th is process. .Not a single terrasouval can be considered a pedim ent form ed by I he “ parallel" re tre a t of a slope, w ith o u t th e p articip atio n of fluvial processes.
Figure 2(A )ashow s som e detailed cross-sections of some terraso u v als of th e n o rth e a ste rn USSR. It should be n o ted first of all th a t colluvial deposits w ithout a n y rounded m a te ria l are w idespread on the surfaces of terraso u v als.
These surfaces are often com pletely covered by colluvia, so th a t a lack of d a ta concerning the stru c tu re of th e veneer of loose deposits m ay lead to false ideas as to the origin of some terrasouvals.
The alluvial-colluvial deposits of th e terraso u v al facies differ sh arp ly from the colluvium in th a t som e rounded m a te ria l is invariab ly present. L ith o lo g ical differences betw een terraso u v al facies and terrace alluvium , often p re s e rv ed on terraso u v als, are n o t so conspicuous. R ounded and an g u lar debris occur together n o t only in th e terraso u v al deposits but also in some v arieties of alluvium . M oreover, some varieties of te rraso u v al deposits will co n tain no angular debris [Fig. 2(B )]. The m ain difference betw een terraso u v al deposits and alluvium is in th e m ode of occurrence, th a t of th e terraso u v al deposits b eing sim ilar to th a t of colluvium .
T errasouval deposits m a y be fully a b se n t from terraso u v als considerably reworked b y slope d en u d a tio n , and te rra c e alluvium will be preserved only in restricted areas, under a cover of colluvium [(Fig.2(C)] At this stage, th e b ed ro ck outcrops often occupv m ost of th e terraso u v al surface. There was a terraso u v al
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w hich was show n as an area u n d erla in b y T riassic shales even on the detailed geological m ap. T h e geologist in charge of p ro specting in th is region was an exceptionally k ee n observer; th is was how th e bu ried alluvium w ith a fairly rich placer in it cam e to be found a t all on th is terrasouval.
The fu rth er re w o rk in g of terraso u v als b y slope denudation can and som e
tim es will lead to a com plete d e stru c tio n of all tra c e s of form er terraces. T e rra souvals becom e g e n tly sloping bedrock surfaces overlain b y ju s t a sheet of
m
F ig. 2. D e tailed c ro ss-s ec tio n s o f p a r ts o f so m e te r r a s o u v a ls in th e d ra in a g e a reas o f th e In d ig irk a (A, C) a n d M a ly A n y u y (B) riv ers
(l) Clay wilh pebbles, (2) Clay with angular debris, (3) Angular debris, Genetical indexes: al — alluvium, c — colluvium, ale — alluvial-colluvial deposits (terrasouval facies), solid lines — boundaries between deposits of different origin: dashed lines — boundaries between lithological varieties of deposits of the same origin, vertical lines — prospecting pits or boreholes; for the rest of signs see Fig. 1
colluvia h ere a n d th ere. S uch terraso u v als can be distinguished from th e
"genuine” p e d im e n ts by m eans of a co m p arativ e analysis of the topographic p attern , an d of correlation b etw een te rra so u v a ls a t different stages of devel
opm ent. The d ire c t genetical relation of such “ pedim ent-like” surfaces to te r
rasouvals of an u n d en iab ly flu v ial-d e n u d atio n a l origin can be proved in m ost cases.
In ad eq u ate co m p arativ e gcom orphological analysis an d som etim es, a p parently, a la c k of d a ta concerning th e loose deposits an d th e bedrock con
figuration are th e m ain reason for th e erroneous opinion t h a t valley pedim ents are w idespread in th e U SSR.
In conclusion, I should like to discuss som e term inological problem s. Tuan (1959), w ho h a s studied in detail th e classical pedim ents of so utheastern
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Arizona, a ttac h es to th e te rm "pedim ent " a purely m orphological m eaning. H e means h v pedim ents g e n tly sloping bedrock surfaces of an y origin, fringing steep slopes. U nlike him , m an y S oviet au th o rs give to th is term a genetical connotation, restrictin g it to such g en tly sloping surfaces as have o rig in ated b \ the “ parallel” re tre a t of steep slopes due to slope d enudation. Since geom orpho
logists are prim arily in terested in th e origin of land form s, the g en etical m eaning appears to be preferable.
The te rm “ d e n u d a tio n ” has two m ain m eanings in th e geom orphological literatu res of various countries. I t is o ften applied to th e removal of loose m aterials b y any exogenous process, resu ltin g in a low ering of th e E a r th 's surface. Thus d en u d a tio n is considered opposite to th e accum ulation of loose deposits w hich in tu rn results in a rise of th e E a r th ’s surface. H ow ever, th e com bination of slope processes, including m ass w asting, creep, sheet an d rill wash, etc., is also referred to as den u d atio n (Tuan, 1959). In th e Soviet U nion, Yu. A. B ilibin has a tta c h e d such a m eaning to th e term “ d en u d a tio n ” as early as 1938. I believe th a t, in order to avoid confusion, th e te rm “ d e n u d a tio n ” should be used only in th is restricted sense. The com bination of all slope p ro c esses has no o th er nam e, and these processes are often inseparable u n d e r in vestigation. Moreover, th e first, b ro ad er m eaning of ih e term has a r a th e r widely used synonym — destruction. A pplying the te rm “ d en u d a tio n ” as recom m ended here, one h as to bear in m ind th a t d enudational processes can be n o t only d estru ctiv e, b u t also accum ulative. The form ation of colluvial tra in s in th e back p a rts of terraso u v als is an exam ple of d enudational accum ulation.
The term “ abrasion” has a very broad m eaning in A m erican lite ratu re , being often used as a synonym of the term “ d e stru c tio n ” . In m o sto f Soviet lite ra l lire, on th e co n tra ry , this te rm has a m eaning re stric ted to w ave action only. F. P.
S avarensky (1939) has som ew hat extended th is m eaning, b y giving the n am e
“ river ab rasio n ” to th e process of lateral u n d ercu ttin g of interfluve slopes b y rivers. 1 believe it reasonable to apply th e te rm “ abrasion” to all processes of d estruction w hich are directed not vertically b u t laterally. Such processes h av e been listed above.
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PEDIMENTS ON TH E NORTHEASTERN B O R D E R