M. V. P t O T H O V S K Y
1. IN TR O D U C TIO N
The problem of pedim ents is stric tly speaking th e problem of th e evolution of the system “ slope-and-pedim ent” . The la tte r problem in its tu rn is p a rt of a bro ad er one— ih a t of “ slope-and-foot surfaces” .
This article considers th e m echanism of slope re treat, th e form atio n of pedim ents by denudation, a n d th e preform ation of pedim ents an d foot s u r
faces by block tectonics. Some inform ation on th e study of pedim ents in th e U SSR is also given.
The above-m entioned system s are of various origins an d h ave various geological stru c tu re s. H ow ever, th e ir elem ents have—even on different contin en ts an d in different te rrito rie s—rem ark ab ly sim ilar features. This sim ilarity shows th a t these form s develop according to the sam e general laws of th e graded profiles of stream s an d slopes.
R eal pedim ents are p lan atio n surfaces form ed by d en u d a tio n a l slope re tre a t u n d e r various clim ates, not lim ite d to desert pedim ents only.
2. FO R M A TIO N OF P E D IM E N T S
F rom the v ery beginning, real pedim ents develop entirely b y d e n u d a tio n according to th e laws of g rad ed profiles. The re treatin g slope can be initially a fault scarp o r the side of a v alley dissecting any uplift. P edim ents will develop below these slopes provided the la tte r h av e a relatively sta b le baselevel of d en u d atio n . Such a base m ay be a subsided block below a fault scarp or I he b ottom of a graded valley.
The dom inant elem ent in the m echanism of slope re treat is, in the a u th o r's opinion, th e underm ining of slope foot. This is caused by th e s a tu ra tio n of th e slope feet w ith w a te r flowing dow n over the slopes and through th e ir scree cover.
This flow m ain tain s a p erm anent or sem i-perm anent horizon of ground w a te r in th e w aste sheet along the foot of the slope. This m oisture intensifies w e ath er
ing and causes an ab ru p t increase in the m echanical m o b ility of th e w aste sheet. Increase in m obility is aid ed b y th e subsurface dow nw ashing of clayey m ateria l. This m ateria l accum ulates in the low er p a rt of th e w a ste profile and, becom ing o v e rsa tu ra te d below th e g round-w ater horizon, it serves as a lu bricant of th e coarser m aterial.
In th is m an n er the slope foot becomes a zone of u n d ercu ttin g . In R ussian th is zone can be nam ed by th e m in ers’ term “ za b o y ”— “ the w orking face” .
The ch a rac te ristic stages and form s of re tre a tin g slopes are th e following:
a non-graded rock scarp of m ax im u m inclination (more th a n 30°), a scree slope o f straig h t profile about 30° (often crowned w ith rem nants of scarp), and a
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''re sid u a l" slope, stra ig h t or slightly concave (18 to 22°), usually strew n with equilibrium progressively shifts tow ards dim inishing indices. These principles were clearly expounded in “ The G eographical Cycle” b y W. M. Davis (1899).
a great pioneer of th e co n c ep t of self-regulating n a tu ra l system s. L. C. K ing is absolutely wrong in say in g th a t pedim ent fo rm ation disproves the D avisian concept. On the c o n tra ry , pedim ents are th e best expression of and proof for
Davis’s general idea of g ra d ed relief profiles and th e ir self-regulation.
The prin cip al outlines of pedim ent form atio n were correctly laid dow n in W. P en ck ’s scheme of w aning developm ent of th e relief. Penck p ointed out th a t a m oisture con ten t increasing dow nslope results in an increased m ob ility of the w aste-sheet. But he believed this increase to he g ra d u al and failed to perceive th e m echanism of underm ining; this is why he could not fully ex p lain I he parallel ret real of slopes. The principle of underm ining by ground w a te r was proposed m uch later by P. B irot (1949).
D uring his investigations in 1961 in th e A ldan region, th e au th o r in d ep en d ently reached conclusions sim ilar to those of P. B irot. Moreover, he noted some ad d itio n al features due to th e presence of p erm afrost which explained the evolution from scarp to residual slope as described above.
The above discussion has been concerned w ith the h u m id ty p e of pedim ent lluvial processes are also present. Subsurface w ash on th e slopes and p edestals, continuing th ro u g h th e scree, concentrates especially along th e stone strips. On descent, form ing peculiar parallel and subparallel p attern s perfectly recogniz
able on airp h o to s and m uch different from th e branching p a tte rn s of norm al th e co ncentration of w ater along the dells intensifies the cryofraction of m ateria l to th e silt fractio n . F u rth erm o re , K. L. M itt (1959) attach es a g re a t significance
m an n er insofar as th ey cause a progressive c o n c en tratio n o f surface w ater in th e w ith slightly step p ed low er edges. These edges can really be terrace s sculptured b y th e lateral m igration of stream s. I. P. K a rta s h o v proposed all terrasouvals
Block fa u ltin g m ay play various roles in p ed im entation. F irstly it m ay depressions of accum ulation, an d in term ed iate oscillating blocks w ith "S chach
telrelief” . In th e la tte r the different surfaces of denudation an d accum ula
tion converge an d often coalesce. These surfaces are of exogenic origin b u t th e entire relief is a system of slope-foot surfaces preform ed b y block faulting.
A fair exam ple of th is kind of relief is provided b y th e m o u n tain s, pediplains an d depressions of th e Lower A m ur basin. H ere, the boun d aries betw een pediplanes and m ou n tain s have rem arkable zigzag courses. W. P en ck suggested th a t such b oundaries were th e results of erosional dissection a n d slope re tre a t blocks including pediplanes, blocks w ith “S chachtelrelief” and p a rts of depres
sions slightly u p lifted after subsidence. P a rts of such system s can h ave th e stru c tu re of u p lifted m ountain blocks, bearing re m n a n ts of th e sam e planated surfaces. O ther p a rts m ay be covered w ith y oung deposits: p roluvial, alluvial, lacu strin e and m arine. I t is possible to re co n stru ct from the surfaces of such v ariously displaced blocks a single common preceding foot surface provided t hey have a com m on graded profile. Such a profile will develop d u rin g periods of relative rest. If th e surfaces of the blocks b e a r young n o n -resista n t deposits (T ertiary sands, clay, argillites, etc.) the g raded profiles and new surfaces of planation will develop very fast. G raded profiles m ay be partly due to aggrada
tion (M andrych eted. 1968, M akkaveev, B erkovich e ta l. 1968, M akkaveev,
T ectonopedim ents include block steps in block-faulted valleys, inclined 123
an d having I lie a p p e ara n ce of ty p ic a l pedim ents. T heir mosaic p a tte rn s explained the step p ed relief of the U rals by dilferential block displacem ents and assum ed th e u p p er levels to be goletz terraces (surfaces of altiplan atio n ). V. A.
The geologists A. X. M azarovich (1939) an d E. V. M ilanovsky (1940) d e
m ents m erge in to stepped goletz terraces. IIence, th e different clim atic types of pedim ents can su rv iv e and m erge into one an o th er.
The p lan ate d surfaces, including pedim ents of the Lower Volga basin and the Pre-U ral region are now studied in detail by m any explorers [A. P. R ozhdest
D. A. Timofeev is noted as one of th e Soviet geom orphologists m ost interested in the problem of pedim ents. H e studied the pedim ents of th e O lekm a—Aldan
te n years pedim ents, slope processes a n d deposits in th e ir relations to m orpho- tecto n ics and geographical conditions h a v e been stu d ied by the T ransbaikal E x p ed itio n of th e G eographical D e p a rtm e n t of Moscow U niversity (Sim onov, 1966, 1968).
In sum m ary, th e p ed im en t concept is being exten siv ely used by Soviet geom orphologists w ho have recognized v a s t expanses of pedim ents of various ty p e s in USSR te rrito ry . The m ain investigations in to pedim ents are being ca rried o u t in Siberia a n d in th e F a r E a s t. They are b ase d on large volum es of prospecting d a ta especially concerning placers.
VALLEY PEDIMENTS OR DENUDED TERRACES