CLYDE WAHRHAFTIG Dept. Geology and Geophysics, University of California, Berkley, California
Stepped Topography of the
Southern Sierra Nevada, California
Abstract: Irregular steps characterize the topogra- much more rapid weathering of granitic rocks where phy on granitic terrane on the west slope of the buried than where exposed. Weathering is predomi- southern Sierra Nevada. These steps are a few nantly by partial alteration and expansion of biotite, hundred leet to a few thousand feet high, one- which shatters the rock. The disintegrated rock can quarter to 5 miles wide, and up to 10 miles long. be moved readily by small streams. The unweath- Most steps face the San Joaquin Valley, but others ered outcrops exposed by accelerated erosion act as line the canyons of the major rivers, facing the local baselevels, because their large joint blocks can- streams. Part of the eastern edge of the San Joaquin not be moved by even the largest streams. Valley is a smooth plain bevelled across granite, and Alternative hypotheses include faulting, differ- has an origin similar to the steps. Outcrops are com- ential erosion due to variations in bedrock lithology mon on the fronts of the steps, and near the outer or in spacing of joints, and parallel retreat of the edges of the step treads, but are rare on the back fronts, with the treads aspiedmonttreppen. Evidence parts oi the treads, which are underlain by disinte- is presented that renders each of these hypotheses grated granitic rock as much as 100 feet thick. doubtful. Treads tend to slope back toward the next higher The proposed hypothesis raises questions about front. the validity of ancient erosion surfaces in the Sierra The stepped topography is confined to granitic Nevada. rocks, and is believed to result primarily from the
CONTENTS Introduction 1166 5. Longitudinal profiles of the basalt-capped Acknowledgments 1167 table mountains ol the lower San Joaquin Previous work 1167 River, and of Auberry and Big Sandy Geologic setting 1168 valleys '.1170 Climate and vegetation 1171 6. Temperature and precipitation at three sta- Description of the steps 1171 tions in the Sierra Nevada 1172 Origin of the steps 1176 7. Topographic map of steps on the north side ot Statement of the hypothesis 1176 the canyon of the San Joaquin River . . 1173 Evidence for differential weathering 1177 8. Topographic map of three steps in the foothills The weathering process 1178 of the Sierra Nevada: Squaw Valley, Hills Origin ot the steps according to the hypothesis 1179 Valley, and Shannon Valley 1174 Discussion of alternative hypotheses 1182 9. Topographic map of the Big Sandy Bluffs and The stepped topography and ancient erosion sur- the valley of Tollhouse Creek 1175 faces 1186 10. Profiles of streams in the stepped topography 1176 Other examples of stepped topography 1187 11. Sketches of roadcuts, showing distribution of Practical implications 1187 fresh and weathered granodiorite .... 1178 References cited 1187 12. Longitudinal profiles illustrating the develop- ment of nickpoints along streams during Figure uplift or tilting 1180 1. Index map of central California 1166 13. Cross profiles illustrating the fixing of a stream 2. Profiles across the foothill part of the Sierra in a solid bedrock notch and nickpoint . 1180 Nevada, California, projected to east-west 14. Development of hillside outcrops 1181 planes 1167 15. Four stages in the capture of a stream by one 3. Index map showing locations ot quadrangles ot its tributaries 1182 and some ot the localities mentioned in 16. The development of trellised drainage and the text 1168 backward-sloping treads by stream cap- 4. Map oJ the western Sierra Nevada between ture 1183 Mariposa and the Kings River, showing 17. Three mechanisms for joint control of stepped general geology and the distribution of topography 1184 step fronts 18. Topographic map showing relationship be-
Geological Society of America Bulletin, v. 76, p. 1165-1190, 19 figs., 2 pis., October 1965 1165
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tween step fronts and master joints east Plate Facing of Raymond, California 1185 1. Jointing and stepped topography in the south- 19. Effect of backward retreat of a front on the ern Sierra Nevada, California 1176 altitude of its crest, according to the pied- 2. Drainage and weathering phenomena in stepped monltreppen hypothesis 1186 topography, southern Sierra Nevada, Cali- fornia 1177
and wherever granitic rocks are exposed, exam- INTRODUCTION ples of stepped topography are to be found. The western slope of the southern Sierra Ne- The steps have been observed throughout vada (Fig. 1), south of Yosemite, rises as a series the southern Sierra Nevada from the Tuolumne of giant irregular steps a few hundred to a few River to the Kern River. They are best devel- thousand feet high, and a quarter of a mile to 5 oped in the region between Mariposa to the miles broad. The fronts of these steps are bold northwest and Camp Nelson to the southeast escarpments with abundant granitic outcrops. (Fig. 3), on the lower part of the western slope, Their treads are nearly level uplands, relatively below 9000 feet. Above 9000 feet the stepped free from outcrops and underlain by a thick mantle ot gruss (disintegrated granite in place). Commonly, each tread declines eastward to the base of the front above it and is drained by a stream flowing along its back edge, so that each step is a little higher on its western side than on its eastern side, and the profiles of the major in- terfluves are slightly sawtoothed (Fig. 2A). The stepped topography contrasts sharply with the northern Sierra Nevada, north of Yose- mite, where broad, flat-topped interfluves cap- ped by remnants of Tertiary gravels and vol- canic rocks rise evenly from the Great Valley al- most to the summit of the range (Fig. 2B) (Lindgren and Turner, 1894;Lindgren, 1911, p. 28-54; Piper and others, 1939, p. 61-84). These J3 •— . 1 !^.-''__ ' 33- flat-topped interfluves and their westward-til ted but otherwise little-detormcd cover are part of Figure 1. Index map of central California, the evidence that the Sierra Nevada is a tilted showing location of Figures 3, 4, 7-9, and 18 fault block (Le Conte, 1886; Lindgren, 1911; Louderback, 1924). The northern and southern Sierra Nevada topography, if present, is obscured by the effects differ not only in topography and extent and of glaciation.1 type of Tertiary cover, but also in the nature of The stepped topography is believed to be the predominant bedrock. Northwest of an ir- caused by differences in the rate of weathering regular line that extends southward from west in the two environments to which granitic rocks of Lake Tahoe (Fig. 1) through Mariposa to in the Sierra Nevada are subject. Where buried Raymond (Fig. 3), the bedrock is predominant- by overburden or gruss, the solid granitic rocks ly mctamorphic, and granitic rocks occur as iso- are moist most of the year, and disintegrate lated plutons (Lindgren, 1911, PI. 1; Jenkins, comparatively rapidly to gruss; where exposed, 1946. PI. 3; Bateman and others, 1963, p. 3). the solid granitic rocks dry after each rain and Southeast of this line the predominant bedrock therefore weather slowly. Small streams, even is granitic, and metamorphic rocks are in narrow overland flow, can transport the gruss, but the septa and roof pendants (Bateman and others, unweathered rock is jointed into blocks so huge 1963; Knopf, 1918, Pis. 1 and 2; Whitney, 1865, p. 217-222, 364-437; Miller and Webb, 1 The stepped topography is clearly shown on topo- 1940, PI. 2; Troxell and Morton, 1962, PI. 1). graphic maps. It is desirable while reading this paper to The stepped topography of the southern Sierra consult the twenty-two 15-mmute quadrangle sheets Nevada is confined to areas of granitic bedrock, shown on Figure 3.
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that not even the largest rivers can move them. stepped topography. The results of some studies Consequently, any exposure of solid rock is a made on drill cores collected by the Depart- temporary base level, to which the country up- ment of Irrigation, University of California, stream or upslope can be leveled, but below Davis, under the direction of R. J. Burgy and which it cannot be reduced. Irregular lines of W. D. McMillan, are incorporated in this such outcrops, formed during periods of down- study. Paul C. Bateman, N. King Huber, and cutting or accelerated erosion, grow into es- Dallas Peck of the U. S. Geological Survey pro- carpments irregular in height and ground plan. vided unpublished information on the distribu- These are the step fronts. The areas between tion of igneous rocks. Richard f. Janda and G. them become flattened by weathering and ero- Brent Dairymplc assisted in some of the field sion to form the treads of the steps. Thus, the work. Harold Johnson of the Johnson Drilling stepped topography is caused by the develop- Company, Fresno, California, and Don Schroe- der of Orosi, California, provided water-well in- formation. John T. Hack, Paul C. Bateman, M. N. Christensen, Garniss Curtis, R. f. Janda, G. B. Dairymple, N. King Huber, and C. B. Hunt criticized the manuscript. I am grateful to all the aforementioned and to Allan Cox, David M. Hopkins, Warren B. Hamilton, Charles Meyer, Richard L. Hay, and Arthur D. Howard for much helpful discussion and many useful ideas. Special thanks are due to Mrs. Virgie T. Noll for careful typing of the manuscript. The responsibility for the ideas expressed in this pa- per is entirely mine; several of those acknowl- Figure 2. Profiles across the foothill part of edged above do not subscribe to all of them. the Sierra Nevada, California, projected to cast-west planes. A, Drainage divide on the PREVIOUS WORK south side of the Kings River; B, Drainage divide on the north side of the American The stepped topography of the Sierra Nevada River received scant attention from earlier investiga- tors. B. F. Hake (1928) was the first geologist to recognize the step fronts. He explained them as ment and propagation of perturbations in the fault scarps, an explanation that is inconsistent rate of weathering and erosion. with geological evidence. In the first part of this paper are described Matthes (1930, p. 24-25, 39-40; 1960, p. 15- the geographic and geologic setting and the 33) thought incorrectly that the summit up- stepped topography. In the second part are pre- lands of the Yosemite and San Joaquin regions sented the theory of origin and evidence to sup- have a gentle westward slope as do the inter- port it. In the third part, alternative explana- fluves of the northern Sierra Nevada. He recog- tions are considered. The paper ends with some nized two kinds of interruption to this slope: implications for the history of the Sierra Ne- (1) subsidiary ranges trending northwest paral- vada and for the management of granitic ter- lel to the main range, which he thought were an ranes, and with some examples of stepped to- inheritance from trellised drainage established pography in other parts of the world. A pre- on Appalachian-type folds long since eroded liminary report on the stepped topography was away (1930, p. 31; 1960, p. 44); and (2) two presented before The Geological Society of west-facing escarpments, the northeast wall of America in April, 1962, at Los Angeles (Wahr- Wawona Valley at the south end of Yosemite haftig, 1963). Park (Fig. 3) and the Big Sandy Bluffs (Fig. 9) near Aubcrrv which he attributed to differences ACKNOWLEDGMENTS in rate of downwasting caused by variations in The ideas on differential susceptibility to the spacing of joints (1930, p. 40). weathering of exposed and buried granitic rocks Krauskopf (1953, p. 8) also called attention were developed on a field trip with Paul C. to the Big Sandy Bluffs and to the line of es- Bateman to the moraines on Bishop Creek (see carpments extending eastward from them to Fig. 3) in 1958 long before I was aware of the Patterson Mountain. He correlated this line
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with the contact between inclusion-rich grano- present, two of which are nearly vertical and diorite (migmatite) on its southern downhill the third, nearly horizontal. Where seen in side and relatively inclusion-free granodiorite quarries, the horizontal joints, apparently un- on its northern uphill side, and attributed the loading joints of the sort described by Jahns escarpment to the greater crodability of the (1943), are more closely spaced near the surface migrnatite. This escarpment is much more com- than at depth. Master vertical joints that ex- plex than Kratiskopf realized (see Fig. 4), and tend for distances of several miles can be recog- the details of the step fronts do not conform to nized on aerial photographs and even on topo- the contacts Krauskopf (1953, PL 1) showed lor the migmatite zone. In fact, some of the es- carpments cross the zone at high angles to join high mountains in the metamorphic rocks to the south (Fig. 4). GEOLOGIC SETTING GRANITIC ROCKS: The rocks of the Sierra Ne- vada batholith lorm plutons from a fraction of a mile to 20 miles across, with sharp crosscutting relationships indicating multiple injection (Ross, 1958; Bateman and others, 1963). They range in composition from hornblende gabbro to alaskite. The most common rock of the west- ern part of the range is slightly to moderately foliated hornblende-biotite quartz-diorite to granodiorite, with an average grain size of 2-4 mm. The next most abundant is biotite quartz- monzonite, which may contain phenocrysts of K-feldspar as much as 3-5 cm across. Although some plutons show compositional zoning, many are si rikingly uniform in composition, but differ Figurc 3. Index map showing locations of markedly in texture and composition from ad- quadrangles and some of the localities men- jacenl plutons. tioned in the text. Dashed lines indicate Many of the plutons, particularly those com- mountain range boundaries; dot-dash lines posed of hornblende-biotite quartz-diorite and indicate national parks. See Figure 1 for granodiorite, show linear and planar parallelism location. of mafic minerals and inclusions, dipping steeply and usually parallel to nearby plutonic contacts with older rocks. The planar parallelism locally graphic maps as alignments of linear segments grades into a pronounced schistosity. of streams with narrow passes. In much ol the region of the stepped topogra- Some extensive domelike exposures of granit- phy, intrabatholithic plutonic contacts have ic rock do not break along orthogonal joints, not yet been mapped. However, good detailed but along broadly curved surfaces parallel to the studies of the granitic rocks exist for some areas surface of the outcrops, forming concentric (Macdonald, 1941; Hamilton, 1956; Durrell, shells a few feet thick. These joints have also 1940; Ross, 1958; N. King Hubcr, 1964, Oral been recognized in roadcuts where the rock is communication; Paul C. Bateman, 1964, Oral not naturally exposed, and in every case are communication), and reconnaissance studies closely parallel to the surface. They are com- exist for much larger areas (Miller and Webb, monly the most prominent joint set in the rock 1940; Krauskopf, 1953; Bateman and others, and the one along which the rock breaks most 1963). Elsewhere, the relations between the readily. Where examined along canyon walls, as stepped topography and plutonic contacts can in the Merced and Kings River canyons, they be estimated from the trend of the foliation. strike parallel to the canyon wall and dip to- Throughout most of the area the granitic ward the river at an angle slightly less than the rocks are jointed. The joints generally are inclination of the slope. Terzaghi (1962) ob- spaced from 2 feet to more than 10 feet apart. served them at Mammoth Pool on the San Three perpendicular sets of joints are commonly Joaquin River. These sheet spalls are similar in
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 lane Formaron J Sranllic rocks
Mctcrnarp? r rdch, - ~---. -~~ Vgrno'tr ne of Klaus* pf
STEP 'ROlu'i --v More thor 2,000 feel tlqh ---+- 1,000~2,000feet hqh ----/- 500 '.000fee' htqh
--+4- Less than 500 feet hqh
Figure 4. Map of n-cstern Sierra Nevada between Mariposa and Kings River, shon-ing general alld distribution ol step [ronts. Hasefronz U. S. Forest Service plr~nimrtricrnap of Sicrra National Forest, scale 1 :125,000. (;eology slightly mocli- fied from Batenla11 and others (1963), Krauskopf (l953), hfacdonalti (19-11), Ulrich and otllers (1962), and N. King Huher (196-1, personal commun.). Step frollts bascd or1 examination of topographic maps and aerial photographs. See Fig. I for location.
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all respects to those described by Jahns (1943) the ridgetops of the western Sierra Nevada. and by Chapman and Rioux (1958). They are This suggests, but does not prove, that the pres- clearly related to the present topography and ent topography of this part of the Sierra Nevada not to the intrusive or tectonic history of the was carved largely after the lone Formation Sierra Nevada batholith. All available evidence was deposited. supports Jahns' explanation of them as unload- CENOZOIC BASALT: Cenozoic basalt is wide- ing joints due to release of confining pressure. spread throughout the southern Sierra Nevada The granitic outcrops that split by sheet as small remnants of formerly more continuous spalling are commonly domes and "bald rocks." flows. The most significant basalt remnants lor Matthes (1930, p. 39-41) regarded them as mas- the interpretation of the stepped topography sive unjointcd monoliths; some appear to be so. cap Kennedy Table, Squaw Leap, and Table However, the orthogonal jointing that is so Mountain along the lower canyon of the San common in the rest of the granitic terrane is Joaquin (Fig. 4) and continue southwest as hill- present in many of the "bald rocks" as faint top cappings southeast of Friant, where as much cracks that did not open (Pi. 1, fig. 1). MKTAMORPHIC ROCKS: The metamorphic rocks, all pre-Cretaceous in age, consist of schist, slate, quart7,ite, marble, calc-silicate hornfels, amphibolite, and serpentine. Bodies of these rocks arc 1-15 miles wide and 5-25 miles long, and they underlie perhaps 15-20 per cent of the west slope of the southern Sierra Nevada below 9000 feet. The bedded and foliated metamor- Figure 5. Longitudinal profiles of the basalt- phic rocks generally strike northwest and dip capped table mountains of the lower San steeply or are vertical. Closely spaced fractures Joaquin River, and of Auberry and Big Sandy perpendicular to bedding and cleavage break valleys (projected onto a vertical plane trend- them into small fragments, generally less than a ing N. 52° E.) few inches across. These fragments litter the surface and arc embedded in the soil. The meta- morphic rocks are more resistant than the as 320 feet of intcrbedded gravel and rhyolite buried granitic rocks, and generally rise above tuff underlies the basalt (Macdonald, 1941, p. the immediately adjacent granitic terrane to 262-263). The basalt and underlying clastic form, rugged, sharp-crested mountains with rocks define an ancient valley floor of the San long even side slopes (MacDonald, 1941, Map Joaquin River, which rises from river level at 1). Other examples arc Thornbury and Goat the edge of the San Joaquin Valley at a rate of mountains (Fig. 4). about 150 feet per mile, to 1500 feet above the IONE FORMATION: The lone Formation of river only 15 miles northeast of the valley mar- Eocene or early Oligocene age emerges from be- gin (Fig. 5). The proj ection of this channel east- neath the alluvium of the Great Valley to cap a ward up the San Joaquin River past Big Sandy discontinuous line of cuesta-like buttes and Bluffs coincides with the basalt remnant of mesas along the west margin of the foothills as a Sugarloaf Hill in Jose Basin (Shaver Lake quad- cemented conglomerate and sandstone caprock rangle), dated by the potassium-argon method 40-110 feet thick (Allen, 1929, p. 361-363; by Dalrymple (1963) as 9.5 million years (m.y.) Macdonald, 1941, p. 260-262). Its southern- old. The continuity of the ancient channel of most outcrop is Little Table Mountain west of the San Joaquin River indicated by these rem- Friant. The sandstone containsabundant anaux- nants of basalt demonstrates conclusively that itic kaohnite and rests on thoroughly decom- there has been no deformation other than tilt- posed granite as much as 40 feet thick, in which ing in this part of the Sierra Nevada lor the last the bright pink colors of lateritic soils pre- 9.5 m. y. dominate. Both the mineralogy of the sediment Other patches of basalt from a few hundred and the character of the weathering profile it feet to 2 miles across are common in the upper buries indicate a tropical climate (Allen, 1929. basin of the San Joaquin River (Matthes, 1960, p. 383-394; Macdonald, 1941, p. 260). The p. 18-19; Birman, 1964, p. 6-7, PI. 1; Frwin, lone Formation dips 2-4 degrees westward to- 1934, p. 45-47, PL 1; Paul Bateman, 1964, Oral ward the Great Valley. Its basal contact, pro- communication; R. J. Janda, 1964, Oral com- jected eastward at these dips, would just graze munication; N. King Huber and C. D. Rinc-
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hart, 1964, Oral communication), and some alluvium that thickens from a feather edge near have been dated by Dairymple (1964) as rang- the west base of the mountains to 1000-3000 ing from 3.3 to 3.9 m. y. old. Krauskopf (1953, feet beneath the valley axis (Davis and others, PL 1) showed basalt flows flooring the head- 1959). The Quaternary alluvium grades down- water basins of Rancheria and Scepter creeks ward into continental deposits of Pliocene and (Tehipitc Dome quadrangle) in the basins of the Miocene age and cannot be differentiated from north and middle forks of the Kings River, and the older deposits. patches perched high on the walls of the canyon of the Kings River from Hume Lake to the CLIMATE AND VEGETATION mouth ol the North Fork (Tehipite Dome and The Sierra Nevada has a Mediterranean cli- Patterson Mountain quadrangles). A flood of mate of winter rains and summer drought. Alti- basalt filled the gorge of the Little Kern River tude imposes striking temperature and precipi- to a depth of more than 800 feet at its junction tation variations on this seasonal regime. with the Kern River (Webb, 1946, p. 368, PI. Records of three stations, averaged tor the 2; Dairy mple, 1963, p. 386) and has been dated period 1952-1961 (U. S. Dept. Commerce, by the potassium-argon method at 3.5 m. y. Weather Bureau, 1951-1961), arc shown on old. Other patches ot basalt lie on the surface of Figure 6. Precipitation amounting to 0.1 inch the plateau between the Kern River and its or more falls on only 26 days per year at Friant south fork (Miller and Webb, 1940, p. 359, PI. on the eastern edge of the Great Valley, on 36 2). days per year at Auberry, in the foothills, and None of these basalt patches has been report- on 46 days per year at Huntington Lake. Below ed to show evidence of oflset by faulting. Many altitudes of 3500 feet, precipitation is largely of them can be traced for considerable distan- rain; snow rarely lasts on the ground. The thick- ces, and variations in altitude can be shown to ness and duration of winter snowpack increase be due to the rough topography over which the with altitude; at Huntington Lake most of the basalt was extruded. They indicate, like the precipitation is snow, which here remains on Tertiary gravels and volcanic rocks farther the ground from late November to late May. north and the lone Formation along the west The vegetation is markedly zoned altitudinal- flank of the range, that the range has not been ly, reflecting variations in the climate. On the broken by faulting in late Cenozoic time. Fur- valley floor and up to altitudes of 500-800 feet, thermore, they place both maximum and mini- natural vegetation is predominantly grass and mum limits on the antiquity of various elements herbaceous flowering annuals, whose growing of the present topography. season coincides with the winter rains. Shortly GLACIATION: The higher parts of the Sierra after rainfall ceases in May or June the grass- Nevada were repeatedly glaciated in Pleistocene lands turn yellow and dry. time (Matthes, 1930; 1960; Blackwelder, 1931; The foothill vegetation, between 800 and Birman. 1964). The firn limit of recognizable 3500 or 4000 feet, is a mixture of grasslands, glaciation was 7800 feet on Shuteye Peak just open oak and pine woodland with grassy under- south ot Yosemite Park and about 10,000 feet story, and sclerophyllous scrub (chaparral). The at the south end of Sequoia Park. The longest open grassy slopes are green during the rainy glaciers descended to about 3000 feet altitude season and dry in summer. on the San Joaquin River and 4000 feet on the Above 3000-4000 feet, tall coniferous forest Kings River, and ended on both rivers 30-35 is dominant, and in the natural condition con- miles east ol the west front of the range. Else- sists ot closely spaced mature pine, cedar, fir, where the ice did not descend below 6000 feet, and Sequoiadendron. and in general was above 7500 feet. Most of the stepped topography was never glaciated. DESCRIPTION OF THE STEPS ALLUVIUM: Alluvial deposits underlie most of ORIENTATION AND NUMBER: Commonly the Great Valley and form discontinuous ribbons steps rise eastward from the Great Valley, and and patches a few feet thick along streams and the frontal escarpments face west and trend beneath flat rneadowlands in the mountains. north to N. 60° W. (Fig. 4). Steps also line the The alluvium consists mainly of micaceous ar- walls of some of the major canyons (Fig. 7). kosic sand derived from the weathering of Some steps are flat valleys surrounded by high- granitic rocks, interbedded with silt; along the er steps on nearly all sides [e.g., Shannon Valley larger streams gravel and boulders are common. (Fig. 8) and Drum Valley in Dunlap quad- The Great Valley is underlain by a wedge of rangle], or are flat-topped mountains, such as
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Buford and Lilley mountains (Fig. 4) in Ray- mond quadrangle. As many as 10 or 11 steps in succession lie be- tween the floor of the Great Valley and the 8500- or 9000-foot contour line. These span a width of 36 miles in the northern part of the area, just south ot Yoscmite Park (Fig. 4), and about 28 miles between the Kings and Kaweah rivers. South of the Kaweah River the west slope steepens considerably, and only 5 or 6 steps spanning a width of 20 miles can be recog- nized from the topographic maps. Individual steps can rarely be traced for more than 5 or 10 miles along the range. Either the fronts lose height so that the treads of two steps merge at an intermediate altitude, or the fronts curve to join in a higher escarpment. Examples of merging treads are on Bald Mountain (Fig. 9) and in the headwaters of Rush Creek (Shaver Lake quadrangle; see Fig. 4); examples of merg- ing fronts are common in Dunlap quadrangle (Fig. 8). Between the Fresno and the Tule rivers the -130 alluvium of the Great Valley is in contact with granite, and the surface of the lowest tread bev- elled across granite is continuous with the sur- face of the alluviated valley floor. This tread is 2-5 miles wide and includes flat-floored embay- ments into the foothills such as Woodlake, An- telope, and Yokohl valleys (Exeter quadrangle) and Hills Valley (Fig. 8). Water wells drilled in this area have reached gruss at 15-30 feet and solid granitic bedrock at depths of less than 100 feet (Donald Schroeder, 1964, Oral communi- J F M A M J cation; Harold Johnson, 1964, Oral communi- cation) scattered granitic outcrops in the form FRIANT of small hillocks surmounted by corestones show Altitude 410 ft. that the plains are formed on granite and are not alluvial surfaces (Pi. 1, fig. 2). STEP FRONTS: The fronts range in height from less than 100 feet to 4500 feet, but are generally between 300 and 2000 feet high. Their over-all slope ranges from 5 to 75 degrees but is com- monly between 15 and 35 degrees. Slopes great- er than 45 or 50 degrees are bare granite walls. Cliffs and benches are locally common on the fronts. The crest of a front may change in altitude as much as 700 feet within 1 mile along its trend (Figs. 4, 7, and 8). The highest parts of the crest tend to be at projecting points of the fronts. Likewise, the frontal slope may change from 10 Figure 6. Temperature and precipi- to 35 degrees (or even 70 degrees on cliffy seg- tation at 3 stations in the Sierra ments) within 1 or 2 miles along the trend (Fig. Nevada (averaged for the period 1952-1961, inclusive). See Figure 9)' The fronts are irregular in plan (Fig. 4) and 4 for locations of stations.
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Figure 7. Topographic map of steps on the north side of the canyon of the San Joaquin River {from U. S. Geol. Survey topographic map of Shaver Lake 15-minutc quadrangle, 1953 edition; contour in- terval 80 feet). See Figure 1 for location.
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have projecting points and shallow re-entrants. to be as great along the length of a tread as The re-cntrants are commonly, but not univer- across it (Figs. 7 and 9). sally, U-shaped and are located where streams Most of the treads slope backward toward the plunge over the fronts (Fig. 9). next highest step, rather than outward toward The step fronts bear abundant outcrops, their own fronts. Because the treads slope back- which are either bouldery outcrops consisting ward, the main stream on each tread commonly of corestones (PI. 1, fig. 3), or are the "bald flows parallel to and near the base of the next rock" and other apparently monolithic cliffs higher front, and the drainage in consequence that break along unloading joints parallel to the has a markedly trellised pattern (Figs. 8 and 9). surface (PI. 1, fig. 5). The bouldery corcstone Toward their fronts, the treads have abun-
Figure 8. Topographic map of three steps in the foothills of the Sierra Nevada: Squaw Valley, Hills Valley, and Shannon Valley (from U. S. Geol. Survey topographic map of Dunlap 15- mmutc quadrangle, and of Orange Cove North 7.5-minutc quadrangle; contour interval 50 feet). See Figure 1 for location.
outcrops arc commonest in the lower foothills, dant bedrock outcrops. On the low treads of the generally below 3000 feet, and the "bald rocks" foothills, these bedrock outcrops may be isolat- dominate above that altitude. ed corestones, rising a few feet out of the sur- Corestones are defined by Linton (1955, p. rounding gruss (PI. 1, fig. 4), picturesque groups 471-472) as detached masses of relatively sound of rounded joint blocks rising 5-50 feet above rock. They are the undecomposed cores of quad- the level of the gruss-covered surface, or conical rangular joint blocks that are commonly em- hillocks covered with corestones. In the higher bedded in decomposed material, the rounded or country (e.g., south of Shaver Lake, Fig. 4) ellipsoidal form of which is the result of more they are generally broad shieldlike areas of bare effective penetration by the decomposing solu- granitic rocks, several acres in extent, which ex- tions at edges and corners than in the middle of foliate along spall cracks spaced a few feet apart. plane faces. Corestones crop out as the decom- The weathered mantle in this part of a tread is posed material around them is eroded. generally thin, and roadcuts commonly expose TREADS : The treads range from a fraction of a fresh corestones. mile to 5 miles in width and are 1-10 times as The outcrops are progressively scarcer inward long as they are wide. They are nearly level or toward the base of the next higher front, and gently rolling; slopes on the tread surfaces range the back parts of many treads in the foothills Irom 0 to 11 degrees and are commonly less than are sufficiently free from granitic outcrops to be 5 degrees. Relief on a tread surface ranges from used as croplands. In these areas of scarce out- a few feet to several hundred feet, and is likely crops, wells and roadcuts have penetrated 25-90
Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 DESCRIPTION OF THE STEPS 1175
2 miles Figure 9. Topographic map of the Big Sandy Bluffs and the valley of Tollhouse Creek (from U. S. Geol. Survey topographic map of Shaver Lake 15-minute quadrangle, 1953 edition; contour interval 80 feet). See Figure 1 for location.
feet of gruss before reaching solid rock. The few group are tributary to the same stream and join outcrops in these parts of the treads are usually it many miles east of the Great Valley. The along streams. nickpoints at the heads of steep stretches are STREAM PROFILES: The longitudinal profiles clearly recognizable, but it is also apparent that of most of the streams in the granitic tcrrane nickpoints on adjacent streams or on tributaries are extremely irregular. Relatively flat reaches to the same stream do not occur at a common across the [reads alternate with steep cascades altitude, nor can they be projected to define or falls in short canyons notched into the fronts. any common base level. Each nickpoint has ap- Longitudinal profiles ol two groups of streams parently developed independently of the others. are shown in Figure 10. The streams of each Across the treads the streams generally flow
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in broad shallow channels, in which sandy, Lake quadrangle. Domes and points mark the "graded," anastomosing reaches alternate with stepped topography of such areas as the Dome reaches in which solid, hard bedrock is exposed Land between the forks of the Kern River, and in the bed and banks (PI. 2, fig. 1). The latter much of the California Hot Springs quadrangle. reaches may have low rapids. The sand is trans- These irregularities, and the irregular character ported gruss. Rarely are pebble or cobble bars of the fronts, treads, and stream profiles, sug- present and the few pebbles and cobbles con- sist of 90 percent or more of metamorphic or ba- saltic rocks, even though these lithologics make up only 5 or 10 per cent of a drainage basin. The canyons notched into the fronts are choked with giant boulders several feet wide, which are joint blocks or corestones, and the streams commonly flow in deep, pot-holed slots in bedrock beneath them (PI. 2, fig. 2). These boulders are too large for the streams to move, even in their steepest segments during periods of flood. Large streams, such as the San Joaquin, the Kings, and the Kern rivers, have smoothly con- cave profiles; only near their headwaters are PLATE 1. JOINTING AND STEPPED TOPOGRAPHY IN THE SOUTHERN SIERRA NEVADA, CALIFORNIA Figure 1. Traces of vertical orthogonal joints on the surface of a "bald rock" outcrop. Tioga Pass Road 1.5 miles west of Yosemite Creek, Yosemite National Park Figure 2. Plat extension of San Joaquin Valley into foothills of the Sierra Nevada, with scattered core- stone outcrops. Note stepped topography in background. View southeast from Fresno-Kings Canyon Highway across Citrus Cove. The hills in the background arc shown on the extreme west side of Figure 8. Figure 3. Low step front armored with corestone outcrops. Near Bates Station, Raymond quadrangle Figure 4. Corestone outcrop embedded in gruss. Sec. 6, T. 7 S., R. 20 E., Bailey Flats, Mariposa quad- rangle Figure 5. Step front armored with "bald rock" outcrop. Big Sandy Bluff on the northwest side of Toll- house Creek, seen from Tollhouse Grade (see Fig. 9) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 ------ ------ ------ JOINTING AND STEPPED TOPOGRAPHY IN THE SOUTHERN SIERRA NEVADA, CALIFORNIA WAHRHAFTIG, PLATE 1 Geological Society of America Bulletin, volume 76 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 ------ ------ ------ DRAINAGE AND WEATHERING PHENOMENA IN STEPPED TOPOGRAPHY, SOUTHERN SIERRA NEVADA, CALIFORNIA WAHRHAFTIG, PLATE 2 Geological Society of America Bulletin, volume 76 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 ORIGIN OF THE STEPS 1177 rapidly than the same rocks exposed, because history of the range, independent of a regional where buried they are kept for most of the year base level. Thus, the stepped topography can in contact with reactive ground-water solutions, be explained as the result of normal erosion in whereas the exposed rock is wet only during granitic rocks of a tilted fault block, and the each rain and is dry most of the year. Weather- treads and fronts cannot by themselves be used ing breaks the granitic rocks into coarse sandy as evidence of former surfaces of low relief or grass. Reaction with solutions proceeds inward for faulting. from joint surfaces grain by grain, so that the Metamorphic rocks lack these steps because fresh core-stones are separated from disinte- they generally weather into small fragments, grated granite by a sharp contact. which roll or creep downslope and can be trans- Weathered grtiss can be moved by small ported by most streams. Hence mountains of streams, even overland runoff; but unweathercd metamorphic rocks have narrow ridge crests joint blocks are so huge that they cannot be and long and even slopes close to the angle of moved, even by the largest rivers. Granitic repose. Streams in the metamorphic terrane bedrock breaks down into clasts of sharply bi- generally have smooth profiles; irregularities modal size distribution, with few or no frag- can be related to contrasting lithologies. ments intermediate between huge corestones A similar hypothesis was proposed by Biidel and grains of gruss. (1957) to account for broad plains and stepped Because exposed granitic rocks weather much surfaces in tropical regions. He recognized that more slowly than buried granitic rocks, and exposed rocks in the tropics are in an essentially because streams cannot move the unweathercd arid environment and weather slowly, whereas rocks, any accidental exposure of Iresh granitic rocks deep in the soil are in a warm humid en- rock from beneath the layer of weathered rock vironment and weather comparatively rapidly. —whether it be in a stream bed or on a hill- On this observation he erected a theory of side—establishes a local, long-persisting base double planation surfaces to account for rapids, level, down to which the area upstream or up- piedmont benchlands, and bornhardts in trop- hill can be flattened relatively rapidly, but be- ical regions. Oilier (1960) applied this concept low which it can be reduced only very slowly. to explain inselbergs in Uganda. The chief dif- Such outcrops, formed during spurts of uplift ference between the present hypothesis and and downcutting during the growth and tilting that of Biidel is that the development of steps, of the range, could grow into escarpments according to the former, is possible at any alti- through differential lowering of the country tude, whereas in Biidel's theory each piedmont above and below them, and at the same time benchland develops near base level. Further- the area above each escarpment would be flat- more, the surface between weathered and un- tened to a base level provided by the lowest weathered rock is not a single irregular surface point on the scarp. The escarpments, growing but is complex, with weathered rock between from randomly distributed points, would coa- and beneath buried corestones (cf. Ruxton and lesce to form a pattern such as that just de- Berry, 1957). Also, weathering takes place be- scribed, with escarpments of variable height, neath exposed solid rock (see Fig. 11 and PI. 2, length, and orientation, and fortuitous patterns fig. 3). of merging treads or fronts; the dominant trend of escarpments would be transverse to the dom- Evidence for Differential Weathering inant direction of drainage. The treads can de- The evidence for differential weathering con- velop at any altitude and at any time in the sists of exposed corestones, bouldery outcrops, PLATE 2. DRAINAGE AND WEATHERING PHENOMENA IN STEPPED TOPOGRAPHY, SOUTHERN SIERRA NEVADA, CALIFORNIA Figure 1. Sandy graded stream bed across tread. Middle fork of Chowchilla River 100 feet upstream from bridge of Bailey Flats-Usona Road in NEJ^sec. 7, T. 6 S., R. 19 E., Manposa quadrangle Figure 2. Boulder-choked canyon of same stream on a step front. Middle Fork Chowchilla River 100 (eet downstream from bridge of Bailey Flats-Usona Road. Same locality as Figure 1 of Plate 2 Figure 3. Solid corestone outcrops on hilltop resting on more than 50 feet of gruss. Note man standing to right of tree. Sand quarry in Orosi-Drum Valley Road, sec. 35, T. 15 S., R. 25 E., Dunlap quadrangle Figure 4. Fresh boulders resting on disintegrated boulders. Mudflow levee on outwash fan of Tahoc Glaciation on Lone Pine Creek. Whitney Portal Road, 6 miles west ot Lone Pine Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 1178 CLYDE WAHRHAFTIG—STEPPED TOPOGRAPHY, CALIFORNIA and glacial erratics of largely unweathered bluish gray in hand specimen and outcrop, be- granitic rocks resting on thick layers of thor- cause of the transparency of the quartz and feld- oughly disintegrated granitic rock. The most spar, and shows in thin section only some ap- convincing evidence is found on moraines and parently deuteric alteration of biotite to chlo- alluvial fans of Tahoe age and older (Black- rite and of anorthite-rich cores of zoned plagi- welder, 1931). The surfaces of these moraines oclase grains to albite and epidote. and alluvial fans bear abundant boulders of Weathering proceeds inward from )omt sur- hornblende-biotite granodiorite and biotite faces toward the cores of joint blocks. The first quartz-monzonite, so little weathered that they sign of weathering is a whitening of the rock, ring to the blow oi a hammer. Excavations in due apparently to development of fine fractures these deposits disclose that to depths of 5 or 10 feet and deeper, these same rocks buried are thoroughly disintegrated to gruss (PL 2, fig. 4). Although dark inclusions projecting several inches from the surficial boulders and a litter of flakes and spalls of weathered granodiorite on the ground around each boulder indicate some loss of volume by weathering, this weathering is insignificant compared to the complete disinte- gration of buried boulders. Nearly all the boul- ders must have been relatively fresh when they were deposited, or they could not have survived glacial or alluvial transport. Their present con- dition proves that the rate of disintegration is much greater where they are buried than where they are exposed. Localities where these relationships can be Figure 11. Sketches of roadcuts, showing seen include Emerald Bay at Lake Tahoe (Fig. distribution of fresh and weathered grano- 1), moraines along Bishop Creek (Fig. 3), mo- diorite. Gruss (disintegrated granodiorite in place)is shown by stipple pattern; solid raines on the upland north of Jackass Creek granodiorite is blank; traces of joints in (R. J. Janda, 1964, Oral communication), just gruss shown by dashed lines. A, Roadcut north of the north edge of the area shown in in NEMsec. 7/T. 6 S., R. 20 E., Mariposa Figure 4, roadcuts in the alluvial cone of Lone quadrangle (Raymond-Usona Road); B, Pine Creek west of Lone Pine (PL 2, fig. 4), Roadcut in NWMsec. 13, T. 6 S., R. 21 and roadcuts in Sequoia National Park (Mat- E., Bass Lake quadrangle (Fresno-Yo- thes, 1956, fig. 54). semite Highway, east-facing cut) In roadcuts and quarries in the western foot- hills of the Sierra Nevada, bouldery outcrops can be seen to rest on thoroughly disintegrated in the feldspars. Little change can be detected gruss (Fig. 11; PL 2, fig. 3). This gruss can be as in thin section in this whitened rock, which is much as 50 or 100 feet thick beneath unweath- still hard and firm. The zone of whitened rock ered corestones (PL 2, fig. 3). Similarly, several is extremely variable in thickness. Some core- highway cuts in the sheeted granodiorite of the stones are completely whitened without losing "bald rock" outcrops along step fronts disclose any of their coherence; in others, the clear, an outermost sheet of relatively fresh hard blue-gray rock is in sharp contact with disinte- granodiorite resting on an unknown thickness grated gruss. of gruss. This may be seen on the Kings Canyon The actual breakdown to gruss involves the highway 2 miles north of Grant Grove (Tehipite development of many closely spaced fractures. Dome quadrangle) and the Auberry-Shaver The most prominent of these parallel the core- Lake road on the west slope of Bald Mountain stone boundaries and give the gruss a schistose (Shaver Lake quadrangle). appearance, which is concentric to the core- stones. Commonly, but not universally, these The Weathering Process fractures are iron-stained, perhaps as a result of Weathering of biotite and, to a much lesser leaching from the soil above. extent, of plagioclase, appears to break down The most noticeable petrographic change is the rock to gruss. The unaltered rock is slightly alteration of biotite at the boundaries of the Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 ORIGIN OF THE STEPS 1179 corestones. In fresh rock, biotite has the pleo- chroic formulae = pale yellow; /3 = 7 = deep Origin of the Steps According to the Hypothesis olive brown with very strong absorption. In We can only infer what the Sierra Nevada gruss, the biotite is bleached to pale reddish looked like in the period between the intrusion brown, almost orange, in the /3 and 7 directions, of the granitic rocks and the beginning of the suggesting oxidation of much of the iron and block faulting that produced the present range. probable considerable leaching of cations. In A possibility is that stepped topography has sections cut normal to cleavage the lattice planes characterized the Sierra Nevada since the first parallel to cleavage fan out at the grain borders exposure of granitic rocks irom beneath their and curl back like peacock feathers. X-ray metamorphic cover. Biidel (1957) and King diflraction studies show that the biotite has al- (1953) have shown that plains and plateaus tered in part to expandable clays. characterize the topography of crystalline ter- The feldspar is much less altered. Many ranes in tropical climates such as that which grains appear completely unaltered, and com- may have prevailed in the Sierra Nevada during monly only the calcic cores of a few plagioclases late Cretaceous and early Tertiary time. Their show signs of extensive alteration, generally to analysis applied primarily to shield areas, not to sericite and clay minerals. Similar alteration of the mountains of orogenic belts. Biidel has fur- plagioclase was observed in weathered granodi- ther shown that a stepped topography would re- onte in the Sudeten Mountains by Jahn (1962). sult from the intermittent uplift of such a region. The alteration veinlets in the plagioclase are Two considerations suggest that this was either parallel to the (010) plane or make probably not the case for the Sierra Nevada: angles of about 60-65 degrees with (010) in the (1) By Budel's hypothesis the major rivers zone of [100]. as well as minor streams would, initially at least, The alteration of biotite to 14-angstrom clays descend the step fronts in falls or rapids, or and of a little of the plagioclase to sericite and would be incised in shallow canyons. The major clay minerals results in expansion of their lat- rivers might subsequently have cut the narrow- tices, particularly with alternate wetting and canyons in which they now flow, armed as they drying. This swelling of the altered minerals are with abundant pebbles and cobbles, for shatters the rock. The fractures, as seen in thin Budel suggests that a supply of coarse clastic section, rarely follow grain boundaries, but materials is the chief reason for the smoothly rather are irregular, radiating in part from ex- concave stream profiles and intricately dissected panding minerals, and in part following cleavage hill lands of arctic and temperate regions. Such directions in the feldspars. The hornblende ap- canyons cut by the major rivers would be nar- pears nearly unaltered. row slots. Their bedrock cliffs would weather Prokopovich (1965) suggested that frost riv- and erode much more slowly than the uplands ing is responsible for gruss in the Sierra Nevada, above them, and there would be no way, ac- but the 50- and 100-foot depths to which dis- cording to either Budel's hypothesis or the hy- integration extends rule this out. It is unlikely pothesis outlined in this paper, for a stepped that the west slope of the Sierra Nevada ever topography to develop on the canyon walls. had a permafrost climate. Lowering of isotherms Therefore the stepped topography, several miles by as much as 7000 feet (several thousand feet wide, that lines the walls of the canyons of the more than necessary to put glaciers in the San Joaquin, Kings, and Kern rivers must have cirques on Shuteye Peak) would bring the cli- developed from more evenly sloping valley mate of Huntington Lake to the valley floor. sides. In other words, the topography was prob- Even under such extreme conditions the aver- ably initially dendritic, with relatively even age temperature at the valley floor would be slopes and narrow ridge crests, probably formed 12°F above freezing, and the annual air-tem- on deeply weathered granodiorite (weathered perature wave would have an amplitude of either to a clay-rich saprolite or to gruss), and 31 °F. If we neglect the effect of snow cover and with few or no exposures of solid unweathered latent heat of water, both of which would re- rock. duce the amplitude of temperature fluctuations (2) The mountains on the lower San Joaquin in the ground, and use Lachenbruch's (1959) capped by the Table Mountain basalt are nar- curves, \ve find the annual penetration of the row and sinuous and appear to be remnants of a Ireezing isotherm into solid granite (with a dif- flow that descended a relatively narrow mean- fusivity of 0.014 cgs units) to be only 1 foot. dering valley floor and that sent a tongue up at Penetration would be much less in gruss. least one tributary, which is now the ridge Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 1180 CLYDE WAHRHAFT1G—STEPPED TOPOGRAPHY, CALIFORNIA southwest of Auberry (Fig. 4). They indicate that, when the upper part of the Big Sandy Blufls was already in existence, the foothill country downstream from the bluffs was not a smooth plain or series of steps but was a hilly area through which the winding valley of the San Joaquin then extended. The present stepped topography on either side of Table Mountain must have developed from that hilly country. From these considerations, we may inter that the Sierra Nevada in early Cenozoic time had a gentlv rolling topography, with a dendritic ar- Figure 12. Longitudinal profiles illustrating the development ol nickpoints along streams ray ol valleys slightly incised in hills of deeply during uplift or tilting. A, B, and C, stages weathered granitic rocks. As the range was tilted in the development of nickpoints succes- westward, beginning in Oligocene or Miocene sively downstream during continued uplift; time, west-flowing streams incised their beds D, E, and F, development of several nick- until they reached solid bedrock. Once solid points simultaneously during one episode of bedrock was exposed at any point, erosion at tilting that point was greatly retarded, unless the stream had abundant gravel bedload, or unless subsequent aggradation buried the bedrock out- While the streams are flowing over nickpoints crop. Probably only the San Joaquin, Kings, on solid exposures (Fig. 13, part A) weathering and Kern rivers had basins large enough with of solid bedrock to gruss continues at depth on sufficient metamorphic and volcanic rocks to either side of the stream. Eventually the stream- provide gravel bedload; and these streams, at bed outcrop would be a ridge or pinnacle of least on their lower courses, have more or less solid rock buried in gruss (Fig. 13, part B). The smoot hly concave profiles which appear to date stream would probably migrate laterally away back to before the Pleistocene glaciations. Ex- from the solid bed, and would then be able to posures of granitic rock in the beds of streams cut down into disintegrated rock to the side. with bedloads predominantly of sand and clay By successive lateral migrations (Fig. 13, parts (such as the Chowchilla, Fresno, Kaweah, and C, D, and E) the stream would eventually be Tide rivers) would act as temporary base levels of long duration. If tilting continued, the streams could deepen their beds upstream from the solid granitic exposures only down to the level of these exposures, or until solid bedrock was exposed upstream. Downstream these streams could deepen their beds in response to tilting and uplift as long as no solid bedrock was exposed. Thus each initial bedrock outcrop in the steam would grow into a pronounced nick- point, and perhaps a rapid or waterfall. T he profiles of these streams would become a scries of flat sandy graded reaches separated by steep rocky reaches. These nickpoints might have developed pro- Figure 13. Cross profiles illustrating the fix- gressively downstream as uplift continued, as- ing of a stream in a solid bedrock notch and suming it to progress westward from the eastern mckpomt (arrow points to center line of margin of the range (Fig. 12, parts A, B, and stream). A, initial exposure of solid granitic C); or they might have developed at several rock in streambed; B, continued weathering on either side of stream; C, lateral migration points on the stream course at the same time, of stream onto weathered rock, exposing assuming bodily tilting and steepening of the solid bedrock on one wall of valley; D, more entire profile (Fig. 12, parts D, E, and F); or lateral migration onto solid bedrock; E, intermediate nickpoints might have developed continued weathering; F, return migration after the initiation of a few widely spaced early and downcutting. fixing stream in a bedrock ones. notch Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 ORIGIN OF THE STKPS 1181 held in a shallow gorge with solid bedrock walls slopes underlain by thick gruss, it is still possible and floor, from which lateral migration would for streams to be captured from their bedrock be impossible. Weathering and lateral migration nickpoints. The stages of such a capture are in- would probably be simultaneous, so that each dicated in Figure 15. The capture would lower stream would be relatively quickly fixed in such the stream bed upstream from the abandoned a gorge. It is only when this has occurred that nickpoint, allowing gradation of a step at con- the development of stepped topography has siderably lower altitude than formerly possible. really begun. The result of such a capture might be the isola- Erosion would be accelerated in intcrstream tion of a cliff-bordered plateau on the edge of a areas as the streams lowered their beds. An area step, such as Lion Point (Fig. 7) or Btiford tributary to a stream downstream from a nick- Mountain (Fig. 4); or of an isolated peak such point would be subject to a wave of gullying or as Black Mountain (Fig. 4) or a rock monu- ment. Such capture probably commonly occurred where several streams poured over the same growing step front. Tributaries to the stream with the lowest nickpoint, eroding downward through the thick gruss beneath the tread, might capture the headwaters of the other streams (Fig. 16). The resulting drainage would be trelhsed, and each tread would slope back- ward to the next higher front. An isolated cliff-bordered plateau would, through this type of capture, eventually be C D drained by a single stream. Its summit would Figure 14. Development of hillside out- become slightly dish-shaped, and the drainage- crops. A, initial exposure in gully floor; divide would lie along the boundary escarp- B, migration of gully; C, exposure of out- ment. Piutc Mountain south of Walker Pass is crop as buttress; D, lateral enlargement of an example. outcrop The irregularity of the step fronts in height and ground plan and the fact that they merge and die out result from the random way in accelerated mass wasting that the area upstream which solid outcrops have been exposed on the from the nickpoint would not experience. Such hillsides, and from the equally random way in accelerated erosion would probably uncover ad- which they have been enlarged and conjoined ditional unweathered rock, and these exposures to produce the step fronts. Because weathering would act as local base level for the hillside to gruss can take place beneath corestonc out- above them. Erosion would continue on either crops, the initial outcrops were probably not side of such outcrops and leave them as but- the ones we see today. As these initial outcrops tresses or projections on the hillsides. Rainwa- were destroyed, the step front may have re- ter would be diverted around the outcrop, ac- treated slightly in the early stages of its devel- celerating erosion at its sides; and the outcrop opment. It is possible that some of the step would tend to grow laterally rather than tip- fronts now present may be destroyed as their slope. Horizontal unloading joints would pro- armour of solid outcrops is undermined by gul- mote this lateral growth. Downslope growth of lying in weathered gruss beneath them. How- the outcrop would take place as the lowering oi ever, the tremendously divergent rates of local base level permitted. The development of weathering and erosion in the two environ- outcrops is illustrated in Figure 14. Several such ments of exposure ensures that a stepped topog- outcrops on the walls of a rejuvenated valley raphy will result from these random exposures. might coalesce to form an irregular cliff on the As long as weathering keeps pace with uplift side of the valley, establishing a temporary base and downcuttmg, and no fresh bedrock is ex- level below which the upper valley walls could posed downstream or downslope, a step front not be lowered. will grow in height through lowering of the low- At this stage in the development of stepped land at its base. The establishment of a down- topography, when the growing escarpments arc stream nickpoint, however, lorms the limit for marked by discontinuous outcrops rising from the height of the step front upstream. The Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 1182 CLYDE WAHRI IAFTIG—STEPPED TOPOGRAPHY, CALIFORNIA highest fronts, therefore, are those that formed second class belong the hypotheses (c) that the when slow uplift or fluctuations in the thickness fronts mark contacts between rocks of different of alluvial fill kept the granitic rock down- susceptibility to weathering and erosion and (d) stream buried. Relatively rapid uplift or tilt- that the stepped topography is controlled by ing, with accompanying rapid downcutting and jointing. To the third class belongs the hypoth- numerous bedrock exposures, would produce a esis (e) that the frontal scarps retreat parallel to large number of steps and, consequently, low fronts. As an example, the Big Sandy Bluffs are a segment of the highest step front between the San foaquin and the Kings rivers, and are in- tersected at half their height by the projected surface of Table Mountain (Fig. 5). The basalt of Table Mountain is underlain by as much as 320 feet of interbedded tuff and gravel (Mac- donald, 1941, p. 262-263) and hence marks the end of a period of aggradation in the ioothill region. This period of aggradation, the only one for which we have evidence in the southern Sierra Nevada, permitted deep weathering in the foothill region to outstrip erosion, enabling the Big Sandy Bluffs, which were already 1500 feet high, to attain their present height of 3000 feet. Although it is believed that much, if not most, of the stepped topography was eroded from a formerly hilly topography, at least one Figure 15. Four stages in the capture of a major step has formed at base level in the man- stream by one of its tributaries. Cross- ner suggested by Biidel. The flat extension of hatched areas are bedrock outcrop?. A, Solid the San Joaquin Valley across granite between bedrock outcrop at x acts as local baselevel the Fresno and White rivers and its extensions for upper segment of main stream, forming into the foothills were probably regraded to a mckpomt; B, tributary y deepens its valley level plane during Quaternary fluctuations in in a segment of the escarpment which is base level and in volume of alluvial fill in the fortuitously deeply weathered; C, The main stream above bedrock outcrop .v is captured San Joaquin Valley, and have an abrupt gra- by tributary y. At about the time of capture, nitic escarpment at their eastern edge (Fig. 7). solid bedrock outcrops appear in bed of trib- Lowering of base level by more than a few hun- utary v, acting as local baselevels for ero- dred feet, and subsequent erosion, would expose sion; D, the upper tread is generally re- a line of outcrops at the western margin of the duced to the level of the upper outcrop in granite platform, where it is overlapped by the bed of tributary v. The original course of Tertiary rocks, and form the front of this step. the mam stream at x is now a promontory. DISCUSSION OF ALTERNATIVE HYPOTHESES themselves and the treads are piedmonttreppen Beiore the proposed hypothesis can be re- as defined by Penck (1924: 1953 translation, garded as a theory for the origin of the steps, it p. 147-227).' is necessary to consider alternative mechanisms THE FAULT-SCARP AND FAULT-LINE-SCARP for step formation in the Sierra Nevada. The HYPOTHESES: The fault-scarp hypothesis was alternative hypotheses belong to three classes. proposed by Hake in 1928. The extreme irregu- They assume either (1) displacements of an orig- larity of the map pattern of the fronts (Fig. 4) inally continuous, nearly level surface; (2) lith- is incompatible with steeply dipping faults. ologic control on the rate of erosion; or (3) Only a series of gently dipping thrusts, for grading to different local or regional base levels which there is no geologic evidence, would fit during uplift of the range. To the first class be- this map pattern. long the hypotheses that the fronts may be (a) The most convincing argument against fault fault scraps or (b) fault-line scarps. To the and fault-line scarps is provided by the Table Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 DISCUSSION OF ALTERNATIVE HYPOTHESES 1183 Mountain basalt of the lower San Joaquin Riv- gion, which during lone time was close to the er, described under the sections "Cenozoic marine shore line, greater height and relief than basalt" and "Origin of the steps according to the Sierra Nevada to the east, which was pre- the hypothesis" (see Figs. 4 and 5). These un- sumably close to the headwaters of the rivers faulted flow remnants are clearly older than the flowing to the sea of lone time. stepped topography above which they rise, Many of the larger canyons are lined with since the mountains they cap arc the result of tiers of steps that face toward the rivers (Figs. topographic inversion by erosion. Therefore 4 and 7). Attributing these steps to faulting re- quires that each canyon, regardless of its sinu- osity, be lined with faults down-dropped toward the river, and that the canyons arc the result of faulting and not erosion. This hypothesis has not been seriously considered for the Sierra Nevada since 1870. In the extreme southern end of the Sierra Nevada, east of Bakersfield, active and inactive- faults are marked by both fault and lault-line scarps. These include the White Wolf fault (Oakeshott, 1955), the Kern River fault (Black- welder, 1928, p. 298 and 308), faults around Tehachapi Valley (Buwalda, 1954), and the Kern Canyon fault (Webb, 1946; 1955). These faults die out northward and could not have caused the stepped topography described in this paper. LITHOLOGIC CONTROL: Rarely do fronts coincide with bedrock contacts. Contacts were mapped by Ross (1958, PL 1), Krauskopf (1953, PI. 1), and Hamilton (1956, PI. 1) and were described by Durrell (1940, p. 4-13). These contacts cross the step fronts, in places at right angles to them; little correspondence between contacts and step fronts can be de- tected. As already noted, the line of escarp- ments that Krauskopf (1953, p. 8) related to his migmatite boundary is a complex of steps, and Figure 16. The development of trelliscd some of the fronts intersect the migmatite drainage and backward-sloping treads boundary at right angles (Fig. 4). Large areas of by stream capture relatively homogeneous rock, such as the Dinky Creek granodiorite (Bateman and others, 1963, PI. 1), the granodiorite of the region around they make a fault origin for the steps in their Oakhurst and Ahwahnee, and the granodiorite vicinity impossible. The prebasalt granitic sur- in the western part of Dunlap quadrangle, show face exposed on the walls of the table mountains relatively little variation in the character of the is parallel to the basalt surface, ruling out a rock, and none to correspond with the stepped fault-line hypothesis as well. topography. Planar and linear parallelism of KIsewhere in the region of stepped topog- minerals and inclusions in these plutons, which raphy, mountains of metamorphic and granitic generally parallel both plu tonic contacts and rocks west of many steps approach these steps internal compositional variations, show no con- in altitude. Examples are Goat and Thornbury sistent relationship to the trends of the step mountains (Fig. 4) whose summits are higher fronts (Fig. 8). than many steps northeast of Oakhurst and On the other hand, N. King Fluber (1964, Ahwahnee. If the step fronts are interpreted as Written communication) finds that the bluffs fault scarps, "unfaulting" the stepped topog- around the basin of Central Camp, the bluff raphy to bring the supposedly displaced surfaces south of Central Camp, and Chiquito Ridge into juxtaposition would give the foothill re- (Fig. 4) are underlain by a quartz-monzonite Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 1184 CLYDE WAHRHAFTIG-STKPPKD TOPOGRAPHY, CALIFORNIA much more resistant to weathering than the more jointed rocks, have excavated in the Can- granodiorite around it; these belts of quartz- adian and Fennoscandian shields and the Coast monzonite appear to be responsible, at least in Range batholith of British Columbia and part, for these escarpments. Other steps in the Alaska an anastomosing or reticulate network of Shuteye Peak quadrangle bear no relationship lake basins, valleys, low passes, and fjords. This to pkitonic or lithologic contacts. As the foot- network appears to follow belts of well-jointed hills are mapped in detail, more step fronts may rock surrounding isolated massifs of presumably be found to coincide with intrabatholithic con- poorly jointed rock. If joint density were re- tacts, but the majority in areas that have been sponsible for the topography of the Sierra mapped do not coincide with lithologic contacts. Nevada, we would expect to find a landscape of JOINT-CONTROL HYPOTHESES: There are three ways in which jointing might account for the steps (Fig. 17): (A) the step fronts are eroded along master joints; (B) each front marks a change from more closely jointed rock in front to more widely spaced jointing behind; and (C) the fronts are upheld by rock with few or no joints and the treads arc underlain by closely jointed rock. Joint-control hypotheses are the most difficult to test because the almost complete lack of exposures over large parts of the treads makes information on joint density nearly impossible to obtain. The master-joint hypothesis can be readily tested. Master joints in an area cast of Ray- mond, mapped irom aerial photographs and topographic maps, are shown on Figure 18. Where they intersect the step fronts (indicated by close spacing of contours), they indent them only slightly. It is unlikely that lesser joints which have no effect on other aspects of the topography could be responsible for the step fronts. Figure 17. Three mechanisms for joint If each front represents a change in spacing ot control of stepped topography (see joints, a tier of 10 or 11 steps requires many text for explanation) changes in joint spacing (Fig. 17B), all increas- ing in the same direction. The likelihood of this situation decreases as the number of steps in- isolated blocklike mountains, separated by a creases. network of valleys. In the stepped topography Where deep excavations in gruss enable the the reverse is the pattern. It is the anastomosing original joint spacing to be measured (lor ex- linear network of fronts that separates isolated ample, in deep highway cuts and on diversion areas which are the treads (Figs. 4 and 8). Thus, ditches), the distance between joints beneath although some steps may he due to differences treads averages about the same as beneath in jointing, joint control of weathering is prob- fronts. These lew examples, however, are not ably not the general explanation tor the stepped enough to draw firm conclusions, particularly topography. as some masses oi granitic rock [for example, PIEDMONTTRKPPEN HYPOTHESIS: According the monoliths of Yosemite Valley (Matthes, to the piedmonttreppen hypothesis of W. Penck 1930, p. 114-117)| appear to be unjomted. The (1924, p. 162-186; 1953 translation, p. 197- hypothesis that the rock beneath the fronts is 227), the step fronts would be backward-re- less jointed than that beneath the treads can treating escarpments and the treads, nearly only be tested by comparing the map pattern of level surfaces graded to base level by retreat of fronts and treads with the map patterns ot the fronts during pauses in the uplift of the jointed and unjointcd rock in other parts of the range. The fronts are supposed to retreat at ap- world. proximately equal rates, so that the steps follow Continental ice sheets, selectively quarrying each other in succession back into the high- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 DISCUSSION OF ALTERNATIVE HYPOTHESES 1185 2 miles i i i i i _J Figure 18. Topographic map showing relationship between step fronts and master joints cast of Ray- mond, California (from U. S. Geological Survey topographic map of Raymond 15-minute quadrangle, 1944 edition; contour interval 50 feet). Heavy dashed lines are master joints recognized from aerial photographs. See Figure 1 for location. lands. This hypothesis has been applied by hypothesis and topography can be resolved only Sauer (1929) to explain topography on granitic by supposing that tilting has counterbalanced rocks in Southern California, which is similar the lowering of the steps, [f this had happened, to the stepped topography described here. Bir- the highest treads, because they are the oldest, man (1964, p. 9) suggests it as a major process should have received the greatest westward re- modeling the Sierra Nevada. versal of tilt and should be nearly level or slope- The treads of the Sierra Nevada slope back- west; they are, however, among those with the ward to the fronts behind them. Hence, as each steepest backward slopes. front retreats, the altitude of its top should de- Implicit also in the piedmonttreppen hypoth- crease (Fig. 19). However, the Sierra Nevada esis (as well as in step development according to rises eastward, and this contradiction between BiidcFs theory) is an accordance in the level oi Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 1186 CLYDE WAHRHAFTIG—STEPPED TOPOGRAPHY, CALIFORNIA the treads, since they are assumed to develop ing to the Chagoopa Plateau upstream. Below at common base levels. However, as Figure 10 the mouth of the Little Kern River discontinu- shows, there is no accordance of nickpoints. A ous benches on the west side of the river are less few of the treads may be piedmonttreppen or than 1000 feet above the stream, and the flat pediments, but most are certainly not. summit of the Bartolas Country is more than 3000 feet above it. None of these downstream THE STEPPED TOPOGRAPHY AND surfaces can be reasonably projected to the ANCIENT EROSION SURFACES Chagoopa surface at the head of the river; the The hypothesis of the origin for the stepped Chagoopa surface cannot have been more ex- topography presented here raises questions tensive than it is now. about the validity of ancient erosion surfaces on Lawson (1904) recognized that the Kern Can- granitic terrane in the Sierra Nevada (and else- yon is eroded for much of its length along a where) and of late Cenozoic uplift deduced north-trending fault. Webb (1946) showed that from them. Three ancient erosion surfaces in this fault ceased activity before the develop- the southern Sierra Nevada have been postu- ment of the present topography and is overlain lated on flat summits and benches and on graded stream profiles between nickpoints (Lawson, 1904; Matthes, 1930; 1937; 1960; Axelrod, 1962; Axelrod and Ting, 1961; for a review see Dalrymple, 1963, p. 384-387), and have been named by Matthes (1937), in the Mt. Whitney region, the Mt. Whitney (oldest), Figure 19. Effect of backward retreat o[ a the Boreal Plateau, and the Chagoopa (young- front on the altitude of its crest, accord est) surtaces. The equivalents of the last two mg to the piedmonttreppen hypothesis in the Yosemite and San Joaquin regions were named (Matthes, 1930; 1960) the Broad Valley by unfaulted basalt on the Little Kern River. (Miocene) and the Mountain Valley (Pliocene) The upper Kern Canyon probably owes its stages. depth and straightness to erosion along the As outlined in the hypothesis presented here, crushed rock of the fault zone, and its straight a bench, summit flat, or nickpoint can develop walls are step fronts that owe their position to in granitic terrane at any altitude at any time. differences in jointing and crushing of the bed- The only requirement is the appearance of solid rock. The lower canyon does not follow the granitic outcrops to act as local base levels. Once fault, and its benches, as well as the benches on formed, these summit flats are long-persisting the tributaries, must be more typical steps. and may accumulate ephemeral alluvial deposits The upper basin of the San Joaquin River is a at any time after their origin. Other evidence broad upland rimmed by high mountains. Mat- must be used to establish that granitic terranes thes (1960) correlated parts of this upland with were once nearly graded plains close to sea level. his (1930) Broad Valley and Mountain Valley The remnants of the older two surfaces in the stages on the Merced River, and reconstructed Mt. Whitney area are flat summits of high peaks Miocene and Pliocene profiles of the San and isolated plateaus. These could have formed Joaquin River by projecting graded reaches of after the uplift of the range and been scraped the tributaries between their nickpoints to their bare of gruss by glaciation or periglacial proc- assumed points of intersection with the San esses. Joaquin (Matthes, 1960, p. 41-44, PI. 2). The The Chagoopa surface is a broad platcaulike upper part of his Pliocene profile, projected benchland on both sides of the headwaters of downstream parallel to the river, coincides with the Kern River in Sequoia Park, into which the the summit of Table Mountain, whose flows and straight U-shaped Kern Canyon is incised 2000- sediments represent a pause in the uplift and 2500 feet. If the Chagoopa Plateau is a remnant downcutting of the range 9-10 m.y. ago (see of an ancient peneplain, it should increase in "Cenozoic basalt"). The lower part of the pro- width downstream and have extensive remnants file diverges from the Table Mountain profile on the lower Kern River. However, for 11 miles and is considerably below it near the San Joaquin between the south boundary of Sequoia Park Valley. The nickpoints on which it is based are and the mouth of the Little Kern River the related to stepped topography in the foothills walls of the inner canyon are 3500-4500 feet and not to stages in the downcutting of the San high, and there is no sign of a bench correspond- Joaquin River. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021 OTHER EXAMPLES OF STEPPED TOPOGRAPHY 1187 contain broad plains and plateaus at different OTHER EXAMPLES OF STEPPED levels, separated by rocky rapids and waterfalls, TOPOGRAPHY and regions of bornhardts and steep-sided domes Many regions of crystalline rocks are char- (for example around Rio de Janiero). Most of acterized by benched landscapes at several these probably originate in the manner proposed levels. The hypothesis outlined for the stepped by Biidel (1957). Some, particularly in the more topography of the Sierra Nevada may also ac- rugged highlands, may originate in the manner count for these benches. suggested in this paper, that is, at high levels, The Peninsular Ranges of Southern Califor- in already uplifted land, not necessarily at or nia, described by Sauer (1929) have many ex- below base level. amples of backward-sloping benches underlain by gruss, separated by steep rocky escarpments, PRACTICAL IMPLICATIONS and the new topographic maps of this region The mechanism outlined for the stepped show these benches to be remarkably like the topography implies that the weathered gruss is steps in the Sierra Nevada. thickest at the back part of the treads and thins Van Tuyl and Lovering (1935) described five to a veneer toward the fronts. Thus, each tread major peneplains and three lower partial pene- may be a separate ground-water basin, and plains in the Colorado Front Range. The re- much of the water flowing between treads may cently published topographic maps of Colorado come to the surface in the short canyons down suggest that the flat summits, benches, and park the fronts. If this is true, problems of ground- lands in crystalline rocks, cited as remnants of water recharge, safe yield, and sewage disposal these peneplains, may actually be steps of the in the foothills of the Sierra Nevada are closely sort described in this paper. Rich (1935) pointed tied with the configuration of the interface be- out that correlation of the remnants of these tween the solid and weathered rock. supposed peneplains presents difficulties. If they Accelerated erosion which results in exposure are in iact si eps, no correlation is to be expected. of unweathered rock produced irreversible The Cevannes and other highlands of the changes in the geomorphic processes. Not only Massif Central have similar benches which will reforestation of such gullied areas be much could also be stepped topography. The numer- more difficult because of lack of soil, but the ous basalts of different ages in this region offer rate of production of new soil will be infinitesi- the opportunity to determine the sequence and mally small. Therefore, gullying resulting from rate of development of the steps. The piedmont- fires, lumbering, road making, and poor farm- treppen of the Fichetelgebirge, Black Forest, ing practice may permanently alter the land- and Vosges may also be stepped topography, as scape of significant parts of the Sierra Nevada Biidel (1957) suggested. and other granitic mountains. The shield areas of the tropic and desert lands REFERENCES CITED Allen, Victor T., 1929, The lone Formation of California: Univ. Calif. Pubs., Bull. Dept. Gcol. Sci., v. 18, p. 347-448 Axelrod, D. I., 1962, Post-Pliocene uplift of the Sierra Nevada, California: Geol. Soc. America Bull., v. 73, p. 183-198 Axelrod, Daniel I., and Ting, William S., 1961, Early Pleistocene floras from the Chagoopa surface, southern Sierra Nevada: Univ. Calif. Pubs. Geol. Sci., v. 39, p. 119-194 Bateman, Paul C., Clark, Lorin D., Huber, N. King, Moore, James G., and Rinehart, C. Dean, 1963, The Sierra Nevada batholith--A synthesis of recent work across the central part: U. S. Geol. Survey Prof. Paper 414-D, 46 p. Birman, Joseph H., 1964, Glacial geology across the crest of the Sierra Nevada, California: Gcol. Soc. America Special Paper 75, 80 p. Blackwelder, Eliot, 1928, The recognition of fault scarps: four. Geology, v. 36, p. 289-311 1931, Pleistocene glaciation in the Sierra Nevada and Basin ranges: Geol. Soc. America Bull., v. 42, p. 865-922 Eiidel, Julius, 1957, Die "Doppclten Einebnungsflachen" in den feuchten Tropen: Zeitschr. fur Gco- morphologie; Neue folge, Band 1. 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W., 1959, Ground-water conditions and stor- age capacity in the San Joaquin Valley, California: U. S. Geol. Survey Water-Supply Paper 1469, 287 p. Durrell,Cordell, 1940, Metamorphism in the southern Sierra Nevada northeast of Visalia, California: Univ. Calif. Pubs., Bull. Dept. Geol. Sci., v. 25, p. 1-118 Erwin, Homer D., 1934, Geology and mineral resources of northeastern Madera County, California: Calif, four. Mines and Geology, v. 30, p. 7-78 Hake, Benjamin F., 1928, Scarps of the southwestern Sierra Nevada. California: Geol. Soc. America Bull., v. 39, p. 1017-1030 Hamilton, Warren, 1956, Geology of the Huntington Lake area, Fresno County, California: Calif. Dept. Nat. Res., Div. Mines Special Rept. 46, 25 p. Jahn, Alfred, 1962, Geneza Skalek Granitovvych (Origin of granite tors): Czasopismo Geograficznc, v. 33, p. 19-44 (in Polish, with English summary) Jahns, Richard H., 1943, Sheet structure in granites, its origin and use as a measure of glacial erosion in New England: Jour. Geology, v. 51, p. 71-98 Jenkins, Olaf P., 1946, Geology of placer deposits: new techniques applicable to the study of placers: Calif. Dept. Nat. Res., Div. Mines Bull. 135, p. 149-216 King, Lester C., 1953, Canons of landscape evolution: Geol. Soc. America Bull., v. 64, p. 721-752 Knopf, Adolph, 1918, A geologic reconnaissance of the Inyo Range and the eastern slope of the Sierra Nevada, California; with a section on the stratigraphy of the Inyo Range by Edwin Kirk: U. S. Geo- logical Survey Prof. Paper 110, 130 p. Krauskopf,Konrad B., 1953, Tungsten deposits of Madera, Fresno, and Tulare counties, California: Calif. Dept. Nat. Res., Div. Mines Special Rept. 35, 83 p. Lachenbruch, A. H., 1959, Periodic heat flow in a stratified medium with application to permafrost prob- lems: U. S. Geol. Survey Bull. 1083-A, 36 p. Lawson, A. C., 1904, The gcomorphogeny of the upper Kern basin: Univ. Calif. Pubs., Bull. Dept. 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D., 1960, The inselbergs of Uganda: Zeitschr. fur Geomorphologie, Band 4, Heft 1, p. 43-52 Penck, Walr.her, 1924, Die morphologische Analyse, em Kapital dcr physicalischen Geologic: Stuttgart, J. Engclhorns Nachf., Gcographische Abh., 2 Reihe, Heft 2, 283 p. (Translated by Hella Czech and K. C. Boswcll, 1953, as Morphological analysis of landforms: London, MacMillan and Co., 429 p.) Piper, A. M., Gale, H. S., Thomas, H. E., and Robinson, T. W., 1939, Geology and ground-water hy- drology of the Mokelumne area, California: U. S. Geol. Survey Water-Supply Paper 780, 229 p. Prokopovich, Nikola P., 1965, Pleistocene periglacial weathering in the Sierra Nevada, California, p. 271 in Geological Society of America Abstracts for 1964: Geol. Soc. America Special Paper 82, 400 p. Rich, John L., 1935, Physiographic Development of the Front Range (Discussion): Geol. Soc. 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MANUSCRIPT RECEIVED UY THE SOCIETY SEPTEMBER 21, 1964 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/76/10/1165/3427644/i0016-7606-76-10-1165.pdf by guest on 23 September 2021