ROBERT P. SHARP Division of Geological Sciences, California Institute of Technology, Pasadena, California Sherwin Till-Bishop Tu

ROBERT P. SHARP Division of Geological Sciences, California Institute of Technology, Pasadena, California

Sherwin Till-Bishop Tuff Geological

Relationships, Sierra Nevada, California

Abstract: A glacial till underlies the Bishop tuff, but it has not been established heretofore on firm geological bases that this is the Sherwin till. Relationships here described demonstrate that the till beneath the Bishop tuff is indeed Blackwelder's-type Sherwin. Radiometric datings of the basal pumice of the Bishop tuff by Dalrymple and others yield an age of 710,000 years. Judging from pre-pumice weathering, the till may be still older, perhaps 750,000 years. These considerations make the Sherwin till-Bishop tuff relationship an important reference datum in the continental Pleistocene sequence of North America.

CONTENTS Introduction 351 Figure General statement 351 1. Place map of Sherwin Grade-Rock Creek area. 353 Physical setting 352 2. Geological map of Sherwin Grade-Rock Creek Acknowledgments 352 area 354 Glacial drift beneath Bishop tuff 352 3. Field sketch of relationships in Big Pumice Cut 355 Pumice on Sherwin till 355 4. Field sketch of Sherwin outwash exposed in Introduction 355 walls of Owens River gorge 355 Pumice-Sherwin till relationships along U. S. 395 355 5. Interpretive cross sections of flattopped pumice Other locations 357 hills 356 Stripping of Bishop tuff 358 6. Field sketch of pumice-Sherwin till relationship Geometrical relationships 358 in Little Pumice Cut 357 No Sherwin till on Bishop tuff 359 7. Map of gravels on Bishop tuff along lower Rock Sherwin outwash and other gravels 359 Creek 360 Speculation on correlation 361 8. Field sketch of old red till on old Sherwin References cited 362 Grade road 361

INTRODUCTION 1964, p. 175; Evernden and Curtis, 1965, p. 355). Since he regarded the Sherwin till as General Statement Illinoian, but older than the tuff, Putnam As the type locality of the Sherwin till (1962, p. 205) flatly rejected this age as too (Blackwelder, 1931, p. 895-900), the Sherwin great. Inclusions of older rock fragments in the Grade-Rock Creek area has long been of local tuff (Gilbert, 193 8, p. 1834-1835; Rinehart and interest. It is now attracting international atten- Ross, 1957; Putnam, 1960, p. 236) permit an tion from Pleistocene chronologists because of inference of contamination, as shown by potassium-argon dating of the associated Dalrymple and others (1965, p. 670-671). Bishop tuff. Blackwelder (1931, p. 918, 899) However, dating of sanidine crystals taken regarded the Sherwin till as possibly Kansan from pumice fragments within the tuff, thus and as lying above the Bishop tuff, but Gilbert presumably uncontaminated, gives the still (1938, p. 1860), Rinehart and Ross (1957), impressively great age of 710,000 years for the Putnam (1960, p. 233), and Wahrhaftig and basal pumice of the Bishop tuff volcanic Birman (1965, p. 310) all place the till beneath episode (Dalrymple and others, 1965, p. 670). the tuff. In view of the use being made of these dates The initially determined K40/A40 age of the (Ericson and others, 1964, p. 731), it is clearly Bishop tuff was 870,000 years (Evernden and desirable to establish as firmly as possible the others, 1957, p. 14). This was subsequently geological relationship between the Bishop tuff revised to 980,000 years (Evernden and others, and Pleistocene glacial deposits. If this can be

Geological Society of America Bulletin, v. 79, p. 351-364, 8 figs., March 1968 351 352 R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA done satisfactorily, and if the potassium-argon ward which departs from the present Rock age of 710,000 years holds up, this becomes a Creek course near the head of Whiskey Canyon most important datum in the Pleistocene (Fig. 1). Differences in thickness of the Sher- geological record (Rinehart and Ross, 1964, p. win till are considerable owing to its irregular 79)' topography and the uneven underlying bed- The geological problem, simply stated, is as rock surface. The greatest exposed thickness, follows. Evidence is compelling that a till 500 feet, is in Rock Creek gorge. underlies the Bishop tuff. Is it the equivalent of Blackwelder's-type Sherwin? Or are there two ACKNOWLEDGMENTS tills here, one older and one (the type Sherwin) Warm hospitality and assistance have been younger than the Bishop tuff (Rinehart and extended by the staff of the Sierra Nevada Ross, 1964, p. 74)? Reference is to pre-Tahoe Aquatic Research Laboratory on Convict tills as the Tahoe and subsequent glaciations are Creek. Discussions in the field with Clyde clearly younger than the tuff (Putnam, 1949, Wahrhaftig and his associates and my Caltech p. 1291). Informed readers will recall that east- colleagues have been most helpful. The manu- side Sierra Nevada glaciations currently script has been improved by critical comments recognized are from youngest to oldest; Tioga, from Brent Dalrymple, Charles Gilbert, and Tenaya, Tahoe, Mono Basin, Sherwin, McGee, Maxwell Gage. and Deadman Pass, with the first three usually- regarded as Wisconsin and the remainder as GLACIAL DRIFT BENEATH Pre-Wisconsin. BISHOP TUFF The tuff-till relationship has been investi- Glacial deposits beneath the Bishop tuff are gated in the field, at intervals, during the sum- known at three sites within the map area (Fig. mers of 1964, 1965, and 1966 through "hands 2). The first, most obvious, and most accessible and knees" tracing of contacts, study of ex- site is a deep cut on U. S. Highway 395 just posures, and repeated visitations to critical east of the crossing with Rock Creek, hereafter locations. The conclusion reached is the op- termed Big Pumice Cut. Exposed here from posite of a view, initially held, that favored the the top down are 10 to 12 feet of fluvial gravel two-till concept. The assembled evidence shows with smooth, rounded stones; 75 feet of loose that Blackwelder's-type Sherwin and the till white pumice and ash; and 45 feet of bouldery beneath the Bishop tuff are one and the same. till (Fig. 3). Enclosing stones from the fluvial The Sherwin till is older than the Bishop tuff gravel, clastic dikes cut the pumice and ex- and thus more than 710,000 years of age, if the tend into the till, transecting boulders. The potassium-argon dates are valid. This has far- pumice consists of two units. One is a lower reaching significance with respect to Pleistocene well-layered, somewhat brownish sequence of chronologies (see discussions in Evernden and ash and pumice, 15 feet thick, with bedding Curtis, 1965, especially by Hopkins, Howell, conformable to the gently inclined (10°) till and Wright). surface. The other is a coarser, looser, poorly- bedded white pumice in horizontal attitude. Physical Setting The lower unit looks like an eolian deposit laid The setting is a 7000-foot tableland lying down as a relatively uniform blanket over an northeast of the Sierra Nevada front in east- uneven terrain. The upper, horizontally-bedded central California (118° 38' W., 37° 33' N.). unit was perhaps emplaced as a series of pumice Here Rock Creek emerges northeasterly from flows. The capping of fluvial gravels is much its mountain canyon and turns abruptly south- younger. Beneath the pumice is an excellent easterly through a narrow gorge cut into the till, unbedded, poorly sorted, with a tight, fine, southwestern flank of the tableland (Fig. 1). silty matrix, and a variety of lithologies repre- Exposed Sherwin till covers a bulbous area of sented in the boulders. Granodiorite and quartz 7 square miles bisected by this gorge. The re- monzonite are the predominating lithologies. gion is shown on the U. S. Geological Survey's The Bishop tuff cliff lies 1000 feet northeast Casa Diablo Mountain, 15-minute quadrangle, of Big Pumice Cut, but there can be little 1953. doubt that the pumice and ash in the cut rep- The location, distribution, and lithologic resent the initial phase of the Bishop tuff vol- constitution of Sherwin drift show clearly that canic episode (Putnam, 1960, p. 235-236). Sim- it was derived from the Rock Creek drainage ilar material is seen below the Bishop tuff in with the glacier following a route directly east- other places, and radiometric dating by Dal- o o

Surge Tank

- = =:== Power 3 House CD z, No.1 w

ffi

S O

Figure 1. Place map of Sherwin Grade-Rock Creek area. 354 R. P. SHARP-SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA PUMICE ON SHERWIN TILL 355

Fluvial gravel

*', 'V^0 0 50 -100 200 300 1 ree|~_ nTi Approximate Scale Figure 3. Field sketch of relationships in Big Pumice Cut, U. S. 395 east of Rock Creek. rymple and others (1965, p. 670) shows the the till exposures on U. S. 395. Unfortunately, pumice and cliff-making tuff to be of one age. Putnam's descriptions (1960, p. 233-234) of Beneath the Bishop tuff, glacial deposits glacial drift in the Bureau of Water and Power were penetrated for some 1100 lineal feet by a tunnel do not distinguish between till and out- tunnel of the Los Angeles Department of Water wash; nonetheless, despite his contrary state- and Power on a line passing 0.75 mile north- ment (1960, p. 250), Sherwin ice need not have east of Big Pumice Cut (Fig. 2). Exploratory extended as far east as Owens River gorge. holes drilled for this project penetrated glacial drift up to 400 feet thick beneath the Bishop PUMICE ON SHERWIN TILL tuff (Putnam, 1960, p. 233-234). Nearly 4 miles east of Big Pumice Cut in the Introduction walls of Owens River gorge upstream from Glacial drift clearly underlies the Bishop powerhouse No. 1 (Fig. 1), a lenticular body tuff, but what is the relationship of this drift of glacial drift, up to 200 feet thick and 1.5 to the Sherwin till? Exposures of the till are miles across, is exposed (Fig. 4) beneath the nearby but not obviously stratigraphically be- Bishop tuff (Gilbert, 1938, p. 1860; Ross and neath the Bishop tuff. If two tills are present, Rinehart, 1957; Putnam, 1960, p. 234). The they cannot be differentiated lithologically as smooth surfaces and rounded shapes of stones both consist of debris derived from the Rock in this material suggested outwash. This sug- Creek drainage. Lithologic variations in a single gestion was confirmed after an extended search exposure of Sherwin till are as great as any dif- within the gorge led to an exposure adequate ferences between Sherwin deposits and the till to establish that the matrix is water-sorted sand in Big Pumice Cut. A contact between two and fine gravel, not the tight, silty matrix of tills could be defined by intervening deposits or by deep weathering of the underlying unit. Both were looked for but not found. To date, Bishop tuff (150ft.) fragments of pumice or Bishop tuff have never (0.7 m.y.) been found as a constituent of Sherwin till nor Sherwin outwash (100 ft.) has Sherwin till ever been shown to rest upon Bishop tuff. Tertiary basalt (75ft.) (3.2 m.y.) Pumice-Sherwin Till Cretaceous quartz Relationships along U. S. 395 monzonite Attention is focused on a strip along U. S. (300-350 ft.) 395 and Rock Creek gorge extending through sections 1, 2, and 34 (Fig. 2). At the northwest end of this strip is Big Pumice Cut exposing till beneath pumice. At the southeast end is a Figure 4. Field sketch of Sherwin outwash ex- large ridge, summit elevation 7246, composed posed in walls of Owens River gorge. of unquestioned Sherwin till. Pumice-till rela- 356 R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA tionships will be traced southeastward down tion is preferred because the band of till can be this strip. traced around the west flank of the hill in the Just northeast of U. S. 395, about 0.5 mile shape of a narrow finger that expands northeast of Big Pumice Cut, is an isolated flat- ward into a bulb which extends to within 100 topped hill and 0.25 mile southeast is a smaller feet of the Bishop tuff cliff (Fig. 2). It seems hill rising to the same level (Figs. 1,2). Putnam unlikely that a glacier could extend halfway (1960, p. 240, map 1) mapped these hills as around this pumice hill, depositing till in this Sherwin till capped with fluvial gravel. They configuration on the pumice, without spread- are gravel-capped, but they are not predom- ing debris all over the nearby landscape. The inantly till. The smaller hill is wholly pumice planimetric and cross-section relationships are as shown by the debris on its slopes, by pole satisfactorily accounted for if pumice overlies holes dug in 1964 for a powerline crossing its the till. flank, and by a cut on U. S. 395. The larger Southwest of these pumice hills in the south- hill does have a band of till, with 20-foot ern part of Sec. 34 and the northern part of boulders, on its southern and western flanks. Sec. 2 (Fig. 2) is a broad, west trending, The till forms a topographic bench on the hill- pumice-filled swale which is transected by the side and is exposed in a long cut on U. S. 395. east wall of Rock Creek gorge. The pumice However, the hill slope above and below the attains a maximum thickness of 50 feet and till bench is covered with pumice. rests upon coarse bouldery till. However, is it Two interpretations of these relationships are primary (in situ) or reworked from the pumice considered. Either a mass of post-pumice till hills one-half mile to the north? Interpretation has been plastered against the flank of this hill of the relationships is further complicated by (Fig. 5B), or the hill consists of pumice that patches of coarse fluvial gravel and scattered has buried an irregular mound of till with sub- large crystalline boulders, both resting on the sequent dissection partially exposing the under- pumice, and by one small island-like till area lying material (Fig. 5A). This last interpreta- (Fig. 2, SE1/4 sec. 34).

A Bishop Tuff Flattopped Hill Cliff

in en Fluvial gravel; ro

Pumice rtZZ^&^'Q o'SJfo ,^> ENE WSW

Approximate Scale Figure 5. Interpretive cross sections of flattopped pumice hills in sec. 34. A, pumice burying till; B, till plastered on pumice. PUMICE ON SHERWIN TILL 357

That the pumice is primary is suggested by fully inspected. The relationships then proved both the present surface slope which is, to some to be as sketched in Figure 6. The top layer is extent, in the wrong direction for secondary grus derived by slopewash and creep from the derivation from the north, and by a small knob extensive till slope above. Extensively weath- in the northernmost part of sec. 2 that appears ered pumice is beneath, and partially mixed

South North

Weathered pumice in situ Feet Approximate Scale Figure 6. Field sketch of pumice-Sherwin till relationships in Little Pumice Cut, U. S. 395, sec. 1.

to represent a disintegrated outcrop of co- with, the grus. Despite weathering, disturb- herent pumice-tuff. This swale is thought to be ance, and mixing by surface wash, creep, and an original topographic feature of the till sur- burrowing animals, enough of the integrity and face that was buried by the erupted pumice. character of the pumice is preserved to show The pumice filling has been preserved by its that it consists of large blocks of bedded ma- lower, protected position and by a partial cap- terial corresponding to units identifiable in the ping of fluvial gravels. basal pumice-ash sequence of Big Pumice Cut. A similar swale is followed in a southerly di- This has been established by repeated back- rection by U. S. 395 along the west side of sec. and-forth comparisons of the field exposures 1. Pumice fragments lie on the swale's floor and is confirmed by a similarity of included and well up on both slopes as shown in Figure foreign rock fragments. This deeply weathered 2. Pumice fragments as much as 5 inches in pumice and ash is essentially in place, and it diameter were found at the south end of the rests upon several feet of brownish-gray grus swale where it is truncated by Rock Creek that cap the Sherwin till. The stratigraphic re- gorge. Secondary deposition of the pumice in lationships duplicate those of Big Pumice Cut. this swale is not impossible, but the develop- Thus, the pumice-on-till relationship can be ment of a pumice filling at least 30 to 40 feet carried 1.5 miles southeast from Big Pumice deep, which was then largely removed for no Cut to Ridge 7246 which is uncontestably obvious reason, would be required. More Sherwin till. The Sherwin till is stratigraph- likely this feature, too, represents a swale on ically below the basal pumice-ash beds of the the original till surface that was buried by the Bishop tuff volcanic episode. initial eruption of pumice. This possibility is strongly supported by the evidence of Little Other Locations Pumice Cut, which is described next. Small fragments of pumice have been found One of the best and most representative ex- on Sherwin till in other places as follows. The posures of Sherwin till in the map area is in the Surge Tank road runs eastward from U. S. 395 long road cut on the east side of U. S. 395 essentially along the Bishop tuff-Sherwin till where it curves around the end of Ridge 7246 contact (Figs. 1,2). The tuff makes a cliff 50 in the SW1/4 of sec. 1. At the north end of to 100 feet high facing outward toward the till this cut is a mass of brownish material initially which composes the slopes rising away from the dismissed as weathered slopewash until care- cliff. In many places along this contact, pumice 358 R. P. SHARP-SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA fragments, mostly 1 to 2 inches in diameter but on Sherwin till demonstrate less than 100 yards occasionally attaining 5 inches, can be found of stripping in most places along the margin of on the till 100 to 200 feet out from the cliff the presently exposed till mass. Almost cer- and 30 to 50 feet above (upslope from) its base. tainly, the cover of Bishop tuff on Sherwin till Most significant is an acre patch of pumice was greater, an inference supported by the fragments at elevation 7180 feet on the crest of small patch of pumice fragments at elevation Ridge 7246 about 0.25 mile northeast of its 7180 feet on the crest of ridge 7246 (Fig. 2). summit (NW1/4, sec. 1, Fig. 2). At the east This pumice is of the type found more abun- end of this ridge, where it abuts against a low dantly in the cliff-making Bishop tuff than in tuff cliff, fragments of coherent tuff, up to 18 the basal airborne layer. Since the Bishop tuff inches in diameter, lie 150 feet out from the represents a series of ash and pumice flows cliff and about 5 feet higher than its base. (Gilbert, 1938, p. 1851-1852), it presumably Pumice fragments up to 5 inches in diameter covered the terrain up to a reasonably ac- lie on till at the east end of a southern spur of cordant height. The pumice patch on ridge Ridge 7246 in the NW1/4 of sec. 6, 100 yards 7246, with allowances for post-tuff warping and from and 60 feet above the nearest Bishop tuff faulting, suggests that most of the exposed cliff. Sherwin till area east of Rock Creek and per- At the southern edge of the till mass, in the haps one third of the area west of that stream center of sec. 12 just west of U. S. 395, abun- were formerly mantled by tuff. This is some- dant fragments of pumice, some up to 6 inches what less than the coverage inferred by Put- in diameter, were found on the till up to 100 nam (1960, p. 234) but possibly greater than yards from and 75 feet above the base of a low anything envisioned by Gilbert (1938, p. 1837- tuff cliff to the south. 1838). In some situations one could perhaps argue that the pumice fragments are small enough to GEOMETRICAL RELATIONSHIPS have been carried onto the till by wind. How- Although the map (Fig. 2) illustrates how ever, the pumice fragments are usually accom- lobes of Sherwin till penetrate re-entrants in panied by pieces of black hornfels and basalt, the edge of the Bishop tuff, it does not show common inclusions in the tuff, whose transport that the tuff cliff rises abruptly above the till by wind is unlikely. All these pumice frag- at these places. Such relationships are best seen ments are considered to be residual from a along the Surge Tank road. Easily visible south former covering of Bishop tuff on those parts of the road at the northwest corner of sec. 6 is of the Sherwin till where they are found. a lobe of bouldery till extending north between low cliffs of Bishop tuff. It would have STRIPPING OF BISHOP TUFF been most difficult for a glacier younger than Gilbert (1938, p. 1833, 1837-1838) regards the tuff to have deposited the till in this rela- the present margin of the Bishop tuff as essen- tionship, although a debris flow from the ice or tially its original edge, feeling that little cliff from marginal glacial deposits might have done recession and stripping have occurred. Putnam the job. Fragments of coherent tuff, up to 18 (1960, p. 234), however, favors considerable inches across, resting on the till at the head of stripping in the Sherwin Grade-Rock Creek this lobe support the inference, drawn from area, inferring that most of the Sherwin till geometrical relationships, that the tuff is exposed there was once buried beneath tuff. younger. Neither author supports his position with con- Farther east, in the north-central part of sec. crete facts, and compelling evidence of strip- 6, the Surge Tank road crosses a low mass of ping is hard to find. Areas of granitic rock, for bouldery till extending northeast to the foot of example that in the SW1/4 of sec. 36 (Fig. 2), the tuff cliff. This till patch is nearly enclosed almost certainly once buried by Bishop tuff, no by the tuff cliff with only a limited opening to longer retain even the smallest remnant or frag- the southwest. It would have been extremely ment of tuff. Evidence of burial should be even difficult for a glacier younger than the tuff to more easily removed from smooth slopes of un- have laid down this till without scattering gla- consolidated till than from craggy exposures of cial debris over the nearby terrain where it is granitic rock. The layer of unconsolidated lacking. The inference of a pre-tuff age for this pumice and ash at the base of the tuff facilitates till is confirmed in Los Angeles Bureau of Water stripping, and the uneven distribution of this and Power drill hole G-1, a scant 100 feet north layer promotes different degrees of stripping. of the till-tuff contact (Fig. 2), that shows The previously described pumice fragments glacial drift beneath Bishop tuff. SHERWIN OUTWASH AND OTHER GRAVELS 359

NO SHERWIN TILL ON BISHOP TUFF the eastern and southern margins of the till body, but no Sherwin outwash is found there An extended search for till or glacial erratics or elsewhere on the tuff. The only Sherwin out- resting on Bishop tuff turned up very little of wash recognized is that exposed in the walls of interest. In the south central part of sec. 6, four Owens River gorge beneath the Bishop tuff scattered granitic boulders, respectively 18, 16, (Fig. 4). Similar gravels are not found beneath 12, and 9 inches in diameter, were found on the the Bishop tuff in Rock Creek gorge, suggest- tuff. All were of rock types that could have ing that the direction of drainage in Sherwin been supplied from high-standing exposures of time was eastward from the mountains, rather Wheeler Crest quartz monzonite just three- than southeasterly, as at present. The present eighths of a mile west. An additional three southeasterly slope of the land is attributed to granitic boulders up to one foot in diameter warping after eruption of the Bishop tuff were found in an Indian stone circle, of which (Rinehart and Ross, 1957; Putnam, 1960, p. there is an abundance in this area. These rela- 244-245). tions, plus the fact that the tuff itself occasion- A considerable area of gravel resting on ally includes granitic boulders, makes glacial Bishop tuff has been mapped, mostly east of emplacement of this material unlikely, espe- lower Rock Creek gorge, beginning 2.5 miles cially in the absence of other debris typical of southeast of the Sherwin till area (Fig. 7). the till. These gravels rest upon the tuff and incor- In the south-central part of sec. 6 is an iso- porate fragments of it. Small exposures can be lated patch of boulder gravel (Fig. 2) covering seen in road cuts along U. S. 395 (sees. 4, 33, about half an acre and lying only 150 yards east Fig. 7), but the best exposed section is at the of exposures of Sherwin till. The deposit con- first curve on the old Sherwin Grade road just tains boulders up to 4 feet in diameter, many west of Rock Creek at Paradise Camp (sec. 29, smaller, worn, and rounded stones, 1 to 4 inches T. 5 S., R. 31 E., Mt. Tom Quadrangle). Here across, and finer gravel and grus. The lithologic are some 40 feet of beds, principally fluvial types present are those of the Sherwin till. Al- gravel with smoothly worn, rounded stones set though the gravel patch lies downslope from in a sandy matrix. Near the top are two lenses, exposures of Sherwin till, it is completely sur- 7 to 8 feet thick, containing larger boulders to rounded by Bishop tuff and there is no con- 6 feet in diameter set in a dense silty matrix. necting trail of debris to the till. These lenses look like debris-flow deposits. Sim- Several interpretations are possible: (1) The ilar material is exposed in gullies along the debris is an outlier of Sherwin till resting on wooden-pole powerline road west of U. S. 395. top of Bishop tuff. (2) It is Sherwin debris, Stones in these deposits, mostly 6 to 18 inches secondarily reworked and transported onto the but occasionally up to 8 feet in diameter, are tuff. This would be possible under existing of lithologies found in the Sherwin till and topographic relations. (3) It is a window of represented in the bedrock of upper Rock Sherwin drift exposed by removal of the over- Creek drainage. lying Bishop tufl. These deposits have considerable antiquity, The fact that the tuff is probably, at most, for granitic boulders are completely disinte- 10 feet thick here; that the gravel is completely grated 15 feet beneath the ground surface. The surrounded by tuff which in places rises a few matrix is also weathered a rich yellow-brown feet above it; and, that any connection on the (10YR5/4) to this depth, and the upper parts tuff surface to the Sherwin till 150 yards west of stripped, debris-flow layers between Rock is lacking, suggests this is probably an unroofed Creek and U. S. 395 are heavily calichefied. exposure of Sherwin drift. However, anyone The gravels are also buried by a blocky creep wishing to argue that the Sherwin overlies the mantle of Bishop tuff fragments derived from Bishop tuff will be interested in visiting this nearby higher outcrops. As much as 300 feet of location. downcutting in Rock Creek gorge has occurred since the initial phase of gravel deposition. SHERWIN OUTWASH Farther up the old Sherwin Grade road west AND OTHER GRAVELS of Rock Creek gorge are large areas of boulder Sherwin till-Bishop tuff geometrical relation- gravel consisting wholly of Wheeler Crest ships are such that, if the till were younger, quartz monzonite and associated dike rocks Sherwin outwash should lie on the tuff or within (Fig. 7). This material has come from the high gullies draining across the tuff. The topographic abrupt face of Wheeler Crest to the west and setting is particularly favorable for this along northwest and need have no genetic relation to 360 R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA

Gravel from Wheeler Crest

Gravel from Q upper Rock Creek 0 -f Scale in Miles Figure 7. Map of gravels on Bishop tuff along lower Rock Creek. the first described gravels. It will not be con- Creek. The lower part of the gorge was not as sidered further. deeply cut then as now, for the gravels spread The distribution (Fig. 7) and constitution of eastward in shallow gullies over the Bishop tuff the first described gravels indicate that they surface from a point about 1.5 miles above the have been transported down the course of Rock present gorge mouth and at a level 300 feet SPECULATION ON CORRELATION 361 higher than its present floor. However, gravels in this region. It is judged, therefore, to have of similar constitution are also found within experienced weathering for a few tens of thou- the gorge to within 80 feet of its floor, suggest- sands of years but not as much as 100,000 years ing that their transport extended over a con- prior to eruption of the Bishop tuff. An esti- siderable part of the gorge-cutting interval. mated age in the neighborhood of 750,000 These gravels are related to the diversion of years for the Sherwin thus seems reasonable. Rock Creek from its former northeasterly path Large areas of Sherwin till previously have into the present southeasterly course, as de- been recognized along the eastern Sierra Ne- scribed by Putnam (1960, p. 249). At that time vada front in the Mono and Bridgeport basins deep dissection of the Sherwin till occurred, and on West Walker River (Blackwelder, 1931, and a large volume of Sherwin debris must have p. 895-900; Putnam, 1949, p. 1290). Although been flushed down the new Rock Creek chan- firm correlation between these occurrences and nel. These gravels are regarded as remnants of the type locality has not been established, the that debris. Putnam (1960 p. 251) dates the topographic setting, the succession of glacia- diversion as Tahoe in age, but the gravels ap- tions, and semi-quantitative data reflecting age, pear much older. Since the Tahoe dating is not support such a correlation. There is scant reason supported by any direct evidence, and since the to adopt the view of Evernden and Curtis Rock Creek gorge is cut 500 to 600 feet into (1965, p. 356) that the Sherwin of the type lo- Bishop tuff and resistant crystalline rocks, it is cality has no established relation to other suggested that the Rock Creek diversion oc- Sierra glacial deposits. Certainly, till beneath curred well before Tahoe time. the Bishop tuff in the Mono Basin has a corre- These gravels are clearly younger than the sponding stratigraphic position (Putnam, 1949, Bishop tuff and probably consist of debris re- p. 1289). worked from the Sherwin till. The possibility The considerable age of the Sherwin makes that they represent Sherwin outwash and that one wonder about the McGee till, a presumably the Sherwin glaciation occurred after eruption still older episode of Sierra glaciation (Black- of the Bishop tuff is highly unlikely. welder, 1931, p. 902-906; Putnam, 1962, p. 192-195). The McGee has a topographic setting SPECULATION ON CORRELATION which is unusual, even for Sherwin, and on that The 710,000-year potassium-argon date for basis alone it could well be older, especially in the basal pumice-ash unit of the Bishop tuft view of the 2.7-3.0 X 106-year age of a nearby volcanic episode (Dalrymple and others, 1965, deposit stated to be of glacial origin (Curry, p. 670) makes the Sherwin till surprisingly old. 1966). The following indications of erosion and weath- In the bottom of Rock Creek gorge along the ering prior to extrusion of the Bishop tuff sug- old Sherwin grade road at the south edge of the gest that the Sherwin till is somewhat older Sherwin till area (sec. 12), a road cut in till still. exposes a prominent reddish zone at its core Granitic knobs capped by Bishop tuff with- (Fig. 8). This looks like two tills separated by out intervening glacial deposits, as in sec. 36 (Fig. 2), where the Sherwin drift beneath the tuff is both extensive and thick (Tunnel 1 section, Fig. 1 in Putnam, 1960) suggest considerable pre-tuff erosion of the Sherwin deposits. In Big Pumice Cut a layer of soily brown grus, much like that on presently exposed slopes of Sherwin till, underlies the basal pumice and in- dicates considerable disintegration, slopewash, and creep on the Sherwin slopes before burial. FigureS. Field sketch of old red (pre-Sherwin?) The till itself is only modestly oxidized to a till on old Sherwin Grade road, lower Rock Creek, depth of a few feet, but disintegration of gra- sec. 12. nitic boulders to a depth of 25 feet below the till-pumice contact is extensive. The buried Sherwin till is more deeply a deeply weathered zone. Gray material above weathered than most Tioga moraines and ap- the reddish zone has all the aspects of a first- proaches, but does not generally attain, the class till and as such it is surely Sherwin. At- weathering displayed by many Tahoe deposits tempts to explain the reddish zone as a ground- 362 R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA water phenomenon fail because some of the urged by Putnam (1962, p. 205). Radiometric reddish debris is reworked into the overlying dating is currently so severely challenging deposit. Deposition of the overlying material temporal correlations with continental glacial by surface creep is not supported by its dense, episodes that it seems pointless at this time to tight, till-like matrix or the inclusion of an un- urge any specific correlation of the Sherwin deformed lens of water-laid debris. A pH pro- with the classical midwestern glacial sequence. file across the contact is of little help as it yields As Damon (1965) notes, Pleistocene chro- values of 8 to 8.5 all the way. Little veinlets of nologists disagree widely about the duration of calcium carbonate extend from the overlying the glacial Pleistocene (Hopkins, in Evernden debris into the reddish zone which has thus had and Curtis, 1965, p. 372). Some are advocating its CaCOs restored by percolation from above. a duration in excess of 1 million (Evernden and The contact is displaced by small faults in sev- Curtis, 1965, p. 343) to 1.5 million years (Eric- eral places (Fig. 8), further evidence of some son and others, 1964, p. 731). A Sherwin age degree of antiquity. of 750,000 years is not inconsistent with a gla- These relations remain something of an cial Pleistocene of this duration, but it is far enigma but for the present are interpreted as out of line with the shorter glacial epochs ad- the contact between two tills. This is favored vocated by Emiliani (1958, p. 271; 1966, p. by the location, at the bottom of a 500-foot 855-856; Emiliani and others, 1961, p. 687) gorge near the edge of the Sherwin till area. among others. The task of bringing interpreta- Just possibly, one sees here the contact between tions of marine and terrestrial Pleistocene Sherwin and McGee tills. records into harmony remains interesting and The Sherwin till was tentatively regarded as challenging, especially in view of Curry's (1966) Kansan by Blackwelder (1931, p. 918). In view recent report of a possible Sierra Nevada glaci- of its probable age of about 750,000 years, this ation radiometrically dated at 2.7-3.0 X 106 seems more reasonable than the Illinoian age years.

REFERENCES CITED Blackwelder, Eliot, 1931, Pleistocene glaciation in the Sierra Nevada and Basin Ranges: Geol. Soc. America Bull, v. 42, p. 865-922. Curry, R. R., 1966, Glaciation about 3,000,000 years ago in the Sierra Nevada: Science, v. 154, p. 770- 771. Dalrymple, G. B., Cox, Allen, and Doell, R. R., 1965, Potassium-argon age and paleomagnetism of the Bishop Tuff, California: Geol. Soc. America Bull., v. 76, p. 665-674. Damon, P. E., 1965, Pleistocene time scales: Science, v. 148, p. 1037-1038. Emiliani, Cesare, 1958, Paleotemperature analysis of core 280 and Pleistocene correlations: Jour. Geology, v. 66, p. 264-275. 1966, Isotopic paleotemperatures: Science, v. 154, p. 851-857. Emiliani, Cesare, Mayeda, T., and Selli, R., 1961, Paleotemperature analysis of the Plio-Pleistocene section at Le Castella, Calabria, southern Italy: Geol. Soc. America Bull., v. 72, p. 679-688. Ericson, D. B., Ewing, Maurice, and Wollin, Goesta, 1964, The Pleistocene Epoch in deep-sea sedi- ments: Science, v. 146, p. 723-732. Evernden, J. F., Curtis, G. H., and Kistler, R., 1957, Potassium-argon dating of Pleistocene volcanics: Quaternaria, IV, p. 13-17. Evernden, J. F., Savage, D. E., Curtis, G. H., and James, G. T., 1964, Potassium argon dates and the Cenozoic mammalian chronology of North America: Am. Jour. Sci., v. 262, p. 145-198. Evernden, J. F., and Curtis, G. H., 1965, The potassium-argon dating of late Cenozoic rocks in East Africa and Italy: Current Anthropology, v. 6, p. 343-364 (with comments, p. 364-385). Gilbert, C. M., 1938, Welded tuff in eastern California: Geol. Soc. America Bull., v. 49, p. 1829-1862. Putnam, W. C., 1949, Quaternary geology of the June Lake district, California: Geol. Soc. America Bull., v. 60, p. 1281-1302. 1960, Origin of Rock Creek and Owens River gorges, Mono County, California: Univ. Calif. Pub. in Geol. Sci., v. 34, p. 221-280. 1962, Late Cenozoic geology of McGee Mountain, Mono County, California: Univ. Calif. Pub. in Geol. Sci., v. 40, p. 181-218. REFERENCES CITED 363

Rinehart, C. D., and Ross, D. C., 1957, Geology of the Casa Diablo Mountain quadrangle, California: U. S. Geol. Survey Geol. Quad. Map GQ-99, scale 1:62,500. 1964, Geology and mineral deposits of the Mount Morrison quadrangle, Sierra Nevada, California: U. S. Geol., Survey Prof. Paper 385, 106 p. Wahrhaftig, Clyde, and Birman, J. H., 1965, The Quaternary of the Pacific mountain system in Cali- fornia: p. 299-340, in Wright, H. E., and Frey, D. G., Editors, The Quaternary of the United States: Princeton, Princeton Univ. Press.

MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 27, 1966 REVISED MANUSCRIPT RECEIVED FEBRUARY 21, 1967 CALIFORNIA INSTITUTE OF TECHNOLOGY DIVISION OF GEOLOGICAL SCIENCES PUB. No. 1433