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.
Physical setting
352 2. Geological map of Sherwin Grade-Rock Creek
Acknowledgments
352
area
Glacial drift beneath Bishop tuff
352 3. Field sketch of relationships in Big Pumice Cut
Pumice on Sherwin till
355 4. Field sketch of Sherwin outwash exposed in
Introduction
355
walls of Owens River gorge
Pumice-Sherwin till relationships along U. S. 395 355 5. Interpretive cross sections of flattopped pumice
Other locations
357
hills
Stripping of Bishop tuff
358 6. Field sketch of pumice-Sherwin till relationship
Geometrical relationships
358
in Little Pumice Cut
No Sherwin till on Bishop tuff
359 7. Map of gravels on Bishop tuff along lower Rock
Sherwin outwash and other gravels
359
Creek
Speculation on correlation
361 8. Field sketch of old red till on old Sherwin
Grade road
References cited
362
INTRODUCTION
General Statement
As the type locality of the Sherwin till
(Blackwelder, 1931, p. 895-900), the Sherwin
Grade-Rock Creek area has long been of local
interest. It is now attracting international attention from Pleistocene chronologists because of
potassium-argon dating of the associated
Bishop tuff. Blackwelder (1931, p. 918, 899)
regarded the Sherwin till as possibly Kansan
and as lying above the Bishop tuff, but Gilbert
(1938, p. 1860), Rinehart and Ross (1957),
Putnam (1960, p. 233), and Wahrhaftig and
Birman (1965, p. 310) all place the till beneath
the tuff.
The initially determined K40/A40 age of the
Bishop tuff was 870,000 years (Evernden and
others, 1957, p. 14). This was subsequently
revised to 980,000 years (Evernden and others,
353
354
355
355
356
357
360
361
1964, p. 175; Evernden and Curtis, 1965, p.
355). Since he regarded the Sherwin till as
Illinoian, but older than the tuff, Putnam
(1962, p. 205) flatly rejected this age as too
great. Inclusions of older rock fragments in the
tuff (Gilbert, 193 8, p. 1834-1835; Rinehart and
Ross, 1957; Putnam, 1960, p. 236) permit an
inference of contamination, as shown by
Dalrymple and others (1965, p. 670-671).
However, dating of sanidine crystals taken
from pumice fragments within the tuff, thus
presumably uncontaminated, gives the still
impressively great age of 710,000 years for the
basal pumice of the Bishop tuff volcanic
episode (Dalrymple and others, 1965, p. 670).
In view of the use being made of these dates
(Ericson and others, 1964, p. 731), it is clearly
desirable to establish as firmly as possible the
geological relationship between the Bishop tuff
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
age of 710,000 years holds up, this becomes a
most important datum in the Pleistocene
geological record (Rinehart and Ross, 1964, p.
79
)'
The geological problem, simply stated, is as
follows. Evidence is compelling that a till
underlies the Bishop tuff. Is it the equivalent of
Blackwelder's-type Sherwin? Or are there two
tills here, one older and one (the type Sherwin)
younger than the Bishop tuff (Rinehart and
Ross, 1964, p. 74)? Reference is to pre-Tahoe
tills as the Tahoe and subsequent glaciations are
clearly younger than the tuff (Putnam, 1949,
p. 1291). Informed readers will recall that eastside Sierra Nevada glaciations currently
recognized are from youngest to oldest; Tioga,
Tenaya, Tahoe, Mono Basin, Sherwin, McGee,
and Deadman Pass, with the first three usuallyregarded as Wisconsin and the remainder as
Pre-Wisconsin.
The tuff-till relationship has been investigated in the field, at intervals, during the summers of 1964, 1965, and 1966 through "hands
and knees" tracing of contacts, study of exposures, and repeated visitations to critical
locations. The conclusion reached is the opposite of a view, initially held, that favored the
two-till concept. The assembled evidence shows
that Blackwelder's-type Sherwin and the till
beneath the Bishop tuff are one and the same.
The Sherwin till is older than the Bishop tuff
and thus more than 710,000 years of age, if the
potassium-argon dates are valid. This has farreaching significance with respect to Pleistocene
chronologies (see discussions in Evernden and
Curtis, 1965, especially by Hopkins, Howell,
and Wright).
Physical Setting
The setting is a 7000-foot tableland lying
northeast of the Sierra Nevada front in eastcentral California (118° 38' W., 37° 33' N.).
Here Rock Creek emerges northeasterly from
its mountain canyon and turns abruptly southeasterly through a narrow gorge cut into the
southwestern flank of the tableland (Fig. 1).
Exposed Sherwin till covers a bulbous area of
7 square miles bisected by this gorge. The region is shown on the U. S. Geological Survey's
Casa Diablo Mountain, 15-minute quadrangle,
1953.
The location, distribution, and lithologic
constitution of Sherwin drift show clearly that
it was derived from the Rock Creek drainage
with the glacier following a route directly east-
ward which departs from the present Rock
Creek course near the head of Whiskey Canyon
(Fig. 1). Differences in thickness of the Sherwin till are considerable owing to its irregular
topography and the uneven underlying bedrock surface. The greatest exposed thickness,
500 feet, is in Rock Creek gorge.
ACKNOWLEDGMENTS
Warm hospitality and assistance have been
extended by the staff of the Sierra Nevada
Aquatic Research Laboratory on Convict
Creek. Discussions in the field with Clyde
Wahrhaftig and his associates and my Caltech
colleagues have been most helpful. The manuscript has been improved by critical comments
from Brent Dalrymple, Charles Gilbert, and
Maxwell Gage.
GLACIAL DRIFT BENEATH
BISHOP TUFF
Glacial deposits beneath the Bishop tuff are
known at three sites within the map area (Fig.
2). The first, most obvious, and most accessible
site is a deep cut on U. S. Highway 395 just
east of the crossing with Rock Creek, hereafter
termed Big Pumice Cut. Exposed here from
the top down are 10 to 12 feet of fluvial gravel
with smooth, rounded stones; 75 feet of loose
white pumice and ash; and 45 feet of bouldery
till (Fig. 3). Enclosing stones from the fluvial
gravel, clastic dikes cut the pumice and extend into the till, transecting boulders. The
pumice consists of two units. One is a lower
well-layered, somewhat brownish sequence of
ash and pumice, 15 feet thick, with bedding
conformable to the gently inclined (10°) till
surface. The other is a coarser, looser, poorlybedded white pumice in horizontal attitude.
The lower unit looks like an eolian deposit laid
down as a relatively uniform blanket over an
uneven terrain. The upper, horizontally-bedded
unit was perhaps emplaced as a series of pumice
flows. The capping of fluvial gravels is much
younger. Beneath the pumice is an excellent
till, unbedded, poorly sorted, with a tight, fine,
silty matrix, and a variety of lithologies represented in the boulders. Granodiorite and quartz
monzonite are the predominating lithologies.
The Bishop tuff cliff lies 1000 feet northeast
of Big Pumice Cut, but there can be little
doubt that the pumice and ash in the cut represent the initial phase of the Bishop tuff volcanic episode (Putnam, 1960, p. 235-236). Similar material is seen below the Bishop tuff in
other places, and radiometric dating by Dal-
o
o
Surge Tank
- = =:== Power
House
No.1
3
CD
z,
w
ffi
S
O
Figure 1. Place map of Sherwin Grade-Rock Creek area.
PUMICE ON SHERWIN TILL
355
F l u v i a l gravel
*',
'V^0
-100
200
300
1 reeT
|~_ n i
Approximate Scale
Figure 3. Field sketch of relationships in Big Pumice Cut, U. S. 395 east of Rock Creek.
0
50
rymple and others (1965, p. 670) shows the
pumice and cliff-making tuff to be of one age.
Beneath the Bishop tuff, glacial deposits
were penetrated for some 1100 lineal feet by a
tunnel of the Los Angeles Department of Water
and Power on a line passing 0.75 mile northeast of Big Pumice Cut (Fig. 2). Exploratory
holes drilled for this project penetrated glacial
drift up to 400 feet thick beneath the Bishop
tuff (Putnam, 1960, p. 233-234).
Nearly 4 miles east of Big Pumice Cut in the
walls of Owens River gorge upstream from
powerhouse No. 1 (Fig. 1), a lenticular body
of glacial drift, up to 200 feet thick and 1.5
miles across, is exposed (Fig. 4) beneath the
Bishop tuff (Gilbert, 1938, p. 1860; Ross and
Rinehart, 1957; Putnam, 1960, p. 234). The
smooth surfaces and rounded shapes of stones
in this material suggested outwash. This suggestion was confirmed after an extended search
within the gorge led to an exposure adequate
to establish that the matrix is water-sorted sand
and fine gravel, not the tight, silty matrix of
Bishop t u f f (150ft.)
(0.7 m.y.)
Sherwin outwash (100 ft.)
Tertiary basalt ( 7 5 f t . )
(3.2 m.y.)
Cretaceous quartz
monzonite
(300-350 ft.)
Figure 4. Field sketch of Sherwin outwash exposed in walls of Owens River gorge.
the till exposures on U. S. 395. Unfortunately,
Putnam's descriptions (1960, p. 233-234) of
glacial drift in the Bureau of Water and Power
tunnel do not distinguish between till and outwash; nonetheless, despite his contrary statement (1960, p. 250), Sherwin ice need not have
extended as far east as Owens River gorge.
PUMICE ON SHERWIN TILL
Introduction
Glacial drift clearly underlies the Bishop
tuff, but what is the relationship of this drift
to the Sherwin till? Exposures of the till are
nearby but not obviously stratigraphically beneath the Bishop tuff. If two tills are present,
they cannot be differentiated lithologically as
both consist of debris derived from the Rock
Creek drainage. Lithologic variations in a single
exposure of Sherwin till are as great as any differences between Sherwin deposits and the till
in Big Pumice Cut. A contact between two
tills could be defined by intervening deposits
or by deep weathering of the underlying unit.
Both were looked for but not found. To date,
fragments of pumice or Bishop tuff have never
been found as a constituent of Sherwin till nor
has Sherwin till ever been shown to rest upon
Bishop tuff.
Pumice-Sherwin Till
Relationships along U. S. 395
Attention is focused on a strip along U. S.
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
large ridge, summit elevation 7246, composed
of unquestioned Sherwin till. Pumice-till rela-
R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA
356
tion is preferred because the band of till can be
traced around the west flank of the hill in the
shape of a narrow finger that expands northward into a bulb which extends to within 100
feet of the Bishop tuff cliff (Fig. 2). It seems
unlikely that a glacier could extend halfway
around this pumice hill, depositing till in this
configuration on the pumice, without spreading debris all over the nearby landscape. The
planimetric and cross-section relationships are
satisfactorily accounted for if pumice overlies
the till.
Southwest of these pumice hills in the southern part of Sec. 34 and the northern part of
Sec. 2 (Fig. 2) is a broad, west trending,
pumice-filled swale which is transected by the
east wall of Rock Creek gorge. The pumice
attains a maximum thickness of 50 feet and
rests upon coarse bouldery till. However, is it
primary (in situ) or reworked from the pumice
hills one-half mile to the north? Interpretation
of the relationships is further complicated by
patches of coarse fluvial gravel and scattered
large crystalline boulders, both resting on the
pumice, and by one small island-like till area
(Fig. 2, SE1/4 sec. 34).
tionships will be traced southeastward down
this strip.
Just northeast of U. S. 395, about 0.5 mile
east of Big Pumice Cut, is an isolated flattopped hill and 0.25 mile southeast is a smaller
hill rising to the same level (Figs. 1,2). Putnam
(1960, p. 240, map 1) mapped these hills as
Sherwin till capped with fluvial gravel. They
are gravel-capped, but they are not predominantly till. The smaller hill is wholly pumice
as shown by the debris on its slopes, by pole
holes dug in 1964 for a powerline crossing its
flank, and by a cut on U. S. 395. The larger
hill does have a band of till, with 20-foot
boulders, on its southern and western flanks.
The till forms a topographic bench on the hillside and is exposed in a long cut on U. S. 395.
However, the hill slope above and below the
till bench is covered with pumice.
Two interpretations of these relationships are
considered. Either a mass of post-pumice till
has been plastered against the flank of this hill
(Fig. 5B), or the hill consists of pumice that
has buried an irregular mound of till with subsequent dissection partially exposing the underlying material (Fig. 5A). This last interpretaA
F l a t t o p p e d Hill
in
en
ro
Bishop Tuff
Cliff
Fluvial g r a v e l ;
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
That the pumice is primary is suggested by
both the present surface slope which is, to some
extent, in the wrong direction for secondary
derivation from the north, and by a small knob
in the northernmost part of sec. 2 that appears
357
fully inspected. The relationships then proved
to be as sketched in Figure 6. The top layer is
grus derived by slopewash and creep from the
extensive till slope above. Extensively weathered pumice is beneath, and partially mixed
South
North
Weathered pumice
in situ
Feet
A p p r o x i m a t e 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 coherent pumice-tuff. This swale is thought to be
an original topographic feature of the till surface that was buried by the erupted pumice.
The pumice filling has been preserved by its
lower, protected position and by a partial capping of fluvial gravels.
A similar swale is followed in a southerly direction by U. S. 395 along the west side of sec.
1. Pumice fragments lie on the swale's floor
and well up on both slopes as shown in Figure
2. Pumice fragments as much as 5 inches in
diameter were found at the south end of the
swale where it is truncated by Rock Creek
gorge. Secondary deposition of the pumice in
this swale is not impossible, but the development of a pumice filling at least 30 to 40 feet
deep, which was then largely removed for no
obvious reason, would be required. More
likely this feature, too, represents a swale on
the original till surface that was buried by the
initial eruption of pumice. This possibility is
strongly supported by the evidence of Little
Pumice Cut, which is described next.
One of the best and most representative exposures of Sherwin till in the map area is in the
long road cut on the east side of U. S. 395
where it curves around the end of Ridge 7246
in the SW1/4 of sec. 1. At the north end of
this cut is a mass of brownish material initially
dismissed as weathered slopewash until care-
with, the grus. Despite weathering, disturbance, and mixing by surface wash, creep, and
burrowing animals, enough of the integrity and
character of the pumice is preserved to show
that it consists of large blocks of bedded material corresponding to units identifiable in the
basal pumice-ash sequence of Big Pumice Cut.
This has been established by repeated backand-forth comparisons of the field exposures
and is confirmed by a similarity of included
foreign rock fragments. This deeply weathered
pumice and ash is essentially in place, and it
rests upon several feet of brownish-gray grus
that cap the Sherwin till. The stratigraphic relationships duplicate those of Big Pumice Cut.
Thus, the pumice-on-till relationship can be
carried 1.5 miles southeast from Big Pumice
Cut to Ridge 7246 which is uncontestably
Sherwin till. The Sherwin till is stratigraphically below the basal pumice-ash beds of the
Bishop tuff volcanic episode.
Other Locations
Small fragments of pumice have been found
on Sherwin till in other places as follows. The
Surge Tank road runs eastward from U. S. 395
essentially along the Bishop tuff-Sherwin till
contact (Figs. 1,2). The tuff makes a cliff 50
to 100 feet high facing outward toward the till
which composes the slopes rising away from the
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
occasionally attaining 5 inches, can be found
on the till 100 to 200 feet out from the cliff
and 30 to 50 feet above (upslope from) its base.
Most significant is an acre patch of pumice
fragments at elevation 7180 feet on the crest of
Ridge 7246 about 0.25 mile northeast of its
summit (NW1/4, sec. 1, Fig. 2). At the east
end of this ridge, where it abuts against a low
tuff cliff, fragments of coherent tuff, up to 18
inches in diameter, lie 150 feet out from the
cliff and about 5 feet higher than its base.
Pumice fragments up to 5 inches in diameter
lie on till at the east end of a southern spur of
Ridge 7246 in the NW1/4 of sec. 6, 100 yards
from and 60 feet above the nearest Bishop tuff
cliff.
At the southern edge of the till mass, in the
center of sec. 12 just west of U. S. 395, abundant fragments of pumice, some up to 6 inches
in diameter, were found on the till up to 100
yards from and 75 feet above the base of a low
tuff cliff to the south.
In some situations one could perhaps argue
that the pumice fragments are small enough to
have been carried onto the till by wind. However, the pumice fragments are usually accompanied by pieces of black hornfels and basalt,
common inclusions in the tuff, whose transport
by wind is unlikely. All these pumice fragments are considered to be residual from a
former covering of Bishop tuff on those parts
of the Sherwin till where they are found.
STRIPPING OF BISHOP TUFF
Gilbert (1938, p. 1833, 1837-1838) regards
the present margin of the Bishop tuff as essentially its original edge, feeling that little cliff
recession and stripping have occurred. Putnam
(1960, p. 234), however, favors considerable
stripping in the Sherwin Grade-Rock Creek
area, inferring that most of the Sherwin till
exposed there was once buried beneath tuff.
Neither author supports his position with concrete facts, and compelling evidence of stripping is hard to find. Areas of granitic rock, for
example that in the SW1/4 of sec. 36 (Fig. 2),
almost certainly once buried by Bishop tuff, no
longer retain even the smallest remnant or fragment of tuff. Evidence of burial should be even
more easily removed from smooth slopes of unconsolidated till than from craggy exposures of
granitic rock. The layer of unconsolidated
pumice and ash at the base of the tuff facilitates
stripping, and the uneven distribution of this
layer promotes different degrees of stripping.
The previously described pumice fragments
on Sherwin till demonstrate less than 100 yards
of stripping in most places along the margin of
the presently exposed till mass. Almost certainly, the cover of Bishop tuff on Sherwin till
was greater, an inference supported by the
small patch of pumice fragments at elevation
7180 feet on the crest of ridge 7246 (Fig. 2).
This pumice is of the type found more abundantly in the cliff-making Bishop tuff than in
the basal airborne layer. Since the Bishop tuff
represents a series of ash and pumice flows
(Gilbert, 1938, p. 1851-1852), it presumably
covered the terrain up to a reasonably accordant height. The pumice patch on ridge
7246, with allowances for post-tuff warping and
faulting, suggests that most of the exposed
Sherwin till area east of Rock Creek and perhaps one third of the area west of that stream
were formerly mantled by tuff. This is somewhat less than the coverage inferred by Putnam (1960, p. 234) but possibly greater than
anything envisioned by Gilbert (1938, p. 18371838).
GEOMETRICAL RELATIONSHIPS
Although the map (Fig. 2) illustrates how
lobes of Sherwin till penetrate re-entrants in
the edge of the Bishop tuff, it does not show
that the tuff cliff rises abruptly above the till
at these places. Such relationships are best seen
along the Surge Tank road. Easily visible south
of the road at the northwest corner of sec. 6 is
a lobe of bouldery till extending north between low cliffs of Bishop tuff. It would have
been most difficult for a glacier younger than
the tuff to have deposited the till in this relationship, although a debris flow from the ice or
from marginal glacial deposits might have done
the job. Fragments of coherent tuff, up to 18
inches across, resting on the till at the head of
this lobe support the inference, drawn from
geometrical relationships, that the tuff is
younger.
Farther east, in the north-central part of sec.
6, the Surge Tank road crosses a low mass of
bouldery till extending northeast to the foot of
the tuff cliff. This till patch is nearly enclosed
by the tuff cliff with only a limited opening to
the southwest. It would have been extremely
difficult for a glacier younger than the tuff to
have laid down this till without scattering glacial debris over the nearby terrain where it is
lacking. The inference of a pre-tuff age for this
till is confirmed in Los Angeles Bureau of Water
and Power drill hole G-1, a scant 100 feet north
of the till-tuff contact (Fig. 2), that shows
glacial drift beneath Bishop tuff.
SHERWIN OUTWASH AND OTHER GRAVELS
NO SHERWIN TILL ON BISHOP TUFF
An extended search for till or glacial erratics
resting on Bishop tuff turned up very little of
interest. In the south central part of sec. 6, four
scattered granitic boulders, respectively 18, 16,
12, and 9 inches in diameter, were found on the
tuff. All were of rock types that could have
been supplied from high-standing exposures of
Wheeler Crest quartz monzonite just threeeighths of a mile west. An additional three
granitic boulders up to one foot in diameter
were found in an Indian stone circle, of which
there is an abundance in this area. These relations, plus the fact that the tuff itself occasionally includes granitic boulders, makes glacial
emplacement of this material unlikely, especially in the absence of other debris typical of
the till.
In the south-central part of sec. 6 is an isolated patch of boulder gravel (Fig. 2) covering
about half an acre and lying only 150 yards east
of exposures of Sherwin till. The deposit contains boulders up to 4 feet in diameter, many
smaller, worn, and rounded stones, 1 to 4 inches
across, and finer gravel and grus. The lithologic
types present are those of the Sherwin till. Although the gravel patch lies downslope from
exposures of Sherwin till, it is completely surrounded by Bishop tuff and there is no connecting trail of debris to the till.
Several interpretations are possible: (1) The
debris is an outlier of Sherwin till resting on
top of Bishop tuff. (2) It is Sherwin debris,
secondarily reworked and transported onto the
tuff. This would be possible under existing
topographic relations. (3) It is a window of
Sherwin drift exposed by removal of the overlying Bishop tufl.
The fact that the tuff is probably, at most,
10 feet thick here; that the gravel is completely
surrounded by tuff which in places rises a few
feet above it; and, that any connection on the
tuff surface to the Sherwin till 150 yards west
is lacking, suggests this is probably an unroofed
exposure of Sherwin drift. However, anyone
wishing to argue that the Sherwin overlies the
Bishop tuff will be interested in visiting this
location.
SHERWIN OUTWASH
AND OTHER GRAVELS
Sherwin till-Bishop tuff geometrical relationships are such that, if the till were younger,
Sherwin outwash should lie on the tuff or within
gullies draining across the tuff. The topographic
setting is particularly favorable for this along
359
the eastern and southern margins of the till
body, but no Sherwin outwash is found there
or elsewhere on the tuff. The only Sherwin outwash recognized is that exposed in the walls of
Owens River gorge beneath the Bishop tuff
(Fig. 4). Similar gravels are not found beneath
the Bishop tuff in Rock Creek gorge, suggesting that the direction of drainage in Sherwin
time was eastward from the mountains, rather
than southeasterly, as at present. The present
southeasterly slope of the land is attributed to
warping after eruption of the Bishop tuff
(Rinehart and Ross, 1957; Putnam, 1960, p.
244-245).
A considerable area of gravel resting on
Bishop tuff has been mapped, mostly east of
lower Rock Creek gorge, beginning 2.5 miles
southeast of the Sherwin till area (Fig. 7).
These gravels rest upon the tuff and incorporate fragments of it. Small exposures can be
seen in road cuts along U. S. 395 (sees. 4, 33,
Fig. 7), but the best exposed section is at the
first curve on the old Sherwin Grade road just
west of Rock Creek at Paradise Camp (sec. 29,
T. 5 S., R. 31 E., Mt. Tom Quadrangle). Here
are some 40 feet of beds, principally fluvial
gravel with smoothly worn, rounded stones set
in a sandy matrix. Near the top are two lenses,
7 to 8 feet thick, containing larger boulders to
6 feet in diameter set in a dense silty matrix.
These lenses look like debris-flow deposits. Similar material is exposed in gullies along the
wooden-pole powerline road west of U. S. 395.
Stones in these deposits, mostly 6 to 18 inches
but occasionally up to 8 feet in diameter, are
of lithologies found in the Sherwin till and
represented in the bedrock of upper Rock
Creek drainage.
These deposits have considerable antiquity,
for granitic boulders are completely disintegrated 15 feet beneath the ground surface. The
matrix is also weathered a rich yellow-brown
(10YR5/4) to this depth, and the upper parts
of stripped, debris-flow layers between Rock
Creek and U. S. 395 are heavily calichefied.
The gravels are also buried by a blocky creep
mantle of Bishop tuff fragments derived from
nearby higher outcrops. As much as 300 feet of
downcutting in Rock Creek gorge has occurred
since the initial phase of gravel deposition.
Farther up the old Sherwin Grade road west
of Rock Creek gorge are large areas of boulder
gravel consisting wholly of Wheeler Crest
quartz monzonite and associated dike rocks
(Fig. 7). This material has come from the high
abrupt face of Wheeler Crest to the west and
northwest and need have no genetic relation to
360
R. P. SHARP—SHERWIN TILL-BISHOP TUFF, SIERRA NEVADA
Gravel from
Wheeler Crest
G r a v e l from
Q upper Rock Creek
0
-f
Scale in M i l e s
Figure 7. Map of gravels on Bishop tuff along lower Rock Creek.
the first described gravels. It will not be considered further.
The distribution (Fig. 7) and constitution of
the first described gravels indicate that they
have been transported down the course of Rock
Creek. The lower part of the gorge was not as
deeply cut then as now, for the gravels spread
eastward in shallow gullies over the Bishop tuff
surface from a point about 1.5 miles above the
present gorge mouth and at a level 300 feet
SPECULATION ON CORRELATION
higher than its present floor. However, gravels
of similar constitution are also found within
the gorge to within 80 feet of its floor, suggesting that their transport extended over a considerable part of the gorge-cutting interval.
These gravels are related to the diversion of
Rock Creek from its former northeasterly path
into the present southeasterly course, as described by Putnam (1960, p. 249). At that time
deep dissection of the Sherwin till occurred,
and a large volume of Sherwin debris must have
been flushed down the new Rock Creek channel. These gravels are regarded as remnants of
that debris. Putnam (1960 p. 251) dates the
diversion as Tahoe in age, but the gravels appear much older. Since the Tahoe dating is not
supported by any direct evidence, and since the
Rock Creek gorge is cut 500 to 600 feet into
Bishop tuff and resistant crystalline rocks, it is
suggested that the Rock Creek diversion occurred well before Tahoe time.
These gravels are clearly younger than the
Bishop tuff and probably consist of debris reworked from the Sherwin till. The possibility
that they represent Sherwin outwash and that
the Sherwin glaciation occurred after eruption
of the Bishop tuff is highly unlikely.
SPECULATION ON CORRELATION
The 710,000-year potassium-argon date for
the basal pumice-ash unit of the Bishop tuft
volcanic episode (Dalrymple and others, 1965,
p. 670) makes the Sherwin till surprisingly old.
The following indications of erosion and weathering prior to extrusion of the Bishop tuff suggest that the Sherwin till is somewhat older
still.
Granitic knobs capped by Bishop tuff without 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 indicates considerable disintegration, slopewash,
and creep on the Sherwin slopes before burial.
The till itself is only modestly oxidized to a
depth of a few feet, but disintegration of granitic boulders to a depth of 25 feet below the
till-pumice contact is extensive.
The buried Sherwin till is more deeply
weathered than most Tioga moraines and approaches, but does not generally attain, the
weathering displayed by many Tahoe deposits
361
in this region. It is judged, therefore, to have
experienced weathering for a few tens of thousands of years but not as much as 100,000 years
prior to eruption of the Bishop tuff. An estimated age in the neighborhood of 750,000
years for the Sherwin thus seems reasonable.
Large areas of Sherwin till previously have
been recognized along the eastern Sierra Nevada front in the Mono and Bridgeport basins
and on West Walker River (Blackwelder, 1931,
p. 895-900; Putnam, 1949, p. 1290). Although
firm correlation between these occurrences and
the type locality has not been established, the
topographic setting, the succession of glaciations, and semi-quantitative data reflecting age,
support such a correlation. There is scant reason
to adopt the view of Evernden and Curtis
(1965, p. 356) that the Sherwin of the type locality has no established relation to other
Sierra glacial deposits. Certainly, till beneath
the Bishop tuff in the Mono Basin has a corresponding stratigraphic position (Putnam, 1949,
p. 1289).
The considerable age of the Sherwin makes
one wonder about the McGee till, a presumably
still older episode of Sierra glaciation (Blackwelder, 1931, p. 902-906; Putnam, 1962, p.
192-195). The McGee has a topographic setting
which is unusual, even for Sherwin, and on that
basis alone it could well be older, especially in
view of the 2.7-3.0 X 106-year age of a nearby
deposit stated to be of glacial origin (Curry,
1966).
In the bottom of Rock Creek gorge along the
old Sherwin grade road at the south edge of the
Sherwin till area (sec. 12), a road cut in till
exposes a prominent reddish zone at its core
(Fig. 8). This looks like two tills separated by
FigureS. Field sketch of old red (pre-Sherwin?)
till on old Sherwin Grade road, lower Rock Creek,
sec. 12.
a deeply weathered zone. Gray material above
the reddish zone has all the aspects of a firstclass till and as such it is surely Sherwin. Attempts 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
reddish debris is reworked into the overlying
deposit. Deposition of the overlying material
by surface creep is not supported by its dense,
tight, till-like matrix or the inclusion of an undeformed lens of water-laid debris. A pH profile across the contact is of little help as it yields
values of 8 to 8.5 all the way. Little veinlets of
calcium carbonate extend from the overlying
debris into the reddish zone which has thus had
its CaCOs restored by percolation from above.
The contact is displaced by small faults in several places (Fig. 8), further evidence of some
degree of antiquity.
These relations remain something of an
enigma but for the present are interpreted as
the contact between two tills. This is favored
by the location, at the bottom of a 500-foot
gorge near the edge of the Sherwin till area.
Just possibly, one sees here the contact between
Sherwin and McGee tills.
The Sherwin till was tentatively regarded as
Kansan by Blackwelder (1931, p. 918). In view
of its probable age of about 750,000 years, this
seems more reasonable than the Illinoian age
urged by Putnam (1962, p. 205). Radiometric
dating is currently so severely challenging
temporal correlations with continental glacial
episodes that it seems pointless at this time to
urge any specific correlation of the Sherwin
with the classical midwestern glacial sequence.
As Damon (1965) notes, Pleistocene chronologists disagree widely about the duration of
the glacial Pleistocene (Hopkins, in Evernden
and Curtis, 1965, p. 372). Some are advocating
a duration in excess of 1 million (Evernden and
Curtis, 1965, p. 343) to 1.5 million years (Ericson and others, 1964, p. 731). A Sherwin age
of 750,000 years is not inconsistent with a glacial Pleistocene of this duration, but it is far
out of line with the shorter glacial epochs advocated by Emiliani (1958, p. 271; 1966, p.
855-856; Emiliani and others, 1961, p. 687)
among others. The task of bringing interpretations of marine and terrestrial Pleistocene
records into harmony remains interesting and
challenging, especially in view of Curry's (1966)
recent report of a possible Sierra Nevada glaciation radiometrically dated at 2.7-3.0 X 106
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. 770771.
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 sediments: 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 California: 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