Soils and subsurface rock-weathering features of Sherwin and
pre-Sherwin glacial deposits, eastern Sierra Nevada, California
-----------------------------,.,------------P. W. BIRKELAND
R. M. BURKE*
A. L. WALKER *
}
Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309
ABSTRACT
Soils and subsurface rock-weathering features developed in
Sherwin and pre-Sherwin tills were studied in the Sherwin Till type
locality and in Bridgeport Basin to determine those characteristics
useful (1) in distinguishing these deposits from those of Tahoe age,
and (2) in subdividing and correlating pre-Tahoe deposits along the
eastern side of the Sierra Nevada. To aid in these two goals, laboratory data were obtained on pH, particle-size distribution, two fractions each of free Fe and free AI, and two P fractions, but only data
on particle size and one Fe fraction proved to be useful. If stable
surface sites are compared, soils formed in Sherwin Till have much
better developed Bt horizons, as indicated by clay content, clay
films, and redness, than those in Tahoe Till. Grusification of granitic clasts is about the same in both Tahoe and pre-Tahoe tills, but
metamorphic and volcanic rocks are much more weathered in
pre-Tahoe deposits. The best developed soils thought to be of
Sherwin age are those in Bridgeport Basin, but the correlation with
the type locality of the Sherwin Till is uncertain. Soils formed in
type Sherwin Till are less well developed than those formed in
Bridgeport Basin deposits, probably either because the former soils
are younger, having formed on an exhumed surface following the
removal of the overlying Bishop Tuff, or because of climatic and
lithologic differences. Data on a weakly developed soil in the uppermost part of the type Sherwin Till, buried by the Bishop Tuff,
help to confirm Sharp's (1968) estimate of the date of the Sherwin
Glaciation at about 0.75 rn.y. B.P. Type Sherwin Till buries a soil
formed in still older till in Rock Creek gorge, and a minimum time
for the formation of that soil is about 100,000 yr.
INTRODUCTION
This study is part of a larger study undertaken to refine
semiquantitative relative dating techniques for dating and correlating Quaternary deposits in the east-central Sierra Nevada where
absolute dating techniques cannot be used. Recently, we defined the
relative dating methods employed and concluded that the Pleistocene tills with distinct morainal form seem better grouped into
Tioga (younger) and Tahoe Tills rather than the previously accepted fivefold subdivision of tills (Burke and Birkeland, 1979).
The methods most useful in subdividing and correlating the above
• Present addresses (Burke) Department of Geology, Humboldt State
University, Arcata, California 95521; (Walker) U.S. Geological Survey,
Mail Stop 75, 345 Middlefield Road, Menlo Park, California 94025.
deposits are those involving the measurement of surface and subsurface rock-weathering features; soils proved rather disappointing
in subdividing the deposits.
In this paper we examine the differentiation of pre-Tahoe deposits from Tahoe deposits. Because surface rock-weathering features are quite advanced on Tahoe deposits and in places the data
overlap with those for older tills (Sharp, 1969), we concentrated on
subsurface rock-weathering and soil data. Our goals were (1) to
define criteria that would clearly differentiate pre-Tahoe from
Tahoe deposits, and (2) to determine if the data generated could be
useful in subdividing and correlating pre-Tahoe deposits. The emphasis was on field criteria backed by a few laboratory methods, so
that these could be integrated into a mapping program. We feel we
have been successful with goal 1, but we have had only limited success with goal 2. Furthermore, our basic conclusions support those
presented earlier by Sharp (1968, 1972) and add more quantitative
data to those already presented by him. This study differs a bit from
an earlier one with somewhat similar goals (Birkeland and Janda,
1971) in that more data are presented here for the soils and we
were able to locate sites that display stronger soil development.
Glacial Stratigraphy
The sequence and ages of glacial deposits in the east-central
Sierra Nevada have been recently reviewed by Birkeland and others
(1976) and Burke and Birkeland (1979). Tioga Till, the youngest
Pleistocene till, predates 9800 ± 800 B.P., whereas the type Casa
Diablo Till, which we consider to have been deposited during the
Tahoe Glaciation, is bracketed by K-Ar dates on basalts of 0.062 ±
0.013 and 0.126 ± 0.025 rn.y, B.P. The next older till is the Sherwin Till, and in the type locality it lies beneath the Bishop Tuff
(Sharp, 1968), for which there is a mean K-Ar date of 0.71 m.y.
B.P. (Dalrymple and others, 1965) that is supported by zircon
fission-track dating (Izett and Naeser, 1976). By adding to this date
the approximate time it took to form a soil in the till prior to burial
by the tuff, Sharp (1968) put the age of the till at about 0.75 rn.y,
The McGee till(?) seems to predate the Sherwin Till and rests on a
basalt K-Ar dated at 2.6 m.y. B.P.
Study Area
Although most of the east-central Sierra Nevada was reported on
by Blackwelder (1931), the detailed work at the key sites has been
done by Sharp (1968, 1972). At the type locality of the Sherwin
Till, these sites include a buried soil in a pre-Sherwin till and surface
Geological Society of America Bullerin, Part I, v, 91, p. 238-244, 3 figs., 1 table, April 1980, Doc. no. 00408.
238
SHERWIN AND PRE-SHERWIN GLACIAL DEPOSITS, SIERRA NEVADA
deposits are decreasing pH, increasing amounts of free Fe and free
AI (Alexander, 1974), and variation in P fractions. Free Fe and free
AI were determined by the dithionite and oxalate extraction
methods (references for methods are in Table 1). The dithionite
method is considered to extract organic-matter-bound, amorphous, and crystalline iron oxides, whereas the oxalate method extracts organic-matter-bound and amorphous iron oxides
(McKeague and Day, 1966; McKeague and others, 1971); these are
denoted Fe" and Fe", respectively. AI was analyzed in the same two
extracts, denoted AI" and AI", but the extractants are less useful for
distinguishing forms of AI than of Fe (McKeague and others,
1971). For the P fractions, H 2S04-extractable P (Pa ) , which consists
of P in apatite and that sorbed on the surfaces of oxides, should
decrease with time as P is leached from the system or is partly converted to organic P (P,,), which will build up (Walker, 1964; Walker
and Syers, 1976). To test the usefulness of these P trends, we
analyzed for both P, and P".
~,
Gre en 'J' ~.\
Cr. ,14,
r-:' ,r":
DUnd8rb~r/
cr. _'
Virginia
Cr.
\S'l/.
~~
~'f
1
SOIL AND ROCK-WEATHERING FEATURES
OF TAHOE TILL
1-~
s:
'fa
"'1
N
0
239
15 Km
Figure 1. Location of soils studied in eastern Sierra Nevada.
and buried soils formed in Sherwin Till (Fig. 1; App, 1). In the
Green-Dunderberg-Virginia Creeks drainage in Bridgeport Basin,
we sampled two surface soils in Sherwin or older till or till-like deposits.
Environmental Setting
The factors important to soil formation in this region have been
given by Sharp (1968, 1972) and Birkeland and Janda (1971), so
they are only briefly summarized here. Rock type varies from predominantly granitic in the Rock Creek area to granitic and
metavolcanic, with some andesite, in the Bridgeport Basin area.
Vegetation is predominantly sagebrush. Mean annual precipitation
varies from about 24 cm in the Rock Creek area to 32 ern or
slightly higher in the Bridgeport Basin area; most of the precipitation occurs during the fall and winter months. Mean annual temperature is probably near 7 to 8 "C.
As a point of reference for this study, we first briefly describe the
post-Tahoe soils and subsurface rock weathering. Soils are weakly
to moderately developed in tills considered to be of Tahoe age, including tills mapped by others as Tenaya, Mono Basin, and Casa
Diablo in age (Birkeland and Janda, 1971; Burke and Birkeland,
1979). The soils are usually oxidation profiles with subdivided Cox
horizons, and some have a cambic B horizon. At the type locality of
the Casa Diablo Till, at Bt horizon has formed that meets the
criteria for the argillic horizon (Soil Survey Staff, 1975), Granitic
clasts commonly are grusified within the soil, and, in places, to
greater depths, and many near the surface are also oxidized to
brown colors.
There are few chemical trends in the post-Tahoe soils (Burke and
Birkeland, 1979; Burke and others, 1979); pH either remains fairly
constant with depth, or for the upper part of the soil it is about 1
unit less than for the unweathered till. Both AI extracts display
slight increases in the soil relative to that in the unweathered till. Fe
trends are a bit more complicated; some profiles show no trends,
others have reversals in the trends, and the greatest build-up is for
Fe" in a post-Tahoe soil in the Green Creek area which increases
from about 0.1 % at depth to about 0.5% in the B horizon. P"
shows a rather systematic increase upward in most profiles, and it
reaches maximum values near 200 ppm. In contrast, Pa displays
few trends and common reversals, perhaps due either to parent material variation in the original till or to a dry climate over a long
interval of time (Birkeland and others, 1979b).
SOIL AND ROCK- WEATHERING FEATURES
OF PRE-TAHOE TILLS
Rock Creek Area
Methods and Expected Trends in Soil Data
In order to use soils to help in the relative dating of deposits, various time-dependent parameters must be measured and compared
with either younger or older soils (Birkeland, 1974, chap. 8). In the
field, the obvious changes with time are increasing thickness of
horizons, greater clay build-up, and redder hues, especially in the B
horizons. Clay build-up can be quantified by laboratory data.
Other laboratory data that might correlate with progressively older
Sharp (1968) reported on a locality in Rock Creek gorge at
which Sherwin Till overlies an older till (loc. 4 in Fig. 1, and App.
1). A deeply weathered reddish zone developed in the latter was
considered to be a buried soil, and Sharp speculated that the lower
till might be McGee in age.
The weathered red zone has all the field attributes of a buried soil
(Working Group on the Origin and Nature of Paleosols, 1971;
Valentine and Dalrymple, 1976). The matrix of the overlying
TABLE 1.
Locality
Horizon
Depth
(em)
DATA ON PRE-TAHOE SOILS, EASTERN SIERRA NEVADA
Particle-size
distribution (%)
sand
silt
clay
Gravel
2.5Y 7/3
7.5YR 5.5/5
2.5Y 7/3.5
10YR 6/4
2.5Y 6/3
2.5Y 5/3
73.3
59.2
62.1
58.0
62.5
66.3
17.9
31.2
30.6
28.5
25.5
22.0
8.8
9.6
7.3
13.5
12.0
11.7
90
10
10
50
50
50-75
0.0
0.6
0.6
0.4
0.5
0.4
7.1
7.3
7.7
7.7
7.8
8.4
0.40
1.01
0.56
0.64
0.58
0.54
0.18
0.06
0.06
0.06
0.04
0.03
0.03
0.07
0.04
0.05
0.04
0.04
0.05
0.07
0.06
0.07
0.07
0.07
364
53
37
125
618
654
4
55
28
22
0
18
10YR 4/4
10YR 4/3
2.5Y 5/3
67.5
69.1
67.4
12.5
8.5
19.6
20.0
22.4
13.0
50-75
50-75
50-75
0.6
0.3
0.4
6.7
6.8
7.1
0.34
0.35
0.34
0.08
0.10
0.05
0.04
0.04
0.02
0.09
0.10
0.08
407
381
417
34
37
22
0.10
0.08
0.07
0.04
0.05
0.04
0.09
0.03
0.05
0.04
0.04
0.03
0.01
0.01
0.02
0.03
0.08
0.08
0.08
0.07
0.07
0.06
0.06
0.06
352
292
409
380
422
467
348
586
70
66
59
50
44
0
150
0
Color
(dry)
(%)
Organic
matter
pH
Fed
Soil in type
Sherwin Till
(loc.5)
B2t
B3t
Cox
Soil in type
Sherwin Till
(loc.6)
A
IIB1
IIB2t
IIB3t
IIIC10x
IIIC20x
IVC30x
IVCn
0-9
9-24
24-80
80-94
110-138
138-238
238-263
263+
10YR 4/3
10YR 5/4
10YR 4/6
10YR 6/4
10YR 6/5
10YR 6/4.5
7.5YR 6/7
2.5Y 7/3
82.9
75.3
69.5
71.3
72.0
80.1
69.0
69.3
11.1
14.4
16.2
16.3
18.1
13.6
23.9
23.8
6.0
10.3
14.3
12.4
9.9
6.3
7.1
6.9
0
75-90
75-90
75-90
50-75
50-75
50-75
50-75
1.1
0.6
0.1
0.2
0.4
0.2
0.4
0.2
6.1
6.6
6.8
6.8
6.9
7.0
7.6
7.8
0.31
0.33
0.41
0.36
0.54
0.28
2.02
0.30
Soil in type
Sherwin Till
buried by
Bishop Tuff
(Ioc, 1a)
C10xb
C20xb
IIC20xb
IIC30xb
IICnb
0-83
83-120 }
120-165
165-210
210+
2.5Y 5/3.5
87.2
6.6
6.2
0
0.3
2.5Y 5/3
86.7
6.8
6.5
5Y 6/2
5Y 7/2
82.7
89.1
11.9
7.9
5.4
3.0
Soil in
Sherwin(?)
Till
(loc.14)
B2t
B3
5YR 6/6
7.5YR 7/6
33.4
48.9
30.0
32.7
36.6
18.4
Soil in
Sherwin(?)
Till
(loc.3)
A
B1
IIB21t
IIB22t
IIB23t
IIB24t
IIB31
IIB32
IIB33
IIB34
I1IC1ox
I1IC2ox
I1IC30x
0-7
7-15
15-43
43-100
100-130
130-160
10YR 6/3
7.5YR 4.5/3
5YR 5/4
5YR 5/3.5
5YR 4/4
5YR 4/4
52.3
46.9
28.6
38.9
44.4
38.5
36.9
33.9
31.2
27.6
26.1
26.7
160 180
7.5YR 5/4
45.V
2~.1
10.8
19.2
40.2
33.5
29.5
34.8
26.9
19.6
20.4
21.8
19.2
20.2
16.2
0-35
35-75+
200-220
240-260
280-300
320-340
360-380
400-420
7.5YR 5/4
7.5YR 5/4
7.5YR 5/4
10YR 8/3
10YR 7/3
10YR 6/3
55.6
55.5
51.0
54.3
51.9
54.9
24.9
25.1
27.2
26.5
27.9
28.9
Note: Standard soil methods were used in the field and laboratory. Soil horizon nomenclature
follows Soil Survey Staff (1975), modified by Birkeland (1974). Parent material layering, denoted
by Roman numerals, is recognized by abrupt variation either in the percentage of gravel or nongravel particle-size distribution that is considered to be of geologic origin. Colors are for dry sampies of the <2-mm fraction. Particle-size distribution of the <2-mm fraction is by pipette (Day,
1965), and the percentage of gravel is a visual estimation. pH is determined by meter on a 2: 1
Pa
P"
(ppm)
(%)
Cn
IIB2tb
IICoxb
I1IB2tb
I1ICoxb
I1ICnb
0-45
45-80
80-100+
AI"
(%)
Buried soil
in preSherwin till,
Rock Creek
gorge
(loc.4)
0-54
54-104
104-144
144-444
1300
Aid
Fe"
7.0
0.22
0.03
0.04
0.05
205
48
0.3
6.9
0.21
0.05
0.03
0.05
166
36
90
90
0.1
0.2
6.5
7.1
0.20
0.14
0.15
0.02
0.04
0.02
0.05
0.02
188
180
46
4
10
10
0.2
0.4
6.0
6.2
3.52
0.16
0.07
0.02
0.12
0.09
0.11
0.07
41
22
152
87
1.4
1.0
0.4
0.8
0.5
0.3
0.8
0.7
0.6
0.7
0.5
0.6
0.0
6.7
6.5
6.7
6.7
6.4
6.2
6.5
7.5
7.6
7.3
7.6
7.5
7.5
0.78
0.38
1.10
1.21
0.86
0.85
0.11
0.11
0.10
0.09
0.08
0.08
0.07
0.08
0.13
0.11
0.12
0.12
274
504
80
110
228
188
102
0
204
170
162
160
0.77
0.09
0.05
0.03
0.09
0.10
0.09
0.09
0.09
0.12
0.17
0.14
0.09
0.09
0.12
A " ..
V.IV
... ...
LJL
11:0
0.10
0.10
0.10
0.09
0.11
0.08
374
210
234
42
48
186
94
86
110
208
146
98
1~ }
50-75
50-75
50-75
50-75
50-75
50-75
50-75
50-75
50-75
50-75
10-20
10-20
10-20
1.31
1.71
1.43
1.10
0.82
1.27
0.12
0.07
0.06
0.03
0.05
0.05
,
.uo
water-to-soil mixture. Organic matter is estimated by loss on ignition, corrected for structural
water loss by subtracting the loss on ignition of organic-matter-free silt plus clay (unpub. method
of Rolf Kihl). Dithionite-extractable Fe and AI (Fed and Aid) and oxalate-extractable Fe and AI
(Fe" and AI,,) were extracted by the methods of Mehra and Jackson (1960) and McKeague and
Day (1966). Acid-extractable P and organic P (P, and P,,) were determined by the methods described in Blakemore and others (1977).
SHERWIN AND PRE-SHERWIN GLACIAL DEPOSITS, SIERRA NEVADA
Sherwin Till is unoxidized, yet the granitic clasts are grusified but
not oxidized, and the contact with the underlying weathered zone is
sharp. In contrast, granitic clasts within the weathered material are
grusified and oxidized brown, a feature common to post-Tahoe
soils. There appear to be two buried B horizons at the site, or alternatively, parent material layering gives the impression of two
buried B horizons; the outcrop was too limited to provide a definite
answer on this. The buried Bt horizon(s) are recognized on the
bases of color, increases in clay content relative to the Cox horizons, and clay films (Table 1; App. 1).
The B horizon(s) do not meet the criteria for argillic horizons because the A horizon(s), with which they are to be compared (Soil
Survey Staff, 1975), are not recognized. Buried soils, however,
rarely have preserved A horizons, so the comparison of A horizon
to B horizon is commonly impossible. Our suggestion in these cases
is to apply the same A:B horizon clay-increase criteria for the
change from the C to the B horizons if one can be reasonably certain that the C horizon material approximates the parent material
for the B horizon. When this is done at locality 4, the soils do not
meet the required 3% increase in clay in the B horizon relative to
the C horizon, so the Bt horizons are not argillic horizons.
Well-developed soils have formed from Sherwin Till at two
localities (locs, 5 and 6 in Fig. 1, Table 1, and App. 1). The relationships with the nearby Bishop Tuff suggest that the latter covered
both sites in the past (Sharp, 1968); hence, soil formation has
probably occurred since the tuff was eroded from the sites. Both
soils have thick (>70 em) argillic horizons, and although that at
locality 5 has the greater amount of clay, that at locality 6 has the
higher chroma, suggesting a somewhat similar degree of development. For contrast, a Bt horizon was not recognized at a different
site in type Sherwin Till in an earlier study (Birkeland and Janda,
1971). Granitic clast weathering in the subsurface is typical for deposits of Tahoe age or older (Fig. 2).
A buried soil in the type Sherwin Till at the contact with the overlying Bishop Tuff also was studied. Sharp (1968) found that the
Figure 2. Granitic clasts near locality 6
that are so weathered to grus that roadbuilding equipment cut through them
readily, leaving weathered boulders intact.
241
characteristics of the buried soil were intermediate between those
of soils formed in Tioga and Tahoe Tills, and concluded that the
Sherwin Till weathered for a few tens of thousands of years before
being buried by the tuff; thus, he put the age of the Sherwin at
about 0.75 m.y. We studied the Little Pumice Cut of Sharp (1968,
Fig. 6) and found that the soil relationships are not clear-cut, so we
concentrated our efforts on the buried soil in the Big Pumice Cut of
Sharp (1968, Fig. 3; loe. 1a in Fig. 1 here; Fig. 3).
The buried soil in the Big Pumice Cut is essentially a weakly
oxidized Cox profile slightly more than 2 m thick (loc, 1a in Table 1
and App. 1). The oxidation colors are less intense than those for
many profiles in Tahoe Till. The granitic clast grusification, however, is as well developed as that in many Tahoe Tills. Thus, the
suggestion of Sharp (1968) that this soil probably has properties intermediate between those in Tioga and Tahoe Tills seems justified.
Bridgeport Basin Area
Sharp (1972) tentatively assigned pre-Tahoe tills of the
Virginia-Dunderberg-Green Creeks drainages to the Sherwin
Glaciation, but he queried the designation because correlation with
the type locality is uncertain. Two soils were studied: that at locality 14 (Fig. 1) is the "old red till" of Sharp (1972), and that at locality 3 (Fig. 1) was previously studied by Birkeland and Janda
(1971, soil samples 8 and 43), who considered the parent material
to be till, and by Sharp (1972), who considered the parent material
to be outwash.
Both soils are characterized by Bt horizons with the highest clay
contents, the best developed clay films, and the reddest hues in the
study area (locs. 14 and 3 in Table 1 and App. 1). In addition, the
weathering of clasts in the soil at locality 3 is extreme. The soils
differ from each other mainly in that the Bt horizon at locality 14 is
thinner, but this could be due more to erosion than to an age difference. Locality 3 is the more stable of the two sites, and this is
reflected by the B2t horizons extending to 160 em and the B3 hori-
242
BIRKELAND AND OTHERS
zons to 300 em. We were not able to determine definitely the parent
material for the soil at locality 3, but the high clay content at depth
seems to rule out a well-sorted fluvial deposit; a mudflow origin
cannot be ruled out, however. Of the soils examined along the central part of the eastern Sierra Nevada by Birkeland and his colleagues over the past 16 yr, these seem to be the best developed soils
formed in till or till-like deposits.
CHEMICAL TRENDS IN SOILS FORMED
FROM PRE-TAHOE TILLS
Soil chemistry was of only limited use for the subdivision and
correlation of tills in this study. Trends in some chemical parameters reflect the variation in soil morphology, but in places there are
some unexplained reversals in trends (Table 1). Variations in pH
with depth follow the expected trends and are not different enough
from those for the post-Tahoe soils (Burke and Birkeland, 1979) to
be useful in age differentiation of tills. Po increases upward in many
profiles, and its main value here seems to be in verifying the existence of the buried soils. Pa shows depletion trends upward in some
soils, but because changes in the I' a often coincide with boundaries
between different parent materials, one cannot be certain of the
pedogenic significance of the trends. Ala, AId, and Fe, all occur in
small amounts, display slight increases upward in most profiles,
and are not too dissimilar to the trends for post-Tahoe soils (Burke
and others, 1979). For some of the soils Fed shows few notable
trends with depth, whereas in three soils (locs. 3, 4, and 14 in Table
1) there is a build-up to 1% or more in the B horizons. Although
some of these increases in Fed could be pedogenic, increases in Fed
at depth in soils at localities 3 to 6 do not coincide with buried B
horizons recognizable in the field; thus, the question of parent material versus pedogenic origin for some high values of Fed is raised.
In summary, the chemical data do not always follow the predicted trends. This was unexpected and unfortunate, as we have
derived much better trends with time in soil chemistry that can be
ascribed to pedogenesis in Holocene soils above tree line (Birkeland
and others, 1979a). Perhaps the dry climate along the base of the
eastern escarpment inhibits the development of consistent soil
chemical trends (Burke and others, 1979).
RELATIVE AGES OF PRE-TAHOE TILLS
The data on soil morphology and particle-size distribution compared with that for post-Tahoe soils can be used to assign relative
and approximate ages to the soils mentioned here. In places, the
chemical data support the morphological data, but the same age interpretation could have been made without the chemical data. The
following interpretations seem valid .
1. The buried pre-Sherwin soil at locality 4 probably took no
less time to form than the post- Tahoe soils in the region, or approximately 100,000 yr. The maximum time necessary to form the soil
cannot be estimated because position in a landscape strongly
influences soil development, and the position of the buried soil in
the paleolandscape is unknown. The buried till could be McGee,
but it does not have to be.
2. Soils formed in Sherwin Till and possible Sherwin Till vary
from south to north. To the south (locs, 5 and 6), argillic horizons
are present, and the strongest color hues are 1OYR. To the north
(Iocs.3 and 14), however, the argillic horizons have more clay, and
the strongest color hues are 5YR. This variation could be the result
of some combination of slightly greater precipitation to the north,
the presence of volcanic: and metavolcanic clasts to the north, and
the additional time available for soil formation to the north. Referring to the latter, the till to the south was buried by the Bishop Tuff
for some unknown length of time, and the present soils probably
formed only after the cover was removed. In contrast, the soils to
the north have undergone pedogenesis since deposition of the tills.
These differences in time available for soil formation could help
explain the differences in the soils, provided the parent tills are indeed the same age. These soils generally display much better development than those formed from Tahoe Till.
3. The buried soil formed from Sherwin Till and subsequently
,....."""J:'_..:__,
Figure 3. Bishop Tuff, with well-bedded
basal layers, overlying Sherwin Till in Big
Pumice Cut of Sharp (1968; loc. 1a here).
Buried soil was sampled near left side of
this view.
SHERWIN AND PRE-SHERWIN GLACIAL DEPOSITS, SIERRA NEVADA
buried by Bishop Tuff (loc. 1a) is no better developed than postTahoe soils in the area. Therefore, the suggestion of Sharp (1972)
that the time for soil formation and rock weathering (100,000 yr or
less) be added to the age of the tuff to derive an approximate age
for the till seems reasonable.
CONCLUSIONS
The soil data here confirm many of the findings of Sharp (1968,
1972), and add some quantitative soil data to back up his conclusions. The soil in the Sherwin Till buried by the Bishop Tuff and
exposed in the Big Pumice Cut (loc. 1a) is basically a Cox horizon
and may have required only several tens of thousands of years to
form, in support of Sharp's (1968) suggested age of about.0.75 m.y.
for the Sherwin Glaciation. Strongly developed soils with Bt horizons have formed from Sherwin Till in the type locality (Iocs, 5 and
6) since the removal of the overlying Bishop Tuff. The two soils in
Bridgeport Basin (locs. 3 and 14) are the best developed for the
area. The interpretation of these soils, compared to the others, is
difficult because they may represent the total development expected
for sites reasonably stable over 0.75 m.y., or the material in which
they formed could be older, or soil development could proceed
more rapidly due to some combination of favorable climate and
lithology. Therefore, we agree with Sharp's (1972) call for caution
in correlation of pre-Tahoe deposits based on relative dating
criteria. The oldest soil studied could be the buried soil in Rock
Creek gorge (loc, 4). Sharp (1972) remarked on the similarity between this soil and the relict ones in Bridgeport Basin (locs. 3 and
14); we disagree with this statement, because the latter soils show
many more strongly developed pedological features than does the
buried soil. Of the soil chemical data, only those for Fed provided
information useful for differentiating pre-Tahoe deposits from
Tahoe deposits.
Correlation with more distant areas in California is even less certain than within this relatively restricted area. The maximum soil
development and rock weathering reported on here have distinct
similarities to those for pre-Tahoe till near Lassen Peak (Crandell,
1972) and Hobart Till near Truckee, if indeed the Hobart Till can
be proven to predate the Donner Lake Till (see Birkeland and
Janda, 1971, Table 1, footnote 5). For now, it seems that soil
morphology and rock weathering information can be used to group
till into a broad pre-Tahoe age designation, but subdivision within
this grouping will have to await further study.
ACKNOWLEDGMENTS
We thank]. P. Whipple for assisting in the field, and R. P. Sharp
and R. R. Shroba for reviewing an earlier draft of this paper. In
addition, this study has benefited from discussions over the years
with D. R. Crandell, R. ]. Janda, R. P. Sharp, and C. Wahrhaftig.
Rolf Kihl did part of the laboratory work. This work was supported by U.S. Geological Survey Grant 14-08-0001-G-202.
APPENDIX I. DESCRIPTION OF LOCALITIES,
AND REMARKS RELATED TO SOILS AND
ROCK-WEATHERING FEATURES AT EACH LOCALITY
Locality 4. The soil was described and collected in a roadcut 0.5 km
south of the bridge over Rock Creek along the Sherwin Grade, at the boundary between sees, 11 and 12, T.SS., R.30E., Casa Diablo Mountain,
243
California, IS-minute quadrangle. The description is for the northern part
of the southern of two outcrops of "old red till" of Sharp (1968, Fig. 8). The
Cn is Sherwin Till, and included granitic clasts are weathered flat-to-theface and are virtually not oxidized (10YR 6/1). In contrast, boulders to
144-cm depth in the buried soil are grusitied and oxidized (maximum oxidation color 7.5YR 6/8). Granitic clasts are slightly oxidized in the lllCoxb
horizon. Clay films are few and thin in the IIB2tb horizon and common and
moderately thick in the IIIB2b. The IIICnb horizon was collected 18 m
south of the rest of the profile, to the south (right) of the fault shown by
Sharp (1968, Fig. 8); granitic clasts there are fresh.
Locality 5. The soil was described and collected in a shallow artificial
cut at the summit of hill 7264', in the center of sec. 1, T.SS., R.30E., Casa
Diablo Mountain IS-minute quadrangle. Vegetation is sagebrush with
some pinon pine. Included granitic clasts are grusified and oxidized. Clay
films in the B2t are common and moderately thick; they seem to be less
common in the B3t.
Locality 6. The soil was described and collected in a large west-facing
roadcut of the southbound lane of Highway 39S, 1.6 km southeast of the
Little Pumice Cut of Sharp (1968, Fig. 1), and just north of the connecting
lane between the southbound and northbound lanes of the highway. It is in
the NW% of sec. 12, T.SS., R30E., Casa Diablo Mountain IS-minute
quadrangle. Vegetation is mainly sagebrush. Granitic clasts are grusified
throughout; those in the soil are oxidized, whereas those in the till are not.
Clay films in the Bt horizons are thin, moderate to few.
Locality La, The soil was described and collected in the Big Pumice Cut
of Sharp (1968, Figs. 1 and 3), 17 m west ofthe first occurrence of Sherwin
Till in the cut. It is in the SW% of sec. 34, TAS., R.30E., Casa Diablo
Mountain 15-minute quadrangle. Parent material I seems to be colluvium,
and II is till. Many granitic clasts throughout the cut are grusified; in addition, those in the top 4 m of the till are oxidized to 2.5Y 6/3.
Locality 14. The soil was described and collected at "old (Sherwin?) red
soilloc." shown in Figure 6 of Sharp (1972), in the south-central part of sec.
34, TAN., R.25E., Bodie, California, IS-minute quadrangle. Vegetation is
sagebrush. Clay films in B2t are thick, abundant, and 2.5YR 6/3 (moist).
Locality 3. The soil was described and collected from a roadcut about
0.1 km north of where the 7,200-& contour intersects Highway 395 in sec.
3S, TAN., R.2SE., Bodie IS-minute quadrangle. A power line crossed the
highway here in 1975. The uppermost 100 ern of soil was described in a
roadcut in the abandoned road just east of, and parallel to, Highway 395,
and the rest of the soil was described in a Highway 395 roadcur. Vegetation
is sagebrush. Many granitic clasts are grusified and oxidized (maximum
color 10YR 6/8) throughout the soil. Percentage of grusified clasts decreases
with depth. Many metamorphic and volcanic clasts can be cut through with
a pick. Clay films in the Bt horizon are moderately thick and common. The
subordinate horizon nomenclature below 100-cm depth reflects various
sampling intervals.
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MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 4, 1979
MANUSCRIPT ACCEPTED OCTOBER 8, 1979
Printed In U.S.A.