|
Appendix A (cont.)
Conclusions
The topographic setting along Crow Creek approximately 1,500 years ago
was similar to that of the present. The P1 terrace and adjoining P1-T2
riser had essentially the same configurations as exist today. This is indicated
by the inclination of buried soil horizons that parallel the modern topographic
surface. Site 48LA277 was situated on a narrow (±15 m), southwest-trending
ridge, the axis and slopes of which averaged 10° in inclination. This
landform was a spur that projected outward from the P1-T2 riser. The "ancestral"
slope that adjoined the spur on the southwest averaged 7° in inclination,
which is slightly greater than the modern, overlying T2 surface which slopes
±4° to the southwest. A southwest-draining, ±10-m-wide,
first-order draw adjoined the ridge on the southeast, while a southwest-draining
swale lay immediately northwest of the site area.
The ground surface was covered by a dense growth of grass, which progressively
covered a succession of thin sheet-like accumulations of CS with the resultant
formation of a Mollisol. The correlative Haplustoll and Argiustoll soils
(members of the Mollisol group), which are present throughout the Crow
Creek Valley, form in a grassland environment (Soil
Conservation Service 1975:303). The pedogenic episode of Mollisol formation
ceased about the same time as the site was abandoned by human occupants.
The Mollisol was subsequently covered by a succession of CS on which an
Entisol is developed.
During the time of human occupation at site 48LA277, the braided channel
of Crow Creek, which was probably similar in size and shape to the modern
channel, "looped" to the northeast and lay only about 50 m from the site,
in contrast to the ±200-m distance that now separates them. With
succeeding time the stream channel continued to "encroach" on the site,
and during the waning stages of human occupation and later, the channel
of Crow Creek lay only ±25 m southeast of the site. A riparian vegetative
environment existed along Crow Creek at the time of human occupation, as
witnessed by the presence of willow, chokecherry, and plum wood in hearths
(Connor 1993).
There is a problem of dating at the site; all 5 radiocarbon dates fall
between approximately 1000 and 1500 B.P. and were secured from within the
Mollisol (Haplustoll). Therefore, accumulation of the 1.5+ m of alluvium
that overlies the Mollisol in Trench 1, the creation of the T1 terrace
and deposition of associated alluvium, and the migration of the channel
of Crow Creek ±150 m to the west, have all occurred in the last
1,000 years. This seems like a short period of time for all of this to
have occurred, but it is possible. Albanese
(1991) presented evidence that the 2-3-tier Holocene terrace system in
the Powder River basin of Wyoming began to form approximately 1400 years
ago and that its initiation coincided with a drop in the regional water
table and the beginning of a major cycle of erosional downcutting. A similar
scenario could have prevailed in the Julesburg Basin. The presence of Late
Archaic-style projectile points at 48LA277 is somewhat puzzling in that
they are usually considered to be older than 1500 B.P. (Frison
1991:34); perhaps the time span in which all of the above listed geologic
events happened is longer than 1,000 years. However, this possibility calls
into question the validity of the radiocarbon dates, without any good reason
to discredit them. More thought may be needed on the matter.
Descriptions of Sediments and Soils
The following descriptions of sediments and soils are based solely on
field observations. No attempt was made to conduct either chemical or grain-size
laboratory analysis. Selected sediment samples were examined under the
binocular microscope. All percentage figures are visual estimates.
The various described lithologic units are numbered sequentially in
descending order. The units do not necessarily correlate between stations
and in most cases they do not. They are used mainly for convenience in
discussion. In cases where units are correlative, it will be specified
in descriptions. All Munsell color designations are for dry samples.
Main Excavation Area - 48LA277
Location E1500-N1497 (Test Pit)
Unit
1
2
3
4
5
(Miocene bedrock) |
Thickness (cm)
2.5
7.6
51
23
23+
|
Description
C soil horizon - sand, loose, (modern); yellowish brown (10 YR4/2),
very fine to granule size (50% of sand fraction is very fine to fine grained),
very poorly sorted, ±25% combined silt and clay content; sand grains
are mainly composed of quartz but some are granite or feldspar; trace of
mica; sand grains are rounded to subangular; ±10% of sample consists
of grass-root fibers. Unit 1 rests on machine-cut surface.
Ck soil horizon - sand, massive; dark yellowish brown (10YR3/2), very
fine to fine grained with secondary amounts of medium-, coarse-, and granule-sized
grains; ±20% of coarse- and larger-sized grains consist of white
feldspar or dark green metamorphic rock; sediment is very poorly sorted;
5-10% of interval consists of 0.5-2.0-cm-long pebbles of granite, feldspar
or metamorphic rock; grass-root fibers are abundant; unit contains high
silt and clay content (±25%). Samples examined under the binocular
microscope contain ±10% dispersed, rounded, charcoal grains that
vary from 0.5 to 1.0 mm in length; basal contact is gradational over 0.5-cm
interval.
Ak soil horizon - sand as above, massive; 5YR2/2 (brownish gray); ±60%
sand content is very fine to fine grained, ±15% is medium grain
sized, while coarse-grained clasts make up the remainder; silt-clay content
is high (estimate ±35%); coarse- and pebble-sized clasts consist
of granite or metamorphic rock. Samples examined under the microscope consisted
of:
(a) 7.6 cm below unit top - sand; ±60% very fine to fine grained,
±15% medium grained, ±10% coarse to granule size; high silt-clay
content; no charcoal;
(b) 15-20 cm below unit top - sand as above; increase in coarse-sized
content (0.5-1.0 mm) to ±15%; granite grains (1-5 mm long) constitute
5% of sample; charcoal grains (0.5-1.5 mm long) constitute ±7% of
sample;
(c) 46-58 cm below top of Unit 3 (interval covers top portion of Unit
4) - sand as above; high silt-clay content; trace of mica; some white CaCO3
cement (Stage I development);
Bk soil horizon - sand, massive; grayish brown (5YR5/2), fine to medium
grained, silty-clayey; ±17% of sand grains are granule size; contains
local concentrations of pebbles (3-7 cm long); 15-25% of sand grains are
composed of granite; Unit displays Stage I CaCO3
development.
Rbk soil horizon - sand, massive; grayish orange (10YR8/44), pre-dominantly
very fine to fine grained; ±5% of sand grains are 0.3-0.5 mm
wide (medium grained); sand is very well sorted; contains white clay
cement; unit is highly weathered, "loose" Miocene bedrock; contains some
"intrusive" fragments of granite at top probably the result of bioturbation. |
All of the above units contain abundant krotovina (rodent holes), which
are 18-25 cm wide and 25-55 cm long. All units are calcareous and react
vigorously with dilute HCl. Units 1-4 are Holocene CS and are very poorly
sorted. They all contain an estimated 25-50% clay-silt content. Sand grains
within Units 1-4 vary from rounded to angular in shape. Unit 1 is historic
in age while Units 2-4 accumulated during the Late Holocene.
Stratigraphic Section at E1499-N1499 (Main Excavation Area 48LA277)
Unit
1
2
3
4
5
(Miocene bedrock) |
Thickness (cm)
2.5
5.1
76
25.4
20+ |
Description
C soil horizon - sand, loose, (modern).
Ck soil horizon; same lithology as Unit 2 at E1500-N1497. Basal contact
gradational over 9 cm.
Ak soil horizon; same lithology as Unit 3 at E1500-N1497.
Bk soil horizon; same lithology as Unit 4 at E1500-N1497. "Patchy" gray
and light brown color due to high degree of rodent burrowing.
Rbk soil horizon, highly weathered; Units 3-5 are highly bioturbated
(krotovina); rodent burrows are 7.6-36.0 cm long; some are horizontal.
Units 4 and 5 "merge" to southwest at E1500-N1497. Both have gray color
due to bioturbation. |
Stadia Station 46
Unit
1
2
3 |
Thickness (cm)
5
30
5+ |
Description
Sand, loose, (modern) - very fine to fine grained, ±10% coarse
to very-coarse-grained size, 5-10% granule-sized grains; few ±0.5
cm long clasts; very poorly sorted; none to weak reaction with HCl.
Ck soil horizon - sand, massive; very fine to fine grained, tr. medium-grained-size,
±5% very coarse grained, silty, ±15% clay; poorly sorted;
upper 18 cm contains ±10% randomly distributed, rounded granule
and pebble sized (2-4 cm) fragments composed of granite.
Ak soil horizon - sand; brownish gray, cumulic soil horizon. |
Backhoe Trench 1 (Figure A9)
Stadia Station 45; surface is machine cut.
Unit
1
2
3
4
|
Thickness (cm)
13
32
91
62 |
Description
Sand, loose (modern).
Ck soil horizon - sand, massive, arkosic; very fine to fine grained,
very well sorted, <10% randomly distributed pebbles (0.5-2.0 cm long),
silty, clayey; slight reaction with HCl.
Ak soil horizon - sand, massive; gray, very fine to fine grained, ±2%
coarse to granule-sized grains, ±40% pebbles (>0.5 cm); poorly sorted;
silty, clayey; pebbles are coated by thin film of CaCO3. Top
of Cultural Feature 10 (hearth) lies 33 cm below top of Unit 3. Feature
10 yielded a radiocarbon date of 1160 ± 90 B.P. (Beta-56859). The
wood charcoal was identified as aspen and birch. The contact between Units
3 and 4 is transitional over an 8-cm interval and is marked by a matrix
change from fine to coarse sand.
Bk soil horizon - sand, massive; matrix is mainly composed of coarse
to granule-sized sand grains; 10-20% of sand grains are very fine to fine
in size; contains silt and clay fractions; ±30% of unit consists
of randomly distributed pebbles (0.5-2.0+ cm long). |
Stadia Station 43; surface is machine cut.
Unit
1
2
3
4
5
(Miocene bedrock) |
Thickness (cm)
0.5
0
70
71
23+ |
Description
Sand, loose, (modern).
Unit 2 has been removed by machine blading.
Ak soil horizon - sand, massive; gray; very fine to medium grained,
poorly sorted, clayey, silty; ±25 percent of unit consists of randomly
distributed pebbles (0.3-2.0 cm long); thin CaCO3 film on base
of pebbles. There is a 10-cm-thick color transition between Units 3 and
4.
Bk soil horizon - sand, massive; very fine to coarse grained, poorly
sorted, clayey, silty; ±30% of unit consists of randomly distributed
pebbles (0.3-2.0 cm long); portions of unit have faint laminated appearance;
Stage II CaCO3 development in lower 10 cm. The northeast edge
of a hearth (Feature 9) lies 9 cm southwest of Stadia Station 43. The dish-shaped
hearth lies at the top of Unit 4 and in profile measures approximately
69 x 15 cm. It yielded a radiocarbon date of 1470 ± 60 B.P. (Beta-56858).
R soil horizon - sand, coarse to very coarse grained; fair to very well
sorted, displays horizontal and cross-laminae (1-2 mm thick) plus planar,
low angle (<5° ) cross-beds and sets of high angle (10-20° )
cross-beds (15-20 cm thick); slight reaction with HCl in upper 16 cm; sharp
contact between Units 4 and 5. |
Stadia Station 43B; surface is machine cut.
Unit
1
2
3
4
5
(Miocene bedrock)
|
Thickness (cm)
5
13
60
37
43+ |
Description
Sand, loose (modern) - massive; brown (5YR 4/2), very fine to fine grained,
some medium- and granule-sized grains, clayey, silty.
Bk soil horizon - sand, laminated; pale brown (10YR 7/2), very fine
to fine grained, very well sorted, silty, clayey, few random pebbles (<5%),
0.5-2.0 cm long; CaCO3 film on bottom of pebbles.
Ak soil horizon - sand, massive; grayish brown (5YR 3/2), very fine
to fine grained, silty, clayey, ±10% granule to 0.5-cm-sized fractions,
<5% 1-7-cm-long pebbles, rounded; horizontal platey pedogenic structure
in upper 15 cm; some blocky (rectangular) structure 15 cm below top of
unit; thin CaCO3 film at base of pebbles; moderate reaction
with HCl.
2Bk soil horizon - sand, massive; yellowish brown, (10YR 6/4), very
fine to fine grained, ±10% medium-grained fraction, <5% pebbles
that are </= 0.5 cm long; silty, clayey, few 2.5-cm-long pebbles (long
axis parallel to base of unit) in lower 20 cm of unit, these pebbles are
completely coated with thin film of CaCO3; strong reaction with
HCl.
Rbk soil horizon - sand, massive; yellowish brown (10YR 6/4), coarse-grained;
±20% fine-grained fraction; ±10% granule to 0.5-cm-long fragments
(rounded to subangular), few 2-4-cm-long pebbles; silty, very clayey, local
CaCO3 cement; strong reaction with HCl. |
Units 4 and 5 are similar in appearance and difficult to distinguish.
The division between them is drawn on the basis of dispersed pebble zone
in Unit 4. Unit 5 (bedrock) is highly weathered.
Stadia Station 41; surface is machine cut.
Unit
1
2
3
4
5 |
Thickness (cm)
7.6
20
66
55
14+ |
Description
Bt soil horizon - sand, massive; brown, fine grained, well sorted, clayey,
silty; ±5% medium-sized grains; 0-5% randomly distributed pebbles
(±0.5 cm long), silty, clay; weak continuous clay skins on breaks.
Bk soil horizon - sand, massive; fine to medium grained, ±15%
coarse grained to granular size silty, clayey; <1% 0.5-1.5-cm-long pebbles;
CaCO3 cement; strong reaction with HCl.
Ak soil horizon - sand, massive; dark gray color, fine to medium sized,
fair sorting, silty, clayey; <5% ±0.5 cm long pebbles, randomly
distributed; contains 5-x-3-cm lens of coarse- to granule-sized particles;
unit has "fluvial" appearance, "hint" of laminae.
2Bk soil horizon - sand, massive; "cream" color, fine-grained, well-sorted,
<5% coarse- to granule-sized grains, ±50% clay/silt content;
few CaCO3 filaments fill ±1 mm wide grass-root molds,
strong reaction with HCl.
R horizon - Bedrock. |
Stadia Station 39; surface is machine cut.
Unit
1
2
3*
4
5
6
7
8
9
10
11
12
13 |
Thickness (cm)
5
18
18
13
16
8
23
18
20
9
6
65
10+ |
Description
Sand, loose.
Abkt soil horizon - sand, massive; very fine to fine grained, with ±15%
pebbles (0.3-1.5 cm long); silty, clayey; blocky structure (2-4 cm spacing);
faint continuous clay skins on peds; good reaction with HCl; colluvium.
Abk soil horizon - sand, massive; very-fine to fine-grained, well-sorted,
±10% rounded, granule-sized grains; clayey, silty; CaCO3
cement and CaCO3 nodules up to 5 mm long; colluvium.
Ak soil horizon - sand; very fine to medium grained, well sorted; silty,
clayey; <5% pebbles, rounded, ±0.5 cm long; fluvial sediment.
Bk soil horizon - sand lenses; fine grained, silty, clayey, with ±20%
coarse-to granule-sized fraction, ±10% randomly distributed, 0.5-2-cm-long
pebbles (coated with thin film of CaCO3); some "patchy" CaCO3
cement.
Ck soil horizon - lithology as above.
2Bk soil horizon - sand; very fine to very coarse grained, very poorly
sorted, <5% scattered pebbles; some laminae (±1 mm) and segregation
of medium or fine grains along 0.3-0.5-cm-thick "horizons"; contains some
CaCO3 nodules and filaments; fluvial sediments.
2Ak soil horizon, fluvial sediment.
Ck soil horizon, fluvial sediment.
3Ak soil horizon, fluvial sediment.
2Ck soil horizon, fluvial sediment.
4Ak soil horizon, colluvium.
R soil horizon - sandstone (bedrock). |
Unit 11 correlates with Unit 3 at Stadia Stations 41-45. All Abk and
Ak horizons are Fluvents with a faint gray cast.
Stadia Station 39A; surface is machine cut.
Unit
1
2
3
4
5
6
7
8
9
10
11
12
13 |
Thickness (cm)
5
13
18
15
8
18
8
15
10
20
15
50
41+ |
Description
Modern sediment - sand, loose, massive; fine grained; ±15% granule-sized
fraction; trace of medium-sized grains, one pebble (1 cm long).
Ck soil horizon - sand, massive; very fine to fine grained, silty, clayey;
±3% scattered, rounded pebbles (±0.5 cm); contains grass-root
molds (±1 mm wide); slight to no reaction with HCl.
Abk soil horizon - sand, massive; light gray mottling; fine to medium
grained, well sorted, clayey, silty; ±3% scattered pebbles (0.5
cm long); a 3-cm-thick, light gray, horizontal band in center of unit;
some CaCO3 cement; few CaCO3 nodules (1-2 mm long).
2Ck soil horizon - sand, massive in upper half; "hint" of 0.5-cm-thick
layers; fine grained, well sorted, clayey, silty; <1% ±0.5 cm
long pebbles; moderate reaction with HCl.
2Ak soil horizon - lithology as in Unit 4.
3Ck soil horizon - lithology as in Unit 4, ±1 cm thick bedding
with laminae; <1% pebbles.
3Ak soil horizon - pebble lens; consists of ±15%, randomly distributed,
0.5-1.5-cm-long pebbles in medium- to coarse-grained sand (some fine grains);
±15% granule-sized grains; silty-clayey; "hint" of bedding and 1-2-mm-thick
laminae; poorly sorted; no grading of pebble size.
4Ck soil horizon - sand, massive; fine-grained, very-well-sorted, >5%
medium and granule-sized grains; few, thin pebble lenses (<60 cm long)
within unit; however bulk of unit is sand with only minor amounts of pebbles;
good reaction with HCl.
4Ak soil horizon - sand as above.
5Ck soil horizon - composite gravel lens; 213 cm long, 1 m of which
extends to southwest; contains some 1-2-cm-thick lenses of very-fine to
fine-grained, silty, clayey sand that are interbedded with 3-6-cm-thick
pebble lenses; moderate reaction with HCl.
5Ck2 soil horizon - sand, laminated (±1 mm), very fine to fine
grained, well sorted, silty, clayey; contains ±15% randomly distributed
very-coarse to granule-sized grains and <1% ±0.5 cm long rounded
pebbles; moderate reaction with HCl.
5Ak soil horizon - sand, massive; very fine to fine grained, clayey,
silty with ±5% very-coarse to granule-sized granite grains and ±5%
randomly scattered pebbles (±1 cm long); moderate reaction with
HCl.
R horizon - sandstone, soft; massive fine grained, very well sorted,
silty, clayey, arkosic. |
Unit 12 is colluvium and correlates with Unit 3 in northeast portion
of Trench 1.
Stadia Station 38; surface is machine cut.
Unit
1
2
3
4
5
6 |
Thickness (cm)
5
15
19.5
11
80
55+ |
Description
Modern sediment - sand, loose; brown (5YR3/4), fine- to medium-grained,
some granule-sized fraction (±10%), silty, clayey; sediment; rests
on machine-cut surface.
Btk soil horizon - sand (fluvial); brown (10YR4/4), laminated, predominantly
fine grained but grain size varies between very fine to medium; contains
±5, rounded, granule-sized fragments of granite and quartz; displays
horizontal, pedogenic, platey structure (2 mm thick), weak continuous clay
skins on some breaks; moderate to strong reaction with HCl, some faint
CaCO3 film on base of granules.
Ck soil horizon - moderate reaction with HCl; fluvial beds as follows:
(a) 2-cm-thick gravel lens; (2.6 m long), ±70% elongate, granite
pebbles (±2 cm long) in fine to coarse-grained, poorly sorted sand.
(b) 3.5-cm-thick sand; yellowish brown (10 YR5/4), massive, fine to
coarse grained (lower), silty, clayey, poorly sorted.
(c) 2-cm-thick gravel; ±25%, rounded, granite pebbles (±1%
cm long) in sand, fine to coarse grained (lower), some granule-sized grains,
poorly sorted; lens is 1 m long.
(d) 12-cm-thick sand interbeds (1.0-1.5 cm thick); composed of sand,
massive, very fine grained, silty, clayey, well sorted, or sand, fine to
coarse grained (predominantly medium sized), poorly sorted, some ±0.5%
cm long pebbles; "hint" of laminae.
Ak soil horizon - sand; grayish brown (10YR3/2), fine to medium grained,
±5% coarse grained, some silt, clayey (±15%); ±5%
2-6-mm-long granules of granite; poorly sorted; moderate reaction with
HCl; some pebbles have thin film of CaCO3 on bottom.
2Ck soil horizon - 4 gravel (arkosic) lenses; 2.5-17.0 cm thick, the
thin (2.5-5.0 cm) gravels are 30-70 cm long; the thickest lens is 5.94
m long; sand lenses occur between gravels, consist of brown (10YR4/2),
very-fine to fine-grained, well-sorted sand; contain some medium-grain
fraction (±10%), few 2-3-mm, granule-sized grains, clayey (±20%),
unit also contains thin "isolate" lenses that are composed of loose, medium-grained,
well-sorted sand; discontinuous laminae 1-2 mm thick are present in sand
lenses that lie between gravels. Unit 5 reacts moderately with dilute HCl.
2Ak soil horizon - sand, massive; grayish brown (5YR3/1), very fine
to fine grained, silty, clayey with 1-15%, granule- to pebble-sized (0.4-3.3
cm) fraction, pebbles are randomly oriented, rounded, elongate, and mainly
composed of granite; sediment has moderate reaction with dilute HCl; faint
CaCO3 film on bottom of pebbles. |
Units 1-6 correlate with same numbered units that are present at Station
39B. Unit 6 correlates with Unit 3 at Stadia Stations 41-45.
Stadia Station 38A; surface is machine cut.
Unit
1
2
3
4
5
6
7 |
Thickness (cm)
5
13
13
10
93
36
41+ |
Description
Sand, loose (modern) - same lithology as at Station 38.
Btk soil horizon - same lithology as at Station 38.
Ck soil horizon - same lithology as at Station 38.
Ak soil horizon (Fluvent) - same lithology as at Station 38.
2Ck soil horizon - same lithology as at Station 38.
2Ak soil horizon - (weak) sand, massive; mottled pale gray; very fine
to fine grained, silty, clayey.
2Ak2 soil horizon - sand; gray, laminated (laminae ±2 mm thick
and discontinuous), very fine to fine grained, silty, clayey. |
Units 1-5 correlate with same numbered units present at Stadia Station
38. Units 6 and 7 are equivalent to Unit 6 at Station 38.
Backhoe Trench 2
In Trench 2, 60-110 cm of "reworked" sandy, pebbly colluvium overlies
±150 cm of exposed Miocene bedrock. The colluvium is not in place
but was "scraped up," redeposited, and compacted by machinery. The Mollisol
(Unit 3) present in Trench 1 has been "redeposited" by machinery in Trench
2. Bedrock exposed in the trench partially consists of ±100 cm of
arkosic coarse-grained, "gravelly" sand with included pebbles (0.5-2.0
cm long), both isolated and in horizontal layers. This facies is overlain
by ±50 cm of sand, very fine to fine grained, very well sorted,
silty with ±40% clay content; contains sets of planar, cross-beds
composed of coarse sand plus 5-18-cm-thick, 1.2-1.5-m-long gravel lenses.
The bedrock sediment was deposited in a fluvial environment.
 Appendix
A (cont.)
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