Late Glacial Environments and Paleoecology at Blackwater Draw ...

82 CHRISTOPHER L. HILL, FRED WENDORF, PAUL B. SEARS AND EDNA PAPAZIAN
while there are higher proportions of Artemisia, Graminae,
Compositae and the Cheno-Ams in the younger
section of the zone. Very low amounts of certain taxa
are not shown on the simplified pollen diagram, including:
Cyperaceae, Rosaceae, Typha, Polygonum,
Rhamnaceae, Plantago, Caryophllaceae, Guttiferae,
Haloragidaceae, Umbelliferae, Ranunculaceae, Rhus,
Anacardiaceae, Moraceae, Sparganum, Potamogeton,
Loranthaceae, Corylus, Rumex, and Cuscuta.
The mollusc assemblage from unit B2 at the nearby
Highway Site (LA 3325) contains freshwater bivalves
(Pisidium casertanum and P. casertanum), aquatic
snails (Gyraulus cf. circumstriatus) and a terrestrial
snail (Vallonia cf. cyclophorella) (Drake 1975:211).
Overlying unit B2 at the Barrow Pit is a facies of
unit D with an age of about 10,600 B.P., based on radiocarbon
measurements of shells (Haynes 1975). Seven
samples from this stratum were analyzed for pollen.
Arboreal pollen is dominated by Pinus; it constitutes
from about thirty to nearly sixty percent of the identifiable
pollen. Other arboreal pollen present in small
amounts include Betula, Quercus, and Picea. There
are minor occurrences of Ulmus, Celtis, Juglans, Platanus,
Acer, Ostrya, Carya, Nyssa, Fraxinus, and Castanea
not shown on Fig. 3. Non-arboreal pollen is
dominated by Ambrosia (about 20% of identifiable
pollen) in the lower section of unit D and Artemisia
(around 30% of identifiable pollen) in the top section.
Compositae, Graminea, and the Cheno-Ams are persistent
throughout unit D. Low frequencies of Rosaceae,
Typha, Polygonum, Rhamnaceae, Caryophllacae, Hypericum,
Myriophyllum, Urtica, Araliaceae, and Cruciferae
(= Brassicaceae) also occur.
At the nearby Highway Site (LA 3325) molluscs
probably from unit D (Haynes 1975) include Sphaerium
sulcatum and Helisoma anceps, suggesting permanent
water, although the presence of Promenetus
exacuous, Helisoma trivolvis, Stagnicola cf. exilis, Planorbula
cf. armigera, and Gyraulus cf. circumstriatus
suggest less permanent water (Drake 1975:209). Some
of the molluscs indicate cooler temperatures than
present (Promenetus exacuous, Gyraulus cf. circumstrates,
Armiger cistra, and Pupilla muscorum). Molluscs
typical of woodland conditions include: Vertigo
ovata, Nesovitrea electrina, Euconulus fulvus, and Vallonia
cf. cyclophorella. The overall molluscan assemblage
appears to reflect ecological conditions associated
with grassy or open woodlands (Drake 1975:209).
A lacustrine sediment from LA 3325/Locality 7 contained
Pinus, Picea, Artemisia, Tubuliflorae, Liguliflorae,
Gramineae, and Chenopodiaceae (Schoenwetter
1975). Pine values reach 35%, while Artemisia values
reach 25%. This sample has been considered as representing
the Tahoka Pluvial (the local equivalent of the
Last Glacial Maximum) or an older climate episode
(Oldfield and Schoenwetter 1975).
OTHER PALEOECOLOGICAL EVIDENCE
FROM UPPER BLACKWATER DRAW
Blackwater Draw Locality No. 1 (Clovis gravel pit;
LA 3324, Locality 12, Hester 1975; Haynes 1975). This
locality has been extensively studied because it contains
a stratigraphic sequence with Clovis, Folsom, and other
artifacts as well as Rancholabrean faunal remains
(Hester 1972). It is situated about 2 miles (3.2 km)
north of the Barrow Pit locality.
There are several dates from deposits stratigraphically
below the speckled sands or unit B deposits at
the gravel pit that appear to constrain the age of the
stratigraphic sequence at the Barrow Pit. Lacustrine
carbonates attributed to the Tahoka Pluvial (designated
stratum B in Holliday 1995) are younger than about
21,100 B.P. (SMU-2533), and other ages of 22,930 B.P.
(AA-7093) and 17,220 B.P. (AA-7092) (Haynes 1995)
limit the age of overlying sediments (cf. Haynes 1975,
Unit B1; also termed the “speckled sand” or the “gray
sand” cf. Antevs 1949, Hester 1972). These ages imply
that the unit B1 and B2 deposits from the Barrow
Pit date to the late Last Glacial Maximum or the early
part of the Late Glacial.
Samples collected from the gravel pit in the 1930s
were submitted to P. Sears for analysis. The samples
contained pine pollen that Sears concluded may have
been present due to long distance transport (Hester
1972:24). Where pollen was preserved in unit B1
(“gray sand”) taxa included Pinus, Ulmus, Artemisia,
Graminae, and Chenopodiacea (Schoenwetter 1975).
Fauna from the “gray sand” contains large and small
vertebrates. In terms of ecological interpretations, the
large vertebrates typically provide regional-scale information
while smaller vertebrates provide information
on local conditions. Carnivores include Canis lupus,
C. latrans, C. dirus, Vulpes velox, Smilodon californicus.
Other large vertebrates include Platygonus,
Camelops, Tanupolama macrocephala, Bison antiquus,
three species of Equus (E. conversidens, E. niobrarensis,
E. scotti), and Mammuthus. Stable isotope measurements
of Bison indicate the regional presence of
C4 (warm climate) grasses, but Mammuthus isotopes
displayed two distinct patterns (Connin et al. 1998;
Hoope 2004). Some of the samples appear to reflect
cooler glacial conditions while other measurements
reflect an increase in warmer climate grasses (Connin
et al. 1998; Hoope 2004).
Some fossils are from extinct animals; their value
as direct environmental indicators is not as strong as
remains from animals that are still living and thus have
climate and range tolerances that can be measured.
The presence of Ondatra zibethica indicates the presence
of permanent water and more evenly distributed
annual rainfall, while the extinct Terrapene canaliculata
may imply some wooded areas (Lundelius 1972).