Page 25 - blm_stickelweinmanroberts1980

year ago range. The classic American Pleistocene chronology recognized
four major ice Incursions: the oldest being the Nebraskan, followed by
the Kansan and Illinoian and terminating after the Wisconsin. In the
meantime, it has been generally recognized that each ice age is a complex
sequence of alternating changes of cold and amelioration of varying de-
grees. The exact chronology of the first three glacial periods is far
from settled inasmuch as geo-chronologic tools and suitable samples are
sadly missing. There is, however, general concensus about the events of
the last 100,000 years which make up the Wisconsin glacial. Until now
radiocarbon dating has been generally possible back to about 40,000 to
50,000 years. Therefore, the prehistory of the Mojave Desert will be
described for that time-range.

For example, typical glacial conditions on the earth existed about
18,000 years ago when the last massive long-term cold spell occurred. The
National Science Foundation during the International Decade of Ocean Ex-
ploration published under its Climap Program a map of climatic conditions
which shows the initial absence of deserts in the American Southwest, in-
cluding the region of the Mojave. Precisely the same results were obtained
in a study by Wells and Berger (1967; see Fig. 1 ) on the presence of late
Pleistocene coniferous woodlands in the Mojave Desert. In this study,
plant remains in packrat (Neotoma) middens were analysed botanically and
simultaneously dated by radiocarbon. As a result, it was determined that
the Mojave was essentially covered by woodland consisting of pihon pine
and juniper due to the tree-line reaching about 600 meters lower. In
another study, King (1976) explored similarly the late Lucerne Valley
Region of the Mojave Desert. The advantages of Neotoma middens containing
macrofossil plant remains lie in the fact that they are found in situ.
Packrats are known to roam only over an area of approximately a few square
kilometers in their lifetime. Thus, the samples are truly representative
of their immediate past geographic environment.

On the other hand, pollen studies have aided prehistoric environmental
reconstruction immensely. Yet the analyst must carefully exclude pollen
blown in by the wind over distances of hundreds of kilometers or washed
down with alluvium from higher vegetation zones to lower elevations. A
major discussion of the Pleistocene pollen record and biogeography of the
American Southwest including the Mojave has been published by Martin and
Mehringer (1965) , in a more regionally specific study by King (1976) and
most recently by Mehringer and Sheppard (1978) . These publications contain
very extensive relevant pollen diagrams and lists which describe in detail
the plant communities of the past.
Despite some inherent difference, macroscopic and microscopic pre-
historic plant analyses agree in their conclusion on the Pleistocene vegeta-
tion cover of the Mojave. A notable difference is found in the level of
depression of the pinon- juniper line held by pollen analysis to be about
1000 meters, whereas Neotoma middens call for about 600 meters or so. But
in the end, both approaches postulate a pifion- juniper woodland cover over
most of the Mojave Desert during the late Pleistocene.
Looking up from the Pleistocene desert toward the San Bernardino
Mountains one would have seen seven valley glaciers (Sharp, Allen and Meier
1959) . Apparently they accumulated at altitudes above 3000 meters on the
north flanks of the highest peaks of the San Bernardino Range and descended

15