GLACIAL GEOLOGY 
423 
careous materials, of uniform texture and yellow color. An upper 
section, of irregular thickness, from 2 to 20 feet, is notably limy, 
white, and more or less indurated in certain layers. The white 
marly upper capping is sharply separated from the yellow lower 
beds by an irregular line of juncture which has every appearance 
of being a marked plane of unconformity. 
The common historical interpretation of the general section is 
briefly this: The lower yellow beds are regarded as representing 
river silts deposited in the lake over a very long period of time 
when an early Bonneville w'ater-level was nearly as high as the 
later Bonneville shore-line. The white marly beds are depositions 
of a shorter high-water stage of the Lake. The irregular line 
between the white and yellow sections is viewed in the light of an 
unconformity, the interval represented being a stage between two 
high-water marks when the old lake-water completely dried up. 
Early Bonneville yellow beds are correlated in time with a first 
epoch of humidity superinduced by conditions of glaciation, while 
the white later Bonneville beds belong to the second Glacial epoch. 
The two parts of the section are thus represented as being separat¬ 
ed by an erosional interval of long duration, occupying a time 
between two epochs of large rainfall and notable ice-forming. 
Two features in particular militate strongly against these de¬ 
posits either being normal stream-silts, or being laid down during 
two distinct epochs separated by a long epoch of excessive dry¬ 
ness. This simpler and very different interpretation for the phe¬ 
nomena presented does not postulate violent and frequent changes 
of climate. It appeals to no other than the ordinary climatic 
conditions and geologic processes that prevail today in the region. 
It takes into account only the familiar geological activities of the 
desert. 
Close examination of the deposits discloses the fact that they are 
not typical stream silts, but that they have a grain very much 
coarser. In size the individual particles appear to‘be about mid¬ 
way between those of normal clay and fine sand. Although ob¬ 
scurely laminated the material in all physical aspects seems to be 
essentially loess, or adobe. Thus, instead of being normal river 
silts swept into still water these deposits really represent dust 
borne by the wind from the neighboring deserts that have dropped 
