464 CHARLES C. MOOK 
sediments of the Sundance formation were laid down; following 
this the sea retreated, the retreat taking place along with final 
Jurassic folding in the Sierra Nevada region. Over the plain 
exposed by the retreat of the Sundance sea the Morrison sediments 
were spread out in the form of a very broad, very flat alluvial fan 
from west to east. This fan must have been crossed by many 
large streams, dotted with lakes, large and small, and characterized 
by an interlacing type of drainage, much after the manner of the 
great alluvial plains of Eastern China at the present time. The 
plain must have been low, and the streams crossing it must have 
been characterized, for the most part, by a low gradient. Locally, 
especially in the western areas, there may, in fact must, have been 
some deposition in relatively swift currents, as indicated by the 
cross-bedded sandstones. These sandstones being rather fine- 
grained for the most part and never conglomeratic, true torrential 
conditions were probably not present in any part of the Morrison 
area so far studied. The round sand grains, associated with aeolian 
type of cross-bedding and sudden variations in thickness, indicate 
that wind deposition was also a factor in the gradual building up 
of the Morrison sediments. The presence of unaltered or little 
altered volcanic ash indicates that volcanic activity must have been 
going on somewhere in the region. Over a plain as broad and flat 
as this one must have been material could not have been transported 
rapidly from the original source to the outer limits of the area of 
sedimentation. The sluggish streams of the plain must have 
deposited material and later picked it up again and carried it 
farther very many times before a selected lot of sediment could have 
reached the outer margins. This will account for the greater 
relative abundance of finer clays in the eastern areas. ‘The inter- 
lacing stream, lake, and swamp conditions on such a plain would 
readily admit of rapid shifting of the courses of streams, areas 
which were at one time stream beds constantly changing to inter- 
stream areas, and the reverse. This would result in the slow, 
gradual shifting of material outward. The end result would be the 
product of alternate deposition and erosion, erosion and deposition, 
for a long period of time, the material being very slowly worked 
eastward. In some such manner as this the relatively thin sheet of 
