48 WILLIS LT. LEE 
eral hundred feet deep, but further to the south and east it 
widens to a broad gently inclined plain, bordered by escarp- 
ments 500 to 1,000 feet or more in height. The preservation of 
the escarpments is due principally to their capping of Dakota 
sandstone, but in some places extrusive sheets. of lava form the 
surface rock. The canyon walls were examined at intervals as 
far south as Bell Ranch. From Canyon Largo, eastward, I fol- 
lowed along the base of the northern escarpment continuously for 
about thirty miles. Sections were studied, in more or less detail, 
at Mill’s Ranch and at the toll road crossing, and at several points 
east of Canyon Largo. A detailed section taken at the edge of 
the escarpment north of Bell Ranch (Sec. 4) may be taken to 
represent the structure of this region. The thickness of the cap- 
ping sandstone was not taken. It is the surface rock, and its 
thickness varies, due to surface erosion. Its original thickness 
does not seem to differ materially from that of the Dakota as 
described for the plains region in general. It was estimated at 
something over 300 feet. Its character differs in no obvious way 
from that of the Dakota described in other places. It was traced 
with little interruption from the Rio Cimarron to the Canadian 
and throughout the region examined. It is massive, quartzitic, 
slightly conglomeratic, and being so much more resistant than 
the shales and softer sandstones beneath, always forms escarp- 
ments in the regions where it has been trenched by the streams. 
The lower parts of the canyon walls are made up of Red 
Beds. No attempt was made to study these further than to 
determine their relation to the overlying shales. A thickness of 
several hundred feet of the Red Beds is exposed where the 
canyon is deepest. They are deep red to purple, easily disinte- 
grated sandstones and shales except the upper 50 to 100 feet or 
more in thickness. This upper series is composed of sandstones 
which differ so materially from the strata beneath that they may 
be considered as possibly representing a separate formation. 
They are massive, red to light pink sandstones, and form a con- 
spicuous cliff throughout the length of the region examined. 
Between the Red Beds and the Dakota occurs a shale and 
