46 LAKAMIE BASIN, WYOMING. 
alluvial plains along the present valleys and represent the undulating 
surface of the basin at various stages of topographic development 
earlier than the present. Possibly some of the higher deposits, such 
for instance as that occupying a wide area northeast of Medicine Bow, 
consist of remnants of Tertiary beds. The deposits are thickest and 
most extensive, however, in the vicinity of Laramie River and were 
laid down by that stream and its branches at no very remote period of 
geologic time. Their distribution is not shown on the map because 
of their intricate and indefinite boundaries. 
ALLUVIUM. 
All the larger streams in the Laramie Basin now through wide val- 
leys filled with alluvial deposits varying in thickness but in many 
places 50 to 60 feet deep. In general they are widest in the 
outcrop zones of the softer rocks and they become narrower, thin, and 
discontinuous in the granite areas, notably along the canyon of 
Laramie River. The widest areas are along Laramie and Little 
Laramie rivers, west and southwest of the city of Laramie, but wide 
alluvial flats also extend up Rock Creek from the railroad to the foot 
of the mountains, and there are alluvial areas of moderate width along 
Medicine Bow and Little Medicine Bow creeks and some of their 
branches. 
STRUCTURE. 
Structurally the Laramie Basin region is a broad syncline trending 
north and south with gentle dips on the east side and steep dips or a 
great fault on the west side. This broad trough is traversed by a 
number of anticlines which start in various directions from the border 
of the basin and die out toward its middle. In the region about 
Medicine Bow these flexures are numerous and cause much com- 
plexity in the structure. The five sections in Plate VIII illustrate 
the principal structural features. 
LARAMIE MOUNTAIN SLOPES. 
The strata lying along the western slope of the Laramie Mountains 
and constituting the summits near Laramie have a general westerly 
dip at moderate angles, averaging 3° near Red Buttes, 4° near Lar- 
amie, 6° east of Wyoming, 14° near Sybille Creek, 4° near McGill, 5° 
near Marshall, and 6° near Little Medicine. The strike is uniformly 
north and south for many miles near Laramie and north-northwest 
and south-southeast from North Laramie River northward, except 
in an area north of Boswell Spring, where an anticline deflects the 
strata to the west, and for a short distance from Pilot Hill north- 
ward, where a small anticline causes a slight offset in the ridge. A 
short distance south of Sybille Canyon the strata are flexed and 
broken for a mile or more. On the great monocline on the western 
