24 BULLETIN 16 9, Lt^ITED STATES NATIONAL MUSEUM 



Stone and Calvert and others have given other sections, but they 

 are too generalized or include too many unexposed parts to be very 

 helpful. 



The Fort Union No. 3 beds (see pi. 6) are a great mass at least 4,000 

 feet thick, and possibly as much as 6,000 if the highest strata of this 

 series in the Crazy Mountains be included. Determinable fossils are 

 known only from the lower 3,000 feet, and this is the part to wliich 

 attention has been particularly directed. In spite of the great thick- 

 ness and heterogeneity, there is no convenient lithologic or paleonto-^ 

 logical basis for subdividing the beds. They consist of shales and sand- 

 stones, most of them lenticular and highly variable. The usual topo- 

 graphic expression is a series of ridges on the sandstones and valleys 

 on the shales. The shales, particularly at the mammal localities, do 

 not differ greatly from those of the No. 1 and No. 2 in appearance, but 

 the sandstones are lighter in color and are generally more resistant than 

 those of the No. 2. Since the shales are poorlj^ exposed, the general 

 impression is of a much paler formation than the underlying Lebo. 

 The massive basal No. 3 sandstone is the most continuous and prom- 

 inent horizon marker in the Fort Union of this field and has been 

 remarked by everyone who has worked here. It almost everywhere 

 forms a prominent scarp, and its resistance to erosion is the cause of 

 the elevation of Bear Butte, Lion Butte, and numerous less notable 

 hills and escarpments. 



The base of this sandstone forms a natural division plane throughout 

 the field and is evidently an erosional disconformity, although it is 

 unlikely that it marks any considerable time gap. The sandstone 

 tends to become less massive in the southern part of the field toward 

 the south end of Lion Butte and where it swings westward to the 

 mountains, and it is here generally more platy and formed by numerous 

 thin beds with shale partings but can be traced continuous^ almost to 

 the mountains. 



The fiuviatile nature of these beds and the presence of numerous 

 channel and flood sandstones make it possible that there are discon- 

 formities at almost any level, but none can be detected as of any regional 

 significance, and the whole series seems to be essentially continuous 

 and without any noteworthy or sudden change in general type of 

 sedimentation. 



The base of the No. 3 is further made noticeable, especially from 

 the air, by almost everywhere supporting a growth of evergreens 

 (pis. 3, 5). This is the more noteworthy because the sandstones of 

 the No. 1 beds, even where almost identical with those of the No. 3 

 in topography and elevation, do not support any trees, and the Lebo 

 in general is almost devoid of large vegetation except for cottonwoods 

 along the stream courses. 



