246 Tertiary. 



quite precipitous sides. Numerous joints occur in many of tlie strata, 

 particularly in the more compact kinds, and fine examples of concre- 

 tionary structure or weathering jire not rare. The tendency of the 

 thick beds of marly sandstone on the banks of Green river, at the 

 crossing, to weather spheroidally, is ver}' noticeable, and this is repeat- 

 ed in various degrees in the ai'gillaceous and calcareous rocks as well. 



The Bridger Group, though succeeding the Green River Group, is 

 closely related to it, for the transition from one to the other is not 

 abrupt, either in the structure of the beds or their contents. The 

 Group is exposed at the surface over a considerable extent of country, 

 northward and eastward from Fort Bridger as far as Little Sandy 

 river and beyond, forming the top laj'ers of numerous isolated buttes. 

 During this epoch it is probable that the land was covered with fresh 

 water in a lake as large as in the previous era, if not more extensive. 

 The beds are mainly composed of dull-colored, indurated clays, and 

 arenaceous layers of considerable thickness, the latter usually brown- 

 ish, or dull yellow or gray, often with more or less of a concretionary 

 structure. The clays are generally compacted, but they become dis- 

 integrated upon exposure to the atmosphere, and readily yield to the 

 eroding forces. Some thinner layers of more calcareous material, with 

 silicious seams, often affording interesting concretions, are interspersed, 

 but they are rather exceptional than otherwise. The Green river and 

 Bridger Groups are readily distinguished by the effects produced by 

 erosion. The former presenting nearly vertical cliffs, so that the im- 

 pression in crossing the country where it forms the surface rock's is 

 that of traveling over an ordinary plain with occasional descents, by a 

 succession of terraces, to the narrow valleys of the streams. On the 

 contrary, where it is concealed, or only occasion all}^ capped by the 

 Bridger Group, the country is very irregular, often simulating the 

 " Bad Lands;" the beds of the latter being eroded without complete de- 

 nudation, so that they stand out in buttes, or rude architectural forms. 



The deposits in the Yellowstone Lake basin, and in the valley of the 

 main river and its tributaries, which may be regarded as Pliocene, are 

 mainly the sediments of an ancient lake, of which the present body of 

 water is the representative on a much reduced scale. Beautiful and 

 highly instructive sections of the old beach formations are exposed in 

 the valleys of the streams, particularly in the lower valley of Pelican 

 creek, and far down the Yellowstone river, where they become more 

 complicated and more interesting. An examination of these shows 

 that the lake formerly extended over a much larger area, and that it 



