INTRODUCTORY NOTE 



GEOLOGY OF THE BRIDGER BASIN. 



FORT BRIDGER is a government military post, situated on the high southern 

 plateau of western Wyoming Territory, in the midst of one of the most interest- 

 ing geological regions of the world. 



The country on all sides was once the bottom of a great eocene lake, the 

 water of which was probably slightly brackish. Whether this lake district had 

 direct communication with the ocean, is undetermined as yet, but there is a pos- 

 sibility that it had.* 



The tributaries of the Green River, which drain this plateau, render the val- 

 leys along the edges of the streams green and wooded. Beyond this fertile 

 strip, wide, barren plains extend, covered by a dense growth of short " sage 

 brush" (Artemisia tridentata). 



From the fragmentary debris lying scattered over the surface of the ground, 

 it would seem as if the various streams formerly were of much greater size and 

 volume than they now are, and that long after the eocene lakes had been drained- 

 rivers of considerable size ploughed up the lake bottoms, excavating an immense 

 area. The formation known as " Mauvaises Terres" rises abruptly from the 

 valleys, and extends in a series of plateaus, one above the other, on either side. 



The high land shows the effect of violent erosion in two forms ; first, the ir- 

 regular and jagged cones that appear upon the sides of the high benches ; and, 

 second, the isolated butte structure, rising directly out of the plain. 



The bad lands of Cottonwood Creek, Henry's Fork, Dry Creek, etc., are ex- 

 amples of the first, and Bridger Butte the best known example of the second. 



Bridger Butte, six miles to the south-west of Fort Bridger, rises to a height 

 of over a hundred feet, and is about two miles long ; its sides slope steeply up, 

 and its level top serves as a landmark that can be seen miles away. 



The stratification throughout this whole formation is nearly horizontal, and 

 across the valley can be distinctly noted, owing to the color and appearance of 

 the various layers. 



No satisfactory explanation has been given of the causes which occasioned 

 the removal of the waters of these lakes, nor of the agencies necessary to ac- 



* Ichthyic fauna of the Green River shales, Hayden's Surveys, vol. iii., No. 

 4, p. 819. 



