GEOLOGIC HISTORY 469 



declivity flowed across the Great Plains region and through many of the 

 wider valleys between the mountains. These, with frequently varying 

 cliannels and extensive local lakes, due to damming and the sluggish flow 

 of the water, laid down the widespread mantle of the White River de- 

 posits of Oligoeene age. The first of these were the sands of the Chadron 

 formation, which occur partly as channels filled with coarse sandstone 

 and show clearl)' the course of old currents. Later, in slack water and 

 areas of overflow, fine sands, clays, and fullers' earth were laid down to 

 a thickness of a hundred feet or more. Some thin but very widespread 

 slieets of limestone at or near the top of this formation indicate the pres- 

 ence of extensive fresh-water ponds. 



The Brule clays, which follow the Chadron beds, indicate a continua- 

 tion of stream deposition, but with currents less strong and with more 

 extensive local lakes and slack-water overflows. The almost general fine- 

 grained character and regular bedding of the Brule sediments indicate 

 that stream declivities either were low or rapidly became so. The original 

 extent of the White Eiver deposition is not known because so much of the 

 formation has been removed by erosion. 



At the beginning of Miocene time the general conditions had not 

 changed materially, but the great silt-depositing streams were given in- 

 creased declivity doubtless by general uplift. They first eroded the sur- 

 face of the Brule clay and other earlier> formations, and then deposited 

 upon them a sheet of sands at first with many local deposits of coarse 

 gravel and boulders. This was the Arikaree formation. It was spread 

 widely over the central plains region, and in some areas attained a thick- 

 ness of nearly 1,000 feet — a flat alluvial fan of wonderful extent. It was 

 deposited far up the east slopes of the Laramie mountains and buried 

 some of the lower ridges. This time was followed by a long epoch of 

 general uplift and gradual erosion, which in the central Wyoming region 

 continued through the remainder of Tertiary time, and some of the ero- 

 sion products Avere carried away by streams and spread over portions of 

 the country east and south. 



QUATERNARY CONDITIONS 



At the beginning of the Quaternary time all the broader topographic 

 features of today were outlined and largely developed. The streams 

 had not cut their valleys so deep and the wide plains of Tertiary deposits 

 were more extensive. During the Glacial epoch there were numerous 

 glaciers of moderate size on parts of Wind Eiver and Bighorn mountains 



