436 HISTORICAL GEOLOGY 



sea level than now, and less intrenched by valleys. Many 

 streams which issued from the newly made mountains on 

 the west were spreading their loads of gravel, sand, and mud 

 far and wide over the low-lying surface. Here and there 

 lakes and marshes doubtless existed temporarily, and the 

 location of these shifted from time to time, so that the de- 

 posits which now represent the Tertiary in the Great Plains 

 are partly such as are laid down in lakes, and partly those 

 which rivers and even winds make. In the Tertiary epochs 

 the climate of the Great Plains region was on the average 

 moister than it is to-day. Coaly layers in the Tertiary 

 strata indicate the existence of swamps, where now only 

 dry prairies are to be found. The Tertiary deposits, which 

 have since been elevated and subjected to a drier climate, 

 are now being rapidly dissected by the growth of ramifying 

 valleys and tributary gullies. In parts of Dakota and 

 Montana the result is an extremely rugged complex of 

 ridges, mesas, and buttes, over which travel is very difficult, 

 and which are therefore known as " Bad Lands " (Fig. 454). 



Changes affecting the western mountains. The young 

 Rocky Mountains and others farther west were being rapidly 

 worn down by the activities of wind, rain, and streams. 

 Some of the material thus furnished found lodgment in the 

 interior basins between the mountain ranges, and there 

 accumulated to great thickness. As in the plains, these 

 deposits were made partly in lakes, but are to be ascribed 

 in large measure to the work of streams which built alluvial 

 fans in front of the valleys they had cut in the mountain 

 slopes. Coalescing with each other, these fans came to form 

 alluvial plains. 



In addition to the sand, gravel, and silt, beds of volcanic 

 ash and sometimes of coarser tuff are found included in these 

 Tertiary strata. They record the eruptions which took 

 place at intervals from the volcanoes in Colorado, Montana, 

 and many other western states while the Tertiary sediments 

 were being laid down. The old volcanic cones have been 



