498 THE TERTIARY PERIOD 



are more consolidated, and are quite hard limestones, sandstones, 

 and shales, with extensive deposits of lignite, formed in ancient 

 peat bogs which followed the low-lying Gulf shores. 



On the Pacific coast a long, narrow arm of the sea occupied the 

 great valley of California, extending northward into Oregon and 

 Washington ; its deposits are at present principally displayed along 

 the eastern flank of the Coast Range. These deposits form a 

 single series, the Tejon, which lies upon the Chico in apparent 

 conformity ; but the lowest Eocene is not represented in the 

 Tejon, and in Oregon an unconformity between the two series 

 has been detected. 



It was in the interior region that the geographical changes 

 wrought by the revolution at the end of the Cretaceous had the 

 most marked effects. Even before that the movement of elevation 

 had converted the interior sea into bodies of fresh and brackish 

 water, in which the latest Cretaceous deposits, the Laramie and 

 Livingstone, or Denver, had been laid down. The same condi- 

 tions appear to have lasted into the Eocene over a part of the 

 Great Plains country. Covering much of North Dakota and Mon- 

 tana and a wide area in Canada was a body of fresh water, in 

 which were formed the Fort Union beds that overlie the Living- 

 stone unconformably. From the evidence of the plants the Fort 

 Union is believed to be the oldest Eocene, but this is still uncer- 

 tain. Fresh-water beds with similar plants are found in Greenland 

 and Alaska. At the end of Fort Union time the Great Plains, 

 from Mexico to the Arctic Sea, were dried up ; but then began 

 the establishment of a series of fresh-water lakes in the region 

 between the Wasatch and Rocky Mountain ranges. At present 

 this is a region of high plateaus, elevated from 5000 to 7000 feet 

 above the sea, but then it must have been much lower and can 

 hardly have been enclosed by such high mountains as now encom- 

 pass it. These lakes were not formed simultaneously, but succes- 

 sively, and together include the whole of Eocene time in an almost 

 unbroken record. 



The oldest of these lakes was the Puerco, a relatively small body 

 of water, which covered the northwestern part of New Mexico and 



