

From a drawing by Charles R. Knight 



A COMMON SIGHT IN NORTH AMERICA MILLIONS OF YEARS AGO 



The most extraordinary feature of this animal was the row of thin plates on either side of 

 the median line, and also the long and heavily armed tail. 



ocean — an inland sea extending from the 

 Gulf of Mexico northward to the Arctic 

 Ocean — covered this area during a long 

 time, in which several hundred feet of 

 strata accumulated. 



Where these rocks flank the mountains 

 they are tilted at an angle of several de- 

 grees, which shows that they were laid 

 down before the complete elevation of 

 the Rocky Mountains. This formation is 

 called the Pierre. (Geologic formations 

 are usually named from the "type" lo- 

 cality in which first recognized, and wher- 

 ever rocks of the same age appear they 

 are designated by.that name.) 



Near the close of the Pierre a part of 

 the inland sea-floor was elevated above 

 the ocean and became a land-mass of low 

 altitude — a vast stretch of jungle-covered 

 delta and coastal swamp, interspersed 

 with bayous and lagoons. 



In the fresh and partly salt or brackish 

 water lake and river beds of this period 

 are preserved leaf impressions of a va- 

 riety of trees, rarely teeth and fragmen- 

 tary bones of mammals, and numerous re- 

 mains of a great variety of reptiles (see 



page 416). This is known as the Judith 

 (Belly) River formation. 



Subsequently this area again sank be- 

 low the sea for a long time and 400 feet 

 of deposits accumulated, in which only 

 sea-shells and marine reptiles are to be 

 found. This ocean deposit, also a part 

 of the Pierre, is designated Beqrpaw. 



Then a long period of elevation began, 

 the rocks and fossils showing a gradual 

 change from salt to brackish water con- 

 ditions, which near the top became quite 

 fresh. These beds, over 700 feet thick, 

 are known as the Edmonton formation. 

 Presumably the area was near sea-level 

 and subject to frequent invasions of the 

 sea — a condition that may be better un- 

 derstood by comparison with the present 

 everglades of Florida (see page 412). 



THE HOME OF A HOST OF RFPTILFS 



In these marshes of prehistoric times 

 dwelt a host of reptiles, some large, 

 some small, and of various forms, flesh- 

 eaters and herb-eaters, but all sharing 

 certain characters in common and known 

 as dinosaurs. Not any were closely re- 



409 



