

MESOZOIC ERA: THE AGE OF REPTILES 515 



from the decay of the underlying formations, which the slow-moving 

 streams of that time were unable to transport. At the beginning of 

 the Lower Cretaceous, however, the Piedmont and Appalachian re- 

 gions were raised, perhaps along the axis of the Appalachian tract, 

 while the land nearer the coast was but little disturbed, either remain- 

 ing comparatively level or being depressed into long troughs, some- 

 what similar to those of the Triassic. Under these new conditions, 

 the streams in the higher regions began again to erode. On account 

 of the abundance of loose, weathered material, the streams in their 

 courses to the sea soon had all the sediment they could carry and as 

 soon as a lower gradient was reached dropped their loads. This 

 resulted in the formation of deltas, flood plains, marshes, and shallow 

 lakes. The deposits formed in this way were gravels, composed of 

 the quartz of quartz veins and quartzites, clay from the decayed 

 feldspar, shales, and slates, and arkose in the immediate vicinity of 

 feldspar-bearing rocks. 



The most striking feature of the Lower Cretaceous geography is 

 the expansion of the Gulf of Mexico towards the west and northwest 

 and the deep subsidence of its floor, upon which were deposited a 

 great thickness of limestones. Large areas in Mexico, Texas, and New 

 Mexico were covered at this time. In this sea the Ouachita Moun- 

 tains stood out as a promontory, as is shown by the ancient shore 

 line which has been traced around their foot. The sediments have a 

 thickness of 5000 feet on the Rio Grande and are even thicker in 

 Mexico. At the base of this marine formation is one which is in part 

 a littoral deposit but is mostly marine (Trinity). 



Western Interior. — In the western interior non-marine formations, 

 sometimes including coal beds, occur (Morrison which may be Juras- 

 sic, Kootenai, Cloverly, Lakota, Fuson). 



Pacific Coast. — On the Pacific coast the conditions were very 

 favorable for erosion, because of the newly raised Sierra Nevadas 

 which were being rapidly cut away, the material derived from them 

 forming a thick deposit in the Sacramento valley. In addition to 

 this area, other narrow strips were submerged east of the present 

 coast of British Columbia and Alaska, during portions of the period. 



Lower Cretaceous of Other Continents. — In Europe, as in North 

 America, the Lower Cretaceous formations are largely of continental 

 origin and are not as widespread as those of the Upper Cretaceous. 

 In general, it can be said that important geographical changes oc- 

 curred in various parts of the earth at the close of the Lower Cretaceous, 



