82 WARREN N. THAYER 



because of its thick volcanic cap. During the early stages of this 

 second cycle the synclinal trough was depressed, allowing the sea 

 to encroach as far north as the Salton Sink, about 200 miles farther 

 northwest than at present. At that time, "the mouth of the 

 Colorado River was at Yuma, sixty miles from its present location, 

 and was gradually building a delta that extended southwest toward 

 the Cocopa Mountains. Deposition continued until the upbuild- 

 ing of the delta had completely separated the head from the rest of 

 the gulf, and converted the floor of the former into an inland sea." 1 

 At numerous times since, the Colorado has alternately emptied into 

 the Gulf and into the lake or inland sea which it created. 



The greater part of the degradation of the province has occurred 

 during recent epochs of the period of second uplift. When once the 

 volcanic capping is worn through, the softer sediments beneath 

 erode with remarkable rapidity, and the larger westward-flowing 

 streams have cut canyons of abysmal depths in them, in their short 

 but steep courses to the sea. The smaller intermittent streams 

 working headward into the plateau are attacking the softer rocks 

 wherever exposed, and, with the sheet floods which follow the 

 torrential downpours characteristic of the province, are doing 

 a work the results of which are not so likely to attract attention 

 as canyon-cutting, but which are infinitely more profound in the 

 degradation of the plateau. 



THE GULF COASTAL PLAIN PROVINCE 



Definition and boundaries. — This province is a plain composed 

 of recent and undeformed marine sediments. Being a southern 

 extension of a similar feature in the United States it has no proper 

 northern boundary, but is continuous through the Gulf states and 

 up the Atlantic as far as Long Island. 



Its western boundary has been described in connection with the 

 Anahuac Desert Plateau province. 



The eastern boundary, in common with that of the remainder 

 of the coastal plain in the United States, may be taken as the border 

 of the continental shelf, and is, of course, many miles east of the 

 present coast line, which is but an incidental feature. 



1 Bowman, op. cil., p. 239. 



