THE PHYSIOGRAPHY OF MEXICO 79 



to an almost entire absence of rainfall, and in places peaks rise 

 almost to the level of the original peneplained surface of the old 

 plateau; for instance, Santa Catalina 10,135 ^ eet an d Mount San 

 Bernardino 10,630 feet. 



In the northern part of the province there are numerous features 

 that have many of the earmarks of glaciation— cirques, moraines, 

 kettle holes, etc. 1 In the San Bernardino Mountains there is 

 scarcely any doubt that these features are of glacial origin, the 

 evidence presented by physical characteristics being supported 

 by a boreal flora and fauna in the higher parts of the range; in 

 Arizona and Sonora, however, there is some question concerning 

 their glacial origin, and able geologists have held different opinions 

 on this point. One holds that they are due exclusively to ice 

 action; 2 another attributes them to volcanic brecciation and 

 subsequent sheet flood erosion. 3 Inasmuch as either process would 

 produce subangular fragments of varying size and would transport 

 them to considerable distances from their point of origin, it is diffi- 

 cult to decide which is the more probable theory. Recent field 

 work has not contributed much to the discussion. 



The prominent drainage feature of this province is the Rio 

 Colorado. It is of interest to physiographers for several reasons. 

 Although it flows hundreds of miles through an arid country, with- 

 out tributaries, and loses much of its original volume by evaporation, 

 it delivers at its mouth a great volume of water, particularly from 

 April to June. In this water is a larger average amount of 

 suspended matter per unit volume than in that of any other 

 North American stream, about twenty thousand parts per million, 

 or enough to cover one hundred square miles of territory to a depth 

 of one foot in a year. This accounts for its rapidly growing delta, 

 and the cutting off of the Salton Sink and the Imperial and 

 Coachilla valleys area. 4 



The lower course of this stream was consequent upon the surface 

 of the great downwarp between the peninsula and the mainland. 5 

 In the bed which it found in this trough the river cut a canyon, 



'Merrill, Science, N.S., XXIV, 117. 



2 Ibid. 4 Bowman, op. cit., pp. 238-40. 



3 W J McGee, Science, N.S., XXIV, 178. « Botsford, he. cit. 



