THE CAHTTILLA BASIN AND DESERT OP THE COLORADO. 6 



Many of these fossil shells were observed in 1853, but have since been described more in 

 detail by other explorers, notably by Dr. E. E. Stearns of California. 



Dr. Stephen Bowers, who describes many of the localities, writes of the region generally 

 as follows: 



"The water of the old Tertiary Sea, which once prevailed here, must have been extremely 

 favorable to the propagation and growth of mollusks, especially oysters. After the vast erosion 

 that has taken place, there are many square miles of fossil beds, especially of oyster-shells, which, 

 in places, are 200 feet thick and may extend downward to a much greater depth. The oysters 

 existed not only in vast numbers but in many varieties, from the small shell which is in evidence 

 over so much of the territory, to varieties nearly a foot long and to others weighing several pounds 

 each. One variety is nearly as round and as large as a dinner plate." 



Fossil oyster-shells are perhaps most abundant in the Coyote Wells District, about 

 7 miles north of the international boundary line and about 375 feet above the level of the 

 sea. Other deposits of marine shells, including shark's teeth, pectens, and univalves, are 

 reported from one of the branches of Carrizo Creek. But the occupation of the valley by 

 sea-water, while comparatively recent geologically, has extreme antiquity and long ante- 

 dates human history, dating back to the Middle Tertiary. 



The continental elevation which followed culminated in the Pleistocene, or Glacial 

 period, when the precipitations of rain and snow are believed to have attained their maxi- 

 mum. At that time, the Colorado of the West had its greatest volume and transporting 

 power. Its silt was distributed far and wide in the interior sea, then only partially cut off 

 from the broad Pacific by a chain of islands which now form the crest of the Peninsula 

 Mountains from San Jacinto to Cape St. Lucas. Entering the Gulf just below where the 

 mouth of the Gila River now is, it began dropping its load of debris and silt, forming the 

 raised delta which gradually extended westward and southerly across the upper end of 

 the Gulf toward the Cocopah Mountains and finally to the higher ridges beyond the 

 Pattie Basin, even to the eastern base of the Peninsula Mountains. 



Aided by the gradual elevation of the land and by the tides of the Gulf, the building 

 up of the delta proceeded rapidly. It assumed the nature of a great dam or levee stretching 

 across the Gulf and diverting the river-water through shifting channels to one side or the 

 other, first to the lower part leading to the Gulf and then to the upper end of the depressed 

 area shut off from the tides. At certain seasons the tides rise to a great height at the head 

 of the Gulf and are accompanied by dangerous bores. Such tides rushing up the mouth of 

 the Colorado have ever been important factors in the formation of the delta. Ordinary tides 

 are said to rise 15 feet, and in extraordinary cases to 37 feet. According to the United 

 States Geological Survey the tidal range is from 14 to 32 feet. (See p. 17.) 



LAKE CAHUILLA. 



The head of the Gulf, being cut off by the Delta from the free access of the sea, became 

 an inland lake of salt water, or at least of brackish water, with the great Colorado River at 

 certain seasons and stages of flood flowing into it. This stream then, as now, was laden with 

 the rich alluvial earths of its upper course, torn from the ravines and canons of the Rocky 

 Mountains and the Grand Canon of Arizona. This influx of river-water, though variable 

 in duration and quantity, must have exceeded the loss by evaporation. Consequently the 

 level of the lake was raised until the excess overflowed to the Gulf by a lower outlet. 



That such conditions continued for centuries appears certain, for the enormous accu- 

 mulation of sediment within the old beach-lines tells the story of long-continued lacustrine 

 conditions, of the displacement of the sea-water, and of the final occupation of the valley 

 by fresh water. This is shown to us by the fresh-water shells, not only on the surface but 

 in the blue-clay sediments, in the banks of ravines and arroyos, and in the deep borings 

 for water, showing that the shells dropped to the bottom and were thus entombed. These 



