BAJA CALIFORNIA AND SONORA SYSTEMS 



4S9 



The following emergence of equal magnitude may still be going on in 

 places. 



Volcanoes were active during the Pleistocene and have continued their ac- 

 tivity to Recent time. Isla Tortuga is the youngest island in the gulf; it erupted 

 from the gulf floor about 6000 feet deep and reached an elevation above the 

 gulf of more than 1000 feet. Its poorly eroded surface and lack of vegetation 

 vouch for its youth. Las Tres Virgenes are said to have been active in historic 

 time. On the west slope of the sierras there are many Quaternary craters and 

 cones, and at San Quintin the volcanic flows, according to Woodford (1928), 

 may be in part historic. Cerro Prieto, near Volcano Lake (Mexicali quadrangle), 

 is a small perfect crater probably formed in Recent time. 



Movement along older fault lines continued during the Quaternary, and 

 probably many new crustal breaks were initiated, whether in early or late 

 Pleistocene is not known, but one may assume that much of this activity oc- 

 curred at the time of the Middle Pleistocene revolution of California. Movement 

 still continues along some of the fault zones in both California and Baja Cali- 

 fornia. The existence of zones of faulting which border the peninsula is indi- 

 cated most strongly by recent phenomena, though activity along some of these 

 zones probably has been nearly continuous from some remote time. 



Quaternary uplift has increased the height of the mountains of the peninsula, 

 rejuvenated streams in regions of low relief, and exposed a considerable area of 

 partially consolidated beach material to erosion, with the resulting development 

 of a coastal slope which appears from a distance to be a plane surface, but 

 which is really an intricate pattern of small arroyos and narrow ridges. Recent 

 erosion has cut deep canyons into the rocks of the peninsula and reduced the 

 height of its mountains, while alluvial deposition has in places half buried some 

 of the ranges in fans of detritus derived from them. The wind has assisted in 

 sculpturing some of the softer rocks in regions of rugged topography, and the 

 oudines of the topography are softened by the addition of aeolian material in 

 the broad low desert regions; giant sand dunes, or medanos, are numerous and 

 (cover large areas in the desert regions. 



At the head of the Gulf of California the Colorado River formed an enormous 

 delta over which it flowed alternately into the gulf and then northward into the 

 Salton Sea, making what is now the Salton Basin into a fresh-water lake. The 

 jCoahuila Indians have handed down legends about this diversion. 



GULF OF CALIFORNIA 



jj 



y Shepard and Emery (1941) and Beal (1948) consider the Gulf of Cali- 

 fornia to be a downfaulted trough complementary to the uplift of the 

 peninsula of Baja California. King (1939) has suggested a relation of 



the faults of adjacent Sonora to those of the Gulf. Beal has described 

 the submerged topography as follows: 



The northern quarter of the gulf is shallow — at no place more than 600 feet 

 deep. The deepest parts of the gulf south of the 30th parallel appear to lie 

 west of its center, and thus probably before the floor was deformed by so much 

 faulting it simulated, in some respects, the westward-tilted block of the penin- 

 sula, suggesting an extension of the basin and range structure of the Sonora 

 area. 



The east side of the gulf appears not to have been affected by faulting; the 

 gulf floor slopes gently from the Sonora coast to the irregular escarpments near 

 the center of the gulf. The most important of these is the great submarine cliff 

 nearly 6000 feet high between the 25th and 26th parallels. Between the 24th 

 and 27th parallels there are many irregularities in the submarine topography 

 between the Ceralbo fault zone and the east coast of the peninsula, but most of 

 them lie west of the Ceralbo fault zone. 



Between 30 and 40 islands varying in size from Isla Angel de la Guarda, 

 between 75 and 80 kilometers long, to very small ones, rise above the surface 

 of the gulf, some to surprising elevations. Other islands such as Consag Bock, 

 San Pedro Martir, Ceralbo, and Santa Catalina have the appearance of wedges 

 uplifted from the granitic floor of the gulf or as stocks or spurs still attached to 

 the granitic batholith. 



Much of the south half of the gulf is occupied by a remarkable depression in 

 the sea floor, extending 400 kilometers southeastward from a point east of Isla 

 Tortuga. It widens into enormous proportions at places and becomes narrow in 

 others, with the closing depression contour 5400 feet below sea level. This 

 great depression area is occupied by three separate smaller basins, the largest 

 and deepest (10,740 feet) of which lies in the center of the gulf between 25 

 and 26° N. Lat. 



A distinctive depression about 250 kilometers long, the origin of which can 

 reasonably be assigned only to faulting, separates the Angel de la Guarda group 

 of islands from the peninsula. The deepest part of the trough is about 5100 

 feet and lies adjacent to Isla Sal si Peudas. The closing depression contour is 

 1200 feet below sea level, thus furnishing a long narrow basin, nearly 4000 feet 

 deep, which widens at its north end. A line indicating the east boundary of the 

 graben is called the Ballenas fault zone, the northwestward extension of which 

 may lie farther west than shown and join the northwestern extension of the 

 Ceralbo fault zone. [This fault, or fault zone, has been drawn on Fig. 30.6 as the 

 western boundary of the depression which the writer interprets as a graben.] 



From the configuration of the gulf floor, there seems good evidence of a 

 fault east of the Isla Ceralbo [Fig. 30.6]. At the sea bottom, north and east of 

 this island, is a submerged island nearly three times as long as Ceralbo, with 

 its crest approximately 1000 feet below sea level and rising about 2500 feet 

 above its base. Topographically, the submerged island appears to ha\ e been 

 once a part of Isla Ceralbo, and both apparently a part of the Southern Cape 



