RECENT GEOLOGIC EVP:NTS 281 



During this epoch of deposition numerous volcanic eruptions oc- 

 curred. Sheets of basalt are included within the gravels at numerous 

 places along the Colorado river, from the mouth of Grand canyon to 

 Yuma. In some places they occur near the base of the gravels and in 

 other places near the top. At the mouth of Williams river a series of 

 eruptions occurred near the close of the epoch, the molten basalt bursting 

 upward through the gravel filling of Detrital-Sacramento valley and 

 spreading over its surface. One of the vents through which the basalt 

 issued is represented by a large volcanic neck exposed in Williams canyon, 

 and some of the lava hills in Detrital-Sacramento valley a few miles 

 north of this canyon appear to be volcanic necks. After five sheets of 

 basalt, each one several feet in thickness, had been formed in succession 

 on the floor of the valley and each one buried in turn by a few feet of 

 sand and gravel, a flood of molten rock was outpoured, forming a thick 

 sheet, which extended completely across the valley and probably dammed 

 the river. The undissected parts of this sheet indicate an original thick- 

 ness of 800 feet or more. 



A dam 800 feet high thrown across the Colorado river at this place 

 would create slack water conditions not only throughout the entire length 

 of Detrital-Sacramento valley, but far into the Grand canyon, and must 

 have facilitated the deposition of river sand and gravel, which had 

 previously accumulated in the valley to a depth of more than 1,000 feet. 

 The altitude of the basalt sheet is essentially the same as that of the sur- 

 face of Detrital-Sacramento and of the sand and gravel remnants found 

 far above the river in the sides of Virgin canyon and at the mouth of 

 Grand canyon. 



CANYON CUTTING (2) 



A second epoch of canyon cutting was brought about by some influence 

 which rejuvenated the streams throughout the Southwest. During the 

 extensive aggradation of the previous epoch some of the streams had wan- 

 dered far from their old channels and in their now invigorated condition 

 cut new canyons, while others reexcavated wholly or in part the old filled 

 valleys. 



The course established by the rejuvenated Colorado river was appar- 

 ently influenced by the volcanic dam. An outlet had evidently been 

 found through the Black Mountain range north of mount Wilson, and the 

 river, abandoning its former course, remained west of this range as far 

 south as Williams river, where it cut through the western edge of the 

 basalt sheet and returned to its old valley after establishing a new 

 course for a distance of about 125 miles. A more difficult course could 

 scarcely have been selected. Instead of reexcavating its old channel 



