CONTENTS. O 
Geographic and geologic details of valleys — Continued. Page. 
Colorado Valley — Continued. 
Las Vegas Wash 37 
Black Canyon. 37 
Cottonwood Valley 37 
Pyramid Canyon 38 
Mohave Valley 41 
The Needles 42 
Chemehuevis Valley 43 
Aubrey Canyon 44 
Great Colorado Valley 45 
Chocolate Mountains 46 
Hualpai Valley 48 
Location and. character 48 
Structure 50 
Big Sandy Valley 50 
Location and character 50 
Rock formations 51 
Structure 51 
Signal Canyon 52 
Detrital-Sacramento Valley • 52 
Location and character 52 
Detrital filling 53 
Williams Valley 53 
Location and character 53 
Rock formations 54 
Other valleys 55 
Location and character 55 
Detrital filling 55 
Geologic history 56 
Introductory statements 56 
Pre-Cambrian conditions 56 
Crystalline rocks 56 
Sedimentation 56 
Base-leveling 56 
Paleozoic and Mesozoic conditions 57 
Tertiary events 57 
Explanatory statement 57 
Denudation 57 
Eruption of andesite 58 
( 'rustal movements and erosion^ 58 
Eruption of rhyolite and younger andesite 59 
Formation of the Grand Wash Trough 59 
Deposition of the Greggs breccia 59 
Erosion of the Detrital-Sacramento Valley 60 
Relation of Colorado River to the Detrital-Sacramento Valley 61 
Uplift of the plateau 61 
Effect of uplift on Colorado River 62 
Quaternary (?) events 62 
First canyon cutting 62 
Deposition of the Temple Bar conglomerate 63 
River diversion 64 
Second canyon cutting Gij 
Deposition of the Chemehuevis gravel 65 
