. Bulletin of the United States Geological Survey--Pre-Cambrian Geology of North America . tical sections it was notedthat the roots of living arrow weeds had been buried to a depth of 6feet or more. In other words, there have been accumulations of silt6 feet or more in depth during the life of this shrub. The materialis well stratified, sometimes with horizontal lamination, but oftenshowing a conspicuous oblique lamination similar to that found inthe Chemehuevis gravel. Much of the silt is very fine and is held insuspension for a long time. When deposited in thick beds it driesand cracks in c
. Bulletin of the United States Geological Survey--Pre-Cambrian Geology of North America . tical sections it was notedthat the roots of living arrow weeds had been buried to a depth of 6feet or more. In other words, there have been accumulations of silt6 feet or more in depth during the life of this shrub. The materialis well stratified, sometimes with horizontal lamination, but oftenshowing a conspicuous oblique lamination similar to that found inthe Chemehuevis gravel. Much of the silt is very fine and is held insuspension for a long time. When deposited in thick beds it driesand cracks in columnar form, some of the columns being 2 to 3 feetin diameter and separated by cracks several inches wide and 2 feet ormore deep. CHOCOLATE MOUNTAINS. South of the Great Colorado Valley, for a distance of about 30miles, Colorado River flows in a canyon or narrow valley through theChocolate Mountains—a name applied generally to a complex groupconsisting of the Spire Range, the Picacho group, the Castle DomeMountains, and the Purple Hills. 48 RECONNAISSANCE OF PART OF WESTERN Fig. 13.—Map and section of a part of Chocolate Canyon, .showing an old debris-filled channel to theeast of the present channel. (Section constructed from boring records.) DETAILS OF VALLEYS. 49 The rock exposed along the river is principally effusive, althoughgranite and metamorphosed sediments occur in some places. Theeffusive rock, consisting of pink andwhite rhyolite in the form of ash,breccia, and flow, extends from the river level apparently to alti-tudes of several thousand feet, and is characterized by conspicuouscliffs, erratic forms of erosion, and rock slopes barren of vegetation. Physiographic changes similar to those at Bulls Head and else-where were noted in Chocolate Canyon. At Lighthouse Rock theriver has apparently left its old channel and cut laterally into therhyolite, leaving Lighthouse Rock standing in the water. A boringin the channel at this point failed to reach bed roc
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