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University of California Publications. [Geology 



very clearly exposed, and a strongly marked angular uncon- 

 formity of at least ten degrees variation in dip is shown. At 

 some points the underlying strata are considerably contorted. 

 The sharp minor folds might possibly be due to local slipping 

 of soft strata below a contact with the gravels. The angular 

 unconformity shown in a long exposure of the cliff can be 

 interpreted only as the result of erosion preceding the deposition 

 of the gravels. 



The gravel deposits resting upon the eroded Virgin Valley 

 Beds consist almost entirely of large, well-rounded rhyolite 

 fragments ranging up to more than two feet in diameter. The 

 rhyolite closely resembles some of the flows in the formation 

 forming the rim of Virgin Valley. A very few well-rounded 

 pebbles of basaltic lava were obtained. Mr. Ileindl, who has 

 examined this basalt, finds it very different from the Mesa Basalt. 

 While basaltic pebbles are rare in the mass of these gravels, 

 large blocks from the basalt capping of the mesa near by are 

 found resting on the top of the gravel beds. 



The unconformity of the rhyolitie gravels on the Virgin 

 Valley Beds might be interpreted as meaning that it represents 

 a stage of accumulation in a valley cut after the period of the 

 Mesa Basalt flow. On the other hand it is noted that basaltic 

 pebbles are quite rare in the rhyolitie gravels, while large masses 

 of basalt from the edge of the mesa are found resting upon 

 the top of these gravels. The edge of the basalt covering the 

 mesa is near at hand, while the flows from which the rhyolite 

 pebbles have been largely derived are much farther removed. 

 It is moreover not probable that the few basaltic pebbles in the 

 gravel are derived from the Mesa Basalt, and basaltic flows are 

 presumably associated with the rhyolites below the Virgin Valley 

 Beds. This evidence seems to show that the accumulation of 

 the gravels occurred before the Mesa Basalt flow, otherwise 

 there should be at least as large a percentage of fragments 

 derived from the mesa cap as we find in other deposits known 

 to have formed during the cutting of the present canon. 



The suggestion that the rhyolitie gravels accumulated at an 

 early resting stage in the cutting of the present valley is 

 probably further negatived by the presence of numerous large 



