450 WALLACE W. ATWOOD 



axes. The surfaces on which these gravels were found were along 

 the crests between the great mountain canyons and on the tops 

 of mesa-like hills near the base of the range. If the gravel-strewn 

 surfaces were extended they would unite and form a plain of gently 

 rolling topography. That plain would slope away from the core 

 of the range, show a distinct warping at the base of the range, 

 and pass off over the upland surfaces of neighboring plateaus. 

 The nature and distribution of the gravels suggested that they 

 were remnants of stream deposits in channels which formerly 

 crossed the present inter-canyon ridges. They appeared to be 

 the deposits of streams which flowed over low gradients and sug- 

 gested further, by their distribution and the distribution and 

 relations of the surfaces on which they were found, a deformed 

 peneplain. In following this ancient erosion surface southward 

 and southwestward over the plateau district it seemed that 

 certain of the outlying mesa surfaces would correspond in age to 

 this peneplain surface, and it was anticipated that on such out- 

 lying surfaces a mantle or scattering of still finer gravels might be 

 found. But these mesa surfaces were found to carry a heavy 

 mantle of bowlder-gravels, in which the larger masses ranged up 

 to three and four feet in diameter. In these bowlder-gravel de- 

 posits certain special bowlders could be recognized which came 

 from outcrops in the mountain areas, and it appeared that they 

 must have been washed out and deposited as a portion of a great 

 alluvial fan about the margin of the mountains. These bowlder- 

 strewn surfaces were followed southward nearly fifty miles from the 

 base of the range, and at that distance the larger bowlders seen 

 ranged to at least three feet in diameter. The interpretation of 

 this bowlder-gravel mantle and its relation to the erosion surfaces 

 upon which it rests and the erosion surfaces in the mountains which 

 seem to correspond in age to those underneath the bowlder-gravels 

 is that at the close of the cycle of erosion during which the pene- 

 plain as described was developed, there was a general uplift in the 

 district, which was emphasized in the San Juan dome. The head- 

 waters of the streams in the uplifted dome were so rejuvenated 

 that they carried together with sands and gravels many large 

 bowlders to the base of the range and spread that material out as 



