218 ILLINOIS ACADEMY OF SCIENCE 



at present or to a cutting off of the water supply from the 

 north. The latter seems entirely adequate to explain all the 

 present canyon features. 



There can be little doubt that formerly the mesa surface 

 reached much farther to the north and northeast extending 

 even over the Dolores Plateau 10 to the north and probably over 

 the La Plata" region to the northeast. After the doming of the 

 La Plata Mountains in Tertiary times active stripping began. 

 The higher areas because of greater precipitation and steeper 

 stream gradients suffered greater erosion than the regions 

 farther out, and the softer formations were readily stripped 

 from the area. This stripping continued from the mountains 

 plateauward, thus gradually removing the upper parts of the 

 valleys, and causing the plateau escarpment to migrate from 

 the mountains. Facts of various kinds support this view. (1) 

 The talus of Mesa Verde sandstone is found all along the 

 northern escarpment for a distance of three or four miles from 

 the foot of the mesa. This material is not waterworn and 

 shows no evidence of landslide. The nearer the mesa the larg- 

 er and more numerous these talus heaps become. These show 

 the recession of the mesa front for this distance at least. (2) 

 Mesa canyons also give evidence of having extended farther 

 to the north. All the main canyons cut through the entire mesa, 

 giving a serrated appearance to the north scarp. Since the 

 entire drainage is still to the south, the canyons, evidently, have 

 been beheaded by a long continued cliff-recession. Morfield 

 canyon offers the best example of this. The upper canyon di- 

 vides into two branches forming a Y. The recession of the 

 scarp has gone on to such an extent that both branches have 

 been almost entirely removed, leaving a semicircular cone of 

 Mesa Verde sandstone surrounded entirely by Mancos shale. 

 What is now mapped as Lone Cone is, therefore, only the last 

 remnant of an inter-canyon strip. The entire drainage is to 

 the south except at the very edge of the escarpment. The re- 

 cession of the cliff is sufficiently rapid to prevent the develop- 

 ment of a drainage in the opposite direction as would seem most 

 natural, especially in those canyons cut into the shale in their 

 upper courses. (3) The entire canyon system implies an enor- 

 mously greater amount of water than now falls on the mesa. 



"Cross: La Plata Folio, p. 4. 

 "Cross: La Plata Folio, p. 10. 



