INTERDEPENDENCE OF TOPOGRAPH! AND GEOLOGY. 29 



southeast corner of the Mosquito map. The base of this hill is 10,000 feet 

 above the level of the sea. 



It were scarcely })Ossible to select an alpine region more admirably 

 adapted to illustrate the interdependence of topographical and geological 

 structure than that chosen for this study. The gentle slopes of the eastern 

 spurs follow the inclination of the easterly dij^ping beds of Paleozoic rocks 

 which form their surface, and which remain in broad sheets, like the covering 

 of a roof, to protect the underlying Archean schists from erosion. Where 

 they have been cut through, first by the erosive action of glaciers and 

 later by the corrasive action of mountain streams, to their stratified structure 

 is due the formation of the almost perpendicular clifts which form the canon 

 walls of their streams. The generall}^ abrupt slope immediately west of 

 the crest is due to a great fault extending along its foot, in virtue of whose 

 movement the western continuation of the sedimentary beds, which slope 

 up the eastern spurs and cap the crest itself, are found at a very much 

 lower elevation on the western spurs ; while tlie jagged outline of the 

 western spurs is due to a series of minor faults and folds, crossing them 

 nearly at right angles. The secondary uplift of the Sheep Mountain ridge 

 on the eastern slopes is the expression of a second great line of fault and 

 flexure, whose direction, like that of the ridge itself, forms an acute angle 

 with that of the main crest. The elevation of the Mount Lincoln massive 

 is the result of a combination of the forces which have uplifted the Mosquito 

 Range and of those which have built up the transverse ridge which sepa- 

 rates the South from the ]\Iiddle Park. 



In the later topography of the range the results of the action of a 

 svstem of enormous glaciers are seen in the immense amphitheaters which 

 form the heads of its main streams, and in the characteristic V-shaped 

 transverse outlines of the valleys descending from them. Finally, the mesa- 

 like character of the lower end of the western spurs toward the Arkansas 

 Valley is due to the existence beneath their surface of comparatively undis- 

 turbed beds deposited at the bottom of a lake formed at the head of that 

 valley by the melting of the ice at the close of the first portion of the 

 Glacial period. 



