GLACIAL DEPOSITS. 93 



Post-Giaciai formations. The Post-Glacial deposits of unstratified gravels 

 are equally prominent, however, on both side* of the range. They result 

 in great part from the redistribution of glacial moraines by the floods which 

 accompanied the melting of the ice at the close of the Glacial period. In 

 the Arkansas Valley they were spread out over the already existing Lake 

 beds, and reach a relatively high level on the mountain spurs. In the 

 western portion of the South Park they form the flood-plain of the larger 

 valleys, which they filled up to a very considerable depth, as has been 

 shown by excavations made at Alma and Fairplay in washing them for 

 gold. Depths of 60 to 100 feet have here been proved of coarse gravel con- 

 glomerate, entirely without stratification. These points are comparatively 

 high up and near the source of supply, and it may be assumed that finer 

 material of the same origin extends to equal if not to greater depths well 

 out on the bottom lands of the park. Within these flood-plains the streams 

 run in alluvial bottoms which widen as one descends and often open out 

 into broad meadows, partially drained lake basins, where some natural ob- 

 stacle has caused a partial damming up of the earlier streams. Of actual 

 moraines no inconsiderable remnants still remain. They can be most clearly 

 seen along the steep sides of the canon gorges through which the mountain 

 streams debouch into the more open valleys, where they often form gravel 

 ridges several hundred feet in height; and on the lower spurs beyond these 

 canons their existence under the forest growth may often be surmised by 

 their characteristic topography of irregular ridges inclosing rounded hollows 

 without exterior drainage, as well as proved by shafts and tunnels made by 

 the misapplied energies of prospectors. 



Archean exposures. To the lithologist no more favorable opportunity could 

 be had for an exhaustive study of the older crystalline rocks which form 

 the backbone of the Rocky Mountain system than that afforded by the 

 exposures in the deep gorges and glacial amphitheaters of the interior of 

 this range. The scope of this work did not admit, however, of any such 

 exhaustive study, which would have required much more time than could 

 have been devoted to the whole region. The utmost that could be done 

 was to grasp the more salient characteristics of the series and to outline on 

 the map such of the more important eruptive masses which intersect them 



