158 GEOLOGICAL SURVEY OF THE TERRITORIES. 



proving the fact conclusively, yet the manner of their occurrence is such 

 as to indicate that they were, in part, contemporaneous with the lake 

 beds, the upper terraces of the latter apparently covering and mantling 

 around their ends. 



Alluvial material occurs here and there adjacent to the streams, and 

 may in places have been taken for lake beds, being probably the same 

 material worked over. Indeed, the alluvium is in places the later pro- 

 duct of the same or similar forces which produced the lake beds. 

 With the alluvium should be classed the meadows inclosed by the gla- 

 cial material. 



Basaltic lava covers wide areas, forming some of the highest points, 

 where it usually caps, as a protective covering, the lignitic sandstones, 

 though it has also run down into the lower regions, covering large areas 

 of the lake beds. 



Having spoken thns generally of the various rocks composing the 

 Middle Park, their more special characters and distribution will now 

 receive attention, taking up in succession the various areas which com- 

 pose the park. 



THE VALLEY OF THE UPPER GRAND. 



The North Fork of the Grand Eiver occupies a profound valley lying 

 between the southern end of the Medicine-Bow range, upon the west, 

 and the Long's Peak group, of the main range, upon the east. Looking 

 up this great valley from the south, the walls of rock seem to rise in great, 

 abrupt, but rather even, massive slopes to a pretty general mountain 

 level on either hand, the bottom of all the lower portion of the valley 

 being quite flat, and averaging about two miles in width. Within the 

 valley, however, the numerous great gorges coming down from either 

 side, chiefly from the east, so break up the apparent regularity of the 

 sides as seen foreshortened from the south, that the impression is of a 

 maze of rugged mountain spurs and canons. Those from the west are 

 of lesser magnitude, the Medicine Bow crest being only from three to 

 four miles west of mid-valley. At the north this crest is a sharp and 

 ragged ridge, but southward it becomes comparatively even and rounded 

 in outline, a massive ridge, falling gradually until covered with lake 

 beds near the junction of the East Fork with the Main Grand. The east- 

 ern valley side retains its ruggedness all the way to the East Fork. The 

 principal drainage of the latter area is that collecting into Grand Lake, 

 and is by a system of the profoundest mountain caQons. 



All this surrounding mountain region is of the archsean rocks, schists, 

 and gneisses. The few dips and strikes that I had opportunity to actu- 

 ally observe, together with the impression obtained from the style of 

 weathering of the rocks, indicated that the valley might be a great syn- 

 clinal, but this is by no means certain. The apparent appropriate uni- 

 formity of form and slope of the valley sides seems hardly a product of 

 erosion, but as if the general surface of the metamorphics had been cov- 

 ered, as at the west, with sedimentary rocks, perhaps the Cretaceous, 

 and as if all had then been folded together to form an anticlinal over 

 the Medicine Bow, a synclinal in the Grand Valley, the latter probably 

 originally determining the course of the drainage, and hence of the 

 greatest erosion which has since removed all of the sediments, but not 

 quite yet destroyed the general impress left by the fold upon the harder 

 underlying rock. 



All the main portion of the valley is occupied by a mass of morainal 

 matter, (see Fig. 9.) Below where the upper caiion i^ortions widen 



