94 GEOLOGICAL SURVEY OF THE TERRITORIES. 



approximately north and south, passing about twenty miles west of 

 Denver. 



Along it, the sedimentary rocks of the plains, instead of remaining 

 horizontal, with their edges thus abutting against the steep faces of the 

 mountain-rocks, like the sands along a rocky shore, are found to be all 

 bent or folded more or less abruptly upward, their worn edges often 

 rising into the air and presenting their scarred faces to the mountains. 



It becomes evident, therefore, that the sedimentary strata rest upon 

 the rocks of the mountains, and that the latter, descending beneath the 

 former, form the foundations on which they rest. 



This upturning of the sedimentary rocks along the mountains is illus- 

 trated in a general way in the accompanying diagramatic sketch. 



Inasmuch as sediments laid down under water cannot end abruptly, 

 it is apparent that these beds must once have been extended much 

 farther up on the central mountain-mass than now, as indicated by the 

 dotted lines. 



We will see later that after this folding of the rocks took j)lace the 

 level of the sea long stood above the present upturned edges of the sedi- 

 mentary rocks, and that, as the land through ages gradually emerged 

 from the water, the wearing action of the slowly-retreating waves and 

 of the usual subaerial erosion gradually removecl all the higher portions 

 of the strata, working back their edges, until they are left as we now 

 find them. 



The surface of the country, as now presented to us, therefore, is due to 

 the action of two causes : first, a folding of the rocks ; second, erosion of 

 those higher and ever-enlarging portions which slowly appeared above the 

 gradually-retiring sea, and which may be progressing as rapidi}^ now as 

 the average erosicai in the past. 



As there is always a variety in the hardness or other characters of the 

 different beds, some will withstand erosion far less readily than others, 

 and, wearing away faster than their harder neighbors, leave the latter 

 rising as ridges above the general surface. These ridges are cut through 

 every few miles by the streams that flow down from the mountains, and 

 while the immediate effect of these streams is to carve abrupt notches 

 or canons through the ridges, general surface-erosion tends to lower their 

 adjacent ends. As seen from the east, therefore, these ridges appear to 

 rise gradually out of the plain from either end to a nearly level center, 

 in long gentle convex curves, and they have thus earned for themselves 

 the unfortunate name of Sog-hacks. 



But there are other beds to claim our attention a moment than those 

 which have been upturned. The erosion which has molded the latter 

 into their present forms has been much longer busy in the higher moun- 

 tains, and much of the material worn from them since the folding of the 

 rocks took place has been deposited in flat beds near the mountain's base, 

 often surrounding and quite covering the hog-back ridges. 



This border region, which varies from one to ten miles in width, may, 

 then, be briefly described as one having abrupt mountain-masses rising 

 boldly and ruggedly upon the one hand, and rolling plains stretching far 

 past the eastern horizon upon the other, Avhile along it the worn edges 

 of the harder upturned strata rise in long parallel ridges, the ends of 

 which may fall either abruptly or in long gentle convex curves to the 

 valleys of the cross-cutting mountain-streams, and which, rising gently 

 from the east, fall in abrupt escarpments on the west to the trough-like 

 valleys of softer beds between, in places all well exposed, and again so 

 nearly covered with the accumulation of more recent material in hori- 

 zontal beds that the higher ridges barely peep above the surface. In 



