batdek.j MORAINAL DEPOSITS. 51 



and operating with greater or less effect at different portions of that 

 period. So far as the drift-deposits are concerned, which at the present 

 time seem to be the only material resting on the granite in the valley, 

 they are undoubtedly of comparatively modern origin, not extending 

 back farther than the Pliocene, but the beginning of the erosion may 

 reach into the past as far as the Jurassic. We have now the evidence 

 that indicates that portions of these mountain-ranges were elevated 

 above the Jurassic seas, and we may suppose that the general outline 

 of the surface continues on the same plan up to the present time. If 

 this was true — and we have no reason to believe the contrary — the erosion 

 may have, and probably did commence far back in the past, and that 

 during the Cretaceous and Tertiary periods the area occupied by the 

 Sawatch range was elevated above the waters ; it is more probable, 

 however, that these formations were deposited to a greater or less ex- 

 tent over this area, and that they have been entirely removed or ground 

 up with the present drift. From the source of the Arkansas at the 

 Tennessee Pass to the canon above Canon City, the distance is about 

 60 miles, supposing this to be mainly a valley of erosion, area worn 

 away would average about eight miles in width, the depth could not have 

 been less than one mile, so that an approximate estimate can be made of 

 the enormous amount of rock-material has been ground up in the excava- 

 tion of this valley. Up to the time of the great glacial period this eroded 

 material may have been swept out on to the plains to assist in forming the 

 vast Cretaceous and Tertiary beds which we find there at the present 

 day. Geologists generally admit that about the close of the Tertiary 

 period, there was an era of intense cold, which they have agreed to call 

 the glacial epoch, and our remarks are based on .that supposition. 

 We believe that at one period this entire valley, with all the side-valleys 

 or canons, was occupied with one vast glacier, diminishing and in- 

 creasing as the temperature was higher or lower, but gradually moving 

 down; that is, the main mass moving southward, and the side-branches 

 moving toward the central mass. As the sides of the mountains are 

 worn smoothly and exhibit signs of glacial action to the height of at 

 least 1,500 feet above the valley, we may arrive at an approximate 

 estimate of the thickness of the glacier. The fissures of the Arkansas 

 and its branches may not have been nearly as large at the commence- 

 ment of the glacial period as at the present time, and the great glacier 

 may have performed the work of erosion for ages, and gradually melt- 

 ing by a change of temperature to the mild climate of the present time, 

 left the numerous mounds, ridges, and other morainal deposits which 

 we find so extensive in this valley, and in many other portions of the 

 Eocky Mountain region. I have spoken of this great ice-mass as a 

 single glacier; there may have been a single one increasing and dimin- 

 ishing through ages with the changes of temperature at different seasons 

 or epochs, or there may have been an unlimited number of glaciers, but 

 the glacial phenomena as indicated by the present surface of that 

 country shows a long and continuous period of action. I have before 

 stated that I regarded the valley as one great lake-basin, commencing 

 near the Tennessee Pass. The valley expands out somewhat for the 

 first ten miles, and gradually closes up below the town of Granite for 

 about four miles, when it opens out again into a broad, level, basin-like 

 form. The bottoms of the main river, as well as the little branches, 

 expose the granite, rocks in such a way that we cannot well avoid the 

 conclusion that they have been worn down to their present position 

 from an elevation not much inferior to the Sawatch or Park ranges. 

 Above the Lake Creek on both sides of the Arkansas, are well de- 



