CHAPTER IV 



Ancient lake basins. — Glacial lakes.— Moeainal deposits in the valley of the 

 Upper Arkansas River and along both planks of the Sawatch Mountains. 



For nearly twenty years I have written more or less in regard to the 

 ancient lake-basins of the West, but it was only within a few years, since 

 the facilities for traveling have so greatly increased that geologists 

 have found that these lake-basins once occupied the entire country 

 from the Arctic Circle to the Isthmus of Darien. In very many in- 

 stances they were merely expansions of river valleys, like the greater 

 number of our lake-basins of the present day. During the early portion 

 of the Tertiary period, the western portion of our continent was covered 

 with immense lakes, some of which occupied a much larger area than 

 any we are acquainted with at the present time. During the Pliocene 

 period, and during the interval to the present time, thousands of small 

 lakes, with a few of large size, were distributed over the great area 

 west of the Mississippi, and the basins with their peculiar deposits are 

 found in the parks, among the mountains, and along every important 

 river- valley. The gathering together of the vast amount of information 

 which is now accumulating on this subject is a task which will, at no 

 distant day, be productive of most interesting results. 



I have made these few remarks to introduce what I may have to say 

 in regard to the valley of the Upper Arkansas Eiver. 



The Arkansas Eiver rises in the Tennessee Pass, nearly west of Mount 

 Lincoln, in latitude 39° 21/ and longitude 106° 19', and flows a little east 

 of south for a distance of about 80 miles in a straight line, when it flexes 

 to the east, and flows through a deep canon in the granite, and emerges 

 into the plains near Canon City. Near the sources of the river are several 

 expansions of the valley from one to two miles in width, oval-shaped, 

 and covered with a deposit of drift-material. Near the junction of the 

 east branch of the Arkansas the valley, with the terraces on either 

 side, continues pretty regularly about five to eight miles in width, but 

 gradually closes up again below Lake Creek, though on either side are 

 vast deposits of the coarse drift-material extending high up on the 

 mountain-sides, especially on the west side of the valley. The valley 

 then gradually expands out and enlarges about five to ten miles in width 

 for a distance of nearly 40 miles. In the annual report for 1873 I 

 have expressed my belief that this valley began in a monoclinal inter- 

 val, with the great Sawatch range on the west side forming the crest of 

 the continental water-shed, and the Park range on the east, which, 

 with its sedimentary rocks and granite basis, formed the east side of a 

 grand anticlinal, the aggregate mass of rocks inclining to the eastward. 

 Our observations over a very extended area only confirmed the opinion 

 expressed in our last report, that the great Sawatch range formed the 

 central portion of a gigantic anticlinal. The west side of the Park 

 range is, for the most part, very abrupt, and for long distances the 

 gneissic rocks show very clearly the direction of the dip. On the east 

 side the sedimentary rocks dip down under the surface of the South 



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