Warren Upham — Pleistocene Climatic Changes. 343 



comprising portions which rise in hills 150 to 200 feet high, all 

 belong to the southern third or half of the entire extent of Lake 

 Agassiz, and probably also to the first third or half of its entire 

 duration. When the forest-covered and almost uninhabited country 

 northward shall be fully explored, probably the number of moraines 

 ascertained to have been formed during the existence of this glacial 

 lake will be at least doubled. Growing from south to north as the 

 barrier of the continental glacier retreated, Lake Agassiz attained 

 a length of about 700 miles, and an area of not less than 110,000 

 square miles, exceeding the combined areas of the great lakes out- 

 flowing by the St. Lawrence ; and its depth above Lake Winnipeg 

 was about 600 feet. 



If now the question be asked how many thousand years ago did 

 the recession of the ice-sheet take place, producing Lake Agassiz, 

 and at halts or slight stages of re-advance forming the moraines, a 

 reply is furnished by the computations of Prof. N. H. Winchell,^ 

 that approximately 8000 years have elapsed during the erosion of 

 the post-Glacial gorge of the Mississippi from Fort Snelling to the 

 Falls of St. Anthony ; of Dr. Andrews,^ that the erosion of the 

 shores of Lake Michigan, and the resulting accumulation of dune 

 sand drifted to the southern end of that lake, cannot have occupied 

 more than 7500 years ; of Prof. G. Frederick Wright,^ that streams 

 tributary to Lake Erie have taken a similar length of time to cut 

 their valleys and the gorges below their water-falls ; of Mr. G. K. 

 Gilbert,* that the gorge below Niagara Falls has required only 

 7000 years, more or less ; and of Prof B. K. Emerson,^ on the 

 rate of deposition of modified drift in the Connecticut Valley at 

 Northampton, Massachusetts, from which he believes that not more 

 than 10,000 years have elapsed since the Glacial period. An equally 

 small estimate is also indicated by the studies of Gilbert ^ and 

 Russell ' for the time since the last great rise of the Quaternary 

 Lakes Bonneville and Lahontan, lying in Utah and Nevada, within 

 the arid Great Basin of interior drainage, which are believed to have 

 been contemporaneous with the great extension of land-ice upon the 

 northern part of the North American continent. These measures of 

 time, surprisingly short, whether we compare them on the one hand 

 with the period of authentic human history, or on the other with the 

 long record of geology, carry us back to the date when the ice-sheet 



' Geology of Minnesota, Fifth Annual Report, for 1876 ; and Final Report, 

 Tol. ii. 1888, pp. 313-341. Quart. Journ. Geol. Soc. vol. xxxiv. 1878, pp. 886-901. 



^ Trans. Chicago Academy of Sciences, vol. ii. James C. SoathaU's Epoch of the 

 Mammoth and the Apparition of Man upon the Earth, 1878, chapters xxii. and xxiii. 



^ Amer. Journ. Science, iii. vol. xxi. pp. 120-123, February, 1881 ; The Ice-Age 

 in North America, 1889, chapter xx. 



^ Proc. Amer. Assoc, for Advancement of Science, vol. xxxv. for 1886, p. 222. 

 "The History of the Niagara River," Sixth Ann. Rep. of Commissioners of the 

 State Reservation at Niagara, for 1889, pp. 61-84. 



5 Amer. Journ. Science, iii. vol. xxxiv. pp. 404-5, November, 1887. 



^ United States Geol. Survey, Second Annual Report, for 1880-81, p. 188 ; 

 Monograph I. Lake Bonneville, 1890, chapter vi. 



' "United States Geol. Survey, Monograph XI. Geological History of Lake 

 Lahontan, 1885, p. 273. 



