966 HISTORICAL GEOLOGY. 



route around the ¥alls near Louisville, which it lost when the ice extended 

 to its southernmost limit. The Ealls are evidence of uncompleted work in 

 subsequent erosion along the valley. 



It is held by some investigators of the drift, and prominently by Chamberlin, that the 

 retreat, instead of ending along the line of the moraine above described, continued until 

 North America had lost the chief part of its ice-sheet, and that this ' ' First Glacial Epoch ' ' 

 was followed by a second advance, of which moraine B was the terminal moraine. This 

 view is sustained on the ground that the erosion produced during the interval, the inter- 

 calation of forest-beds and stratified clays, and the weathering and oxidation of the 

 lower tills would have required a very long period of time. It is, however, an important 

 consideration in favor of the shorter retreat, that the beds eroded were, to a great extent, 

 soft ; that the amount of water discharged was very large ; and that interstratified sand- 

 beds and forest-beds are such as modern glaciers are now producing. The arguments and 

 facts favoring the theory of two glacial epochs and an intei'glacial are presented by Cham- 

 berlin in his Report on the Geology of Wisconsin ; also in the 3d and 7th Reports of the 

 U. 8. Geol. Siirv., and in later publications, in part of which Leverett is joint author ; by 

 G. M. Dawson in his Memoir on Rocky Mountain Geology in the Trans. Boy. Soc. Canada, 

 vol. viii., 1890, etc. Upham, Hitchcock, "Wright, and others favor the idea of a continuous 

 succession of recessions and halts during the retreat. 



In northeastern Iowa, according to McGee, the successive glacial deposits are : (1) the 

 lower till, which is overlaid by stratified sands and clays (called locally gumbo) ; (2) a 

 forest-bed, with unconformity beneath through erosion and decomposition ; (3) an upper 

 till of small extent, from ice that was of short duration ; (4) the loess, which contains 

 some bowlders, and graduates at base into the till. These are supposed to be anterior to 

 what is called by Chamberlin the Second Glacial Epoch. The loess is stated to have been 

 formed in an ice-bound lake, which he names Lake Hennepin, made by the meeting of 

 two lobes of ice, advancing either side of the Driftless area. The loess makes a fertile 

 soil, which appears to be evidence that there was abundant vegetation in the waters in 

 which it was deposited, and thus throws doubt over the presence of the ice. The depau- 

 perate condition of the shells shows only that the waters were cold ; and their great 

 numbers, that conditions of growth were still not very unfavorable. 



The great distance of transportation of glacial drift over the Continental Interior in 

 British America, and the remarkable uniformity in the drift deposits over the vast area — 

 " 250,000 square miles " — has led to the view that the region was submerged under fresh 

 or salt waters, and that floating ice was the transporter. But the flow over such waters, 

 whether tidal or not, would have been north and south, and not across the area j- and 

 there is no evidence of marine conditions. Moreover, if floating ice worked there, it was 

 the agent to the south in the United States ; and this is not in accordance with the facts 

 there observed. 



Land and freshwater shells and other fossils of the loess of the Mississippi valley. — 

 Erom Galena, 111. : Succinea avara, 8. obliqiia, Patula striatella ; Vallonia pulchella, 

 Limnophysa Mimilis, L. desidiosa, Pupa contracta, P. muscorum (R. E. Call). — From 

 Davenport, la. : Succinea avara, 8. obliqua, Helicina occulta, Pupa fallax, Helix stria- 

 tella. Also tusk and molars of Elephas primigenius (Pratt). — From Muscatine, la. : 

 Helix striatella, H. fulva, H. pulchella, H. lineata, H. Cuperi, Pupa Blandi, P. quarti- 

 caria, P. muscorum, P. simplex, Succinea avara, 8. obliqua, Helicina occulta, Limncea 

 humilis, Unio ebenus, U. ligamentinus, U. rectus, Melantho subsolida, Margaritina con- 

 fragosa. Also teeth, bones, and antlers of Cervus Muscatinensis (Witter, in McGee's Iowa). 



From Hickman, in Kentucky : Conulus chersina, Hyalina arborea, Helicina orbicu- 

 lata, H. profunda, Limncea (Limnophysa) desidiosa, Mesodon profundus, M. albolabris, 

 Macrocyclis concava, Patula alternata, P. perspectiva, P. solitaria, Stenotrema (Helix) 



