350 W. UPHAM — GLACIAL ACCUMULATION AND INVASION. 



rarely perhaps 10 or 20 miles. A much farther extension of new drift 

 over an old drift surface by gradual snow and ice accumulation, with some 

 glacial inflow from the north, is known in Ohio, Indiana and Illinois, and 

 especially in northeastern Iowa, where a forest bed between the successive 

 till deposits has been mainly preserved, undisturbed by erosion during 

 the later ice accumulation, upon large areas reaching 10 to 30 miles or 

 more back from the margin of the newer drift. 



Irregularity of gla^cial Invasion and its meteorologic Explanation. 



From my exploration of the moraines and other drift deposits of the 

 Minnesota lobe of the ice-sheet, it is learned that, while the south end 

 and western side of this ice-lobe were being melted back from the first 

 or Altamont moraine to the fifth and sixth or Elysian and Waconia 

 moraines, its eastern side became absolutely or relatively thicker than 

 before in comparison with the ice outflowing southwestward from the 

 Lake Superior area. The southeastward and eastward currents of the 

 central part of the lobe therefore pushed back the southwestward cur- 

 rents, so that the gray and blue drift from the Cretaceous shales of west- 

 ern Minnesota and from the Silurian limestones of Manitoba was spread 

 over the red drift derived from the Cambrian and Keweenawan sand- 

 stone and eruptive rocks of the Lake Superior basin. An overlap of the 

 gray drift, with much limestone, above the red drift, containing no lime- 

 stone, is observed along an extent of fifty miles from Wright and Anoka 

 counties northeastward to the Saint Croix river on the boundary of 

 Wisconsin.* 



At a somewhat earlier time, preceding and during the formation of the 

 Altamont moraine, while much melting and recession of the ice-sheet 

 were taking place in Iowa, South and North Dakota and southeastern 

 Minnesota, with rapid deposition of loess, the ice border on the east side 

 of the Wisconsin driftless area appears to have increased in thickness 

 and to have encroached at least several miles on that area, covering 

 ground which had not previously been glaciated. 



During the same time and later, while the ice-sheet in the upper Missis- 

 sippi basin and the region of the Laurentian lakes eastward to lake 

 Nipissing and the east end of lake Erie was melting away, its southern 

 border farther east still reached approximately to its early maximum 

 limits, and in certain portions, probably including the whole extent of 

 Long island, Marthas Vineyard and Nantucket, there was apparently an 

 advance somewhat beyond the earlier boundary .f 



* Proe. Am. Assoc. Adv. Sci., vol. xxxii, 1883, pp. 231-234. Geology of Minnesota, vol ii, 1888, pp. 

 254-256, 409-413. 



fBull. Geol. Soe. Am., vol. vi, p. 26, November, 1894; Am Jour. Sci., iii, vol. xlix, pp. 1-18, with 

 map, January, 1895. 



