24 W. UPHAM — DRUMLINS AND MARGINAL MORAINES. 



occupied more than a tenth part of the whole duration of the lake, or by 

 rough estimate a hundred years, more or less. The conspicuous belts of 

 morainic hillocks, hills and ridges, consisting of very bouldery till, fre- 

 quently with much kame gravel and sand, of which I liave mapped 

 twelve in Minnesota and North Dakota, and Leverett a still larger number 

 in Illinois, Indiana and Ohio, were therefore probably each amassed 

 within a few years, or at the longest probably no more than 25 or 50 years, 

 even for the accumulation of the prominent Leaf hills, rising 200 to 350 

 feet above the surrounding countr3^ How could such rapid drift trans- 

 portation and deposition take place ? If this question can be satisfac- 

 torily answered, with reference of the moraines, both in North America 

 and Europe, to the time of retreat from the lowan glacial boundaries, a 

 chief argument, which has been much relied upon by the defenders of 

 the theory of two or several distinct glacial epochs, will be set aside. 



Conditions of Formation of marginal Moraines. 



Englacial drift had been carried by the ice currents in some important 

 amount into the basal quarter or third of the ice-sheet; and when the 

 su]3erficial melting or ablation reduced the ice border to a less thickness, 

 this drift was gradually uncovered upon the ice surface. The rates of 

 ascent of the frontal slope may be assumed, in accordance with the upper 

 limits of glacial action on mountains, and after careful consideration of 

 the surface gradients of the Alpine glaciers and of the Greenland ice-sheet, 

 as 400 feet in the first mile, 200 feet in the second mile, and 150, 120, 100, 

 85, 75, 67, 60 and 55 feet in the third to the tenth miles, respectively, 

 attaining an altitude of 1,312 feet, or about a quarter of a mile. Thence 

 we may suppose the ascent to average 50 feet per mile for the next nine 

 miles, by which the altitude of a third of a mile, the probable upper limit 

 of the englacial drift on any area where the ice-sheet had been about a 

 mile thick, would be reached. 



On areas where the ice-sheet built up large marginal moraines, and 

 also wherever its drainage from ablation brought exceptional volumes of 

 modified drift, or stratified gravel, sand and clay, directly supplied by 

 the ice melting, we must believe that the amount of the englacial drift 

 was greater than on other tracts having smaller moraines and little modi- 

 fied drift. Let us assume, therefore, for the definite illustrative case in 

 which We are seeking to account for prominent moraine accumulations, 

 that the total englacial drift in the lower third or 1,760 feet of the ice-sheet 

 was equal to a thickness of 15 feet. This may have been distributed, as 

 shown in the accompanying table, so that the basal ice stratum, 400 feet 

 thick, terminating within the first mile from the front, should contain 

 5 feet of englacial drift ; the stratum, 200 feet thick, terminating in the 

 second mile, 2 feet of drift; the 150 feet of ice terminating in the third 



