POET HURON MOBAINIC SYSTEM AND PEOBABLE COEEELATIVES. 313 



ward, or where the clrumlins lie on a horizontal surface, the lamination and evidences of build- 

 ing up by slow accretion are best displayed. It thus appears probable that the attitude of the 

 surface passed over by the ice determined to a large extent the method of drumlin formation. 



After the above description of the drumlins was written Alden presented a paper before the 

 Pittsburgh meeting of the Geological Society of America, an abstract of which has reached the 

 writer, in which he discussed radiation of glacial flow as a factor in drumlin accumulation. Radi- 

 ation was about as marked in the Grand Traverse region as it was in the Green Bay region studied 

 by Alden, so the drumlins under discussion may have had a like origin. The Grand Traverse 

 region, however, does not afford so good a field as the Green Bay for testing the hypothesis. The 

 abstract presented by Alden is as follows: 



This discussion is based on the relations of the moraines and drumlins of the south half of the Green Bay glacier of 

 eastern Wisconsin. There is a very notable development of drumlins which are grouped in three more ur less distinct 

 sets. Corresponding to each is a set of marginal moraines believed to mark the limits of the glacial lobe during the 

 stages when the drumlins were being formed. The outer moraine of each set marks the limit of a readvance of the ice 

 following an interval of recession. The drumlin belts in each case are confined to distances of 30 to 35 miles from the 

 south end of the lobe, that is, to that part where the ice was radiating widely to the curved margin of the lobe and where 

 it was thinned in consequence of the radial spreading and of loss by melting and ablation. 



The computations of the probable elevation of the surface of the glacier were based on the known thickness of the 

 ice within 4 miles of the margin where the Baraboo quartzite range was overridden and on an estimated average slope 

 for this surface of 50 feet per mile in the first 25 miles, including the initial rise of 700 feet in the first 4 miles and allow- 

 ing an average of 20 feet per mile thereafter. Deducting from these the present elevation of the land gives a thickness 

 of ice varying from 1,450 feet over the initial part of the first drumlin belt to 450 to 830 feet where drumlins ceased to be 

 formed within 5 miles of the limit of ice advance. The drumlin-forming ice stream had an initial width at A of 17 

 mil es where radiation began. In advancing 15 miles this expanded to a width of 32 miles at B and at the terminal 

 moraine the expansion gave a marginal arc of about 100 miles. Computations show that in spreading to the width at B, 

 while at the same time maintaining the requisite thickness and low surficial slope, the cross section of the stream must 

 have increased to 1.603 times the area of the initial section and, inasmuch as no tributary stream had added to the 

 volume, the rate of flow must have been faster at A than at B. At C, where the drumlin formation ceased, the arc of 

 the stream had expanded to 85 miles and the cross section was 2.325 times that at A and 1.45 times that where drumlin 

 formation began. From these figures it is evident that the spreading of the ice under its own weight alone would not 

 account for the remarkable expansion. Only the forward crowding of the more rapidly moving ice in the rear could 

 have supplied the requisite volume. Though the stream expanded greatly so that friction was much increased, the 

 remarkable development of drumlins indicates that the basal ice did not clog. Such basal movement was probably 

 due to the ice being shoved bodily forward by the vigorously advancing ice in the rear, this forward shove being super- 

 imposed on such internal flowage as was taking place. It is believed that the application of such propulsive force in 

 the region of the center of radiation of such a mass, which was tending to lag in every part, owing to great friction, would 

 tend to cause the longitudinal lines to spread and so develop stress along transverse lines. These stresses, though per- 

 haps not causing the actual opening of longitudinal crevasses, would facilitate spreading of the ice about obstructing 

 piles of drift and their being shaped into drumlins rather than their obliteration by erosion. It might also induce 

 localized deposition in piles or ridges which would later be shaped and might be added to by the plastering on of drift. 

 Computations based on the ice stream forming the second set of drumlins show the radiation to have been even more 

 marked than in the first case, with corresponding greater crowding forward of the faster moving ice in the rear and more 

 marked development of drumlins. Comparison with segments of the glacier which had equal initial widths but did 

 not form drumlins shows that in the latter there was very moderate radiation and that, unless the ice in the rear was 

 moving more slowly than that in front, there was a decrease in the volume of the stream as it advanced, as opposed to 

 the increase in volume of the drumlin-forming stream. With the radiation fully accounted for by the ice under its own 

 weight and with no forward crowding of the ice in mass there would be absence of lateral stress and of the tendency to 

 longitudinal crevassing, and this may explain the absence of drumlins. Other factors than radiation are probably 

 involved. 



Structure.- — -The material of which the drumlins of the Grand Traverse area are composed is 

 in large part a reddish till. This is especially true of the drumlins in the region of least relief 

 and especially in the district shown in figure 2, between the north part of Torchlight Lake and 

 the south arm of Pine Lake, where the drumlins are the most numerous and best formed of any 

 in this field. Southeastward toward the moraine a change takes place from clayey to looser- 

 textured material, and the drumlins on the extreme southeastern outskirts of the field are gen- 

 erally very loose textured. This change in texture is a natural result of an approach to the edge 

 of the ice, for more or less fine material is carried away from the ice border by the waters dis- 

 charging from the border, thus leaving the coarser material to be deposited in the moraines and 

 other deposits made near the edge of the ice. Pressure is also markedly less near the edge than 



