THE LOCAL ORIGIN OF GLACIAL DRIFT 427 



sources has always seemed a stumbling block to those who are 

 familiar with the facts, but not with their meaning. 



In Fig. 1, a represents the center of the sector of an ice-cap, 

 and b d its circumference. The areas marked I, 2, 3, and 4 rep- 

 resent successive and equally wide belts of rock of equal resis- 

 tance and like topography. The ^ 

 center of movement is assumed to 

 be at a. When the ice has advanced 

 from a to b d the deposit of till 

 made at that point is made by ice 

 which, in so far as it has moved 

 from the center, has passed over 

 formations I to 4 in succession. In 

 such situations the drift is normally 

 found to contain more material from 



4 than from 3, more from 3 than 



H J ' J Fig. 1. 



from 2, and more from 2 than from I. 



It will be understood that if the width of the exposures of 

 the several formations were unequal, or if the several formations 

 were of unequal resistance, the case might be very different. It 

 is also clear that the topography of the several belts will influ- 

 ence the amounts of their contributions, and in view, first, of the 

 varying widths of the various belts of rock passed over by the 

 ice, second, of their varying topographies, and third, of their 

 varying degrees of resistance, many exceptions may arise to the 

 generalization that the contributions of various formations to the 

 till of any locality are in inverse proportion to their distances 

 from the point concerned. 



The explanation of the markedly local character of the drift 

 appears to involve several considerations. Fig. 1 illustrates one 

 point involved. Suppose the ice passing over the formations 1, 

 2, 3, and 4, successively, gathers material with equal facility 

 from each of them. By the time the ice from 1 has spread over 

 2, the average thickness of the basal layer of drift derived from 

 I, supposing none of it to have been deposited, would have been 

 reduced to one third its original thickness, since the area of 2 is 



