DEPTH OF THE ESKER 421 



Xeil J. McGregor's well in Birds Hill village, about 15 rods southwest 

 from the edge of the esker ridge near its west end, bored last summer, 

 has this section: Soil, 1 foot; bluish till, with few boulders, 11 feet; gray 

 till, 8 feet ; coarse gravel, all water-worn, with nearly all the cobbles lime- 

 stone, very abundant, 1 to 10 inches in diameter, 40 feet; less coarse 

 gravel, or more probably till, with only few stones, 10 feet; limestone, 

 tbe bedrock, 28 feet, to the bottom of this well, at 98 feet, whence water 

 I'ises in tbe well pipe 70 feet, to 20 feet below the surface. 



Here we may think that an englacial tunnel, occupied by a torrent 

 that had fallen through crevasses and moulins of the ice, thence flowing 

 through this tunnel to join the esker river, became filled with the tor- 

 rential gravel, similar to that chiefly forming the west end of the esker. 

 The bottom of the coarse gravel in this well is at a depth of 60 feet from 

 the surface, being somewhat lower than the bottom of the wells in the 

 excavation. The bluish gray color of the till, extending to the surface, 

 is due to its relatively unoxidized and unleached condition, in contrast 

 with the yellowish gray till of the esker, where its height above the ad- 

 joining land has permitted infiltration of the water of rains, favoring 

 the change of its iron element from the protoxide combinations to the 

 rust-coloring sesquioxide. The same bluish gray till forming the surface 

 is seen at the post-office kept by George Chudleigh, at the school-house, 

 and indeed in all the cellars and wells of the village. 



Eelationship to the Glacial Lake Agassiz 



If the overlying till in Mr. McGregor^s well, having a thickness of 20 

 feet, represents an average of the amount of englacial till above the level 

 in. the ice-sheet at which the esker was formed, the total quantity of 

 englacial drift was apparently equal to a thickness of 30 feet or more, 

 some two-thirds of it being held and carried along higher in the ice than 

 the highest level of Lake Agassiz here, which was about 500 feet above 

 the present land surface. 



Much of the interest of this subject depends on the relation of Birds 

 TJill to Lake Agassiz. The esker is situated near the center of the area 

 of the ancient glacial lake. On the east border of its area the Herman 

 Beach, the earliest and highest of a series marking the boundaries of the 

 old lake, is mapped from the southward outlet at Browns Valley, Minne- 

 sota, in its course north and northeast to the vicinity of Eed Lake, being 

 there 1,210 to 1,215 feet above the sea; and on the west it extends through 

 North Dakota and onward across the Assiniboine Eiver to Eiding Moun- 

 tain, west of the south part of Lake Manitoba, there lying at an altitude 

 of 1,300 to 1,320 feet above the sea. 



I 



