TILL OR BOWLDEE-CLAY. 135 



nearly to the Red River and Winnipeg and south to the Canadian Pacific 

 Railway, from East Selkirk eastward along Ijhis railway, and 10 miles east 

 of Emerson, where the flat plain of the Red River Valley is bordered by 

 slightly higher land. Till also forms the surface of the terrace along the 

 foot of the Pembina Mountain escarpment between the international 

 boundary and Thornhill. Beneath the delta deposits of gravel and sand, 

 and along the central portion of the Red River Valley, where the surface 

 is commonly fine silt or clay, a sheet of till lies between these sediments 

 and the bed-rock. 



The till is the direct deposit of the ice-sheet, as is shown by its 

 consisting of clay, sand, gravel, and bowlders, mingled indiscriminately in 

 an unstratified mass, without assortment or transportation by water. Very 

 finely pulverized rock, forming a stiff, compact, unctuous clay, is its prin- 

 cipal ingredient, whether at great depths or at the surface. It has a dark, 

 bluish-gray color, except in its upper portion, which is yellowish to a depth 

 that varies from 5 to 60 feet, but is most commonly between 15 and 30 

 feet. This difference in color is due to the influence of an- and water upon 

 the iron contained in this deposit, changing it in the upper part of the till 

 from protoxide combinations to hydrous sesquioxide. Another important 

 difi^erence in the till is that its upper portion is commonly softer and easily 

 dug, while below there is a sudden change to a hard and compact deposit, 

 which must be picked and is far more expensive in excavating. The 

 pi'obable cause of this diff"erence in hardness was the pressure of the vast 

 weight of the ice-sheet upon the subglacial till, while the upper part of 

 the till was contained in the ice and di'opped loosely at its melting. Upon 

 each side of Lake Agassiz the till has a moderately undulating and rolling 

 surface. Within the area that was covered by this lake it has a much 

 smoother and more even contour, and its upper portion, owing to its 

 manner of deposition in this body of water, sometimes shows an imperfect 

 stratification, with a scantier intermixture of bowlders and gravel. Yet 

 even where it has distinct lamination it usually is more like till than like 

 ordinary modified drift, and contains stones and gi'avel tlu'ough its entire 

 mass. 



