342 E. M. KINDLE 
feet below the surface of the Cretaceous plateau. The Athabasca, 
where it joins the Clearwater, has cut its valley into the Cretaceous 
rocks to a depth of about five hundred feet. The lower portions 
of both streams, however, flow for considerable distances across 
a very broad, low, flat plain of recent origin, which is composed 
Frc. 1.—Sketch map of the Mackenzie River basin 
chiefly of fine silts and sands. These beds are of both lacustrine 
and fluvial origin and evidently of postglacial age. Near the river 
the fluvial beds are more in evidence than the lacustrine, but the 
latter probably have a far greater extent than the former. In the 
Peace River region the cut banks of sand rising in places 80 feet 
high or more above the water represent the bottom deposits of a 
