PHYSIOGRAPHY OF THE BEAVERDELL MAP-AREA. 
15 
trees do not grow. Lakes large and small occur upon the valley 
floors as well as upon the uplands. Two of the larger ones are 
Okanagan lake, which is about 70 miles long by 2 wide, and 
Kamloops lake, which lies in the valley of the South Thompson. 
These lakes are partly caused by damming of the valleys by 
glacial debris. 
TOPOGRAPHIC FORMS DUE TO GLACIERS. 
In conclusion we may refer to certain special types of topog- 
raphy which occur upon the plateaus and are generally thought to 
be due to glacial action. 
In certain places in the valley bottoms which for some 
reason are protected from erosion, one sometimes finds long low 
ridges lying along the sides of the valleys. These often have 
a narrow smoothly rounded top with the sides sloping evenly 
from it, and a small draw between it and the hillside proper. 
The ridges are made up of unconsolidated rounded boulders and 
sand. Similar ridges may lie across a valley and with them 
may occur rounded depressions without outlet, and irregular 
mounds, the whole making up an irregular hummocky surface. 
These ridges, depressions, and mounds, are similar to deposits 
laid down by valley glaciers in mountain regions to-day, and are 
called moraines. The greater part of the blanket of round boul- 
ders, sand, and clay which is spread over the upland to-day is 
thought to have been laid down by a continental ice sheet which 
covered all of the Interior plateaus at one time. A number of 
high terraces which have been found up to elevations of over 
5,000 feet have been described as the remnants of deposits laid 
down in lakes which were formed during the melting of this ice 
sheet. Finally the steep sides and flat bottoms of the large 
valleys all over the plateaus resemble known glaciated valleys 
very closely. The steep smooth valley walls which are perhaps 
the most striking features of the Plateaus topography are without 
doubt the work of the glaciers. 
