24 
MUSEUM BULLETIN NO. 11. 
ridges are, however, often covered with loose material for long 
distances. Rounded pebbles and boulders differing in character 
from the underlying rock are invariably found mixed in the loose 
soil. There is probably very little loose detritus within the 
quadrangle which does not contain foreign pebbles. What pro- 
portion of it is residual soil in place we do not know, but the rock 
outcrops are very seldom much weathered, and one can generally 
obtain fresh rock within a few inches of the surface, which would 
indicate that most of the loose detritus Is transported material. 
This blanket of detritus is with some exceptions probably no- 
where more than a few feet thick. It is ascribed to deposition 
from a large ice sheet. 
Relation of the Rock Structure to the Upland Surface. 
Among rocks of pre-Oligocene age there is no very evident 
relation of the upland topographic surface to rock structure. 
Contacts between the formations cross the slopes at all angles, 
and sometimes one and then another will occupy the higher 
ground. There is, however, some parallelism between the di- 
rection of valleys or dry troughs upon the upland and the strike 
of planes or zones of fracture in the rocks, which indicates that 
the valleys are located upon zones of weakness in the rocks. There 
is also a definite relation between topography and rock structure 
in the areas underlain by Tertiary sediments and lavas. 
That the courses of the streams are largely determined by 
fracture zones in the underlying rocks is indicated by the follow- 
ing facts : the valleys in plan are generally made up of straight 
stretches. Straight stretches in adjacent valleys are often 
parallel to each other, and to parts of valleys lying below the 
upland surface. When the strikes of all the fracture zones and 
fault planes determined within the quadrangle are plotted on a 
projector chart it is found that they are bunched in four or five 
zones, and that these lie between N. 8° W. and N. 55° E., with a 
subordinate group which strike N. 85° E. If the directions of the 
straight stretches in the valley bottoms lying within the quad- 
rangle be measured they are found to be divisible into groups 
which trend roughly parallel and strike from N. 6 C W. to N. 45° 
