2o8 Tlic American Geologist. October, 1898 
posite, although it is especially guarded at that point by a 
mountain rampart 2500 feet above the river. Arrived upon 
this great upland plain, the glacier deployed to the south and 
east, reaching as far as Coulee City in Douglas county. The 
evidence of this occupation is furnished by the bowlder-strewn 
prairies, miles in extent. Great masses of basalt were wrenched 
o'H from the edge of the plateau and deposited over the prairie 
in blocks up to 60 feet in diameter. The underlying granite 
as well is represented in the boulders. 
The appearance of the Okanogan glacier was fraught with 
the gravest consequences to the Columbia river. Its waters 
were effectually dammed until they attained a hight sufficient 
to begin their glacial drainage southward through what is 
now called the Grand coulee. This valley, with a width of 
from one to four miles, bounded by perpendicular walls of 
basalt 400 feet high, though at present unoccupied except by 
stagnant lakes, was originally a fracture line in the lava, which 
subsequent preglacial streams drained to the north and south, 
from near Coulee City. The waters of the Columbia deep- 
ened this channel and planed down the divide. It had, how- 
ever, to enter the southern portion by a waterfall approxi- 
mately 400 feet high, in comparison with which Niagara is 
a mere bagatelle. 
This brief account of the Grand coulee makes a fitting in- 
troduction to the subject of coulee formation. Recalling to 
mind our first definition of coulees as valleys which are unoc- 
cupied or at least not adequately occupied by running water, 
and which have comparatively level floors, we may need to 
add further that, in general, we shall expect to find these 
coulee floors with a low divide, sending the superficial drain- 
age waters toward either end. This is done that we may ex- 
clude mere "draws" and dry creeks, and also valleys whose 
streams have been depleted by the capture of their head- 
waters. Thus limited, it will appear that the coulees of Okan- 
ogan and adjacent counties are ice formed, — either directly 
by ice action, or indirectly on account of the glacial occupa- 
tion. With this in view, we may distinguish with reference 
to their origin three forms of coulees: i. Valleys occupied 
temporarily by preglacial streams. 2. Preglacial valleys 
whose waters were permanently diverted by the ice. 3. Val- 
