) 
TERRACES OF OKANOGAN VALLEY 51 
out on the lava plain as far at least as the Grand Coulee. Un- 
mistakable signs of profound glacial action abound on every 
hand (see Plate I). 
The high-level terraces (Plate I) which are so conspicuous in 
the valley of the Okanogan river are composed almost entirely 
of worked-over gravels with little or no fine materials. This fact 
seems to indicate lacustrine origin. They extend for many miles 
above the. mouth of the stream. The fact to be emphasized in the 
present connection is that the main Okanogan terrace is to be 
connected with the Great terrace on the Columbia river, in which 
case their origin is to be sought in a common cause — the damming 
of the latter stream by advancing ice of perhaps the Chelan 
glacier when the lake thus formed overflowed at the Grand 
Coulee. 
Des Moines. 
4 
