646 J HARLEN BRETZ 



DEPTHS OF SNAKE AND COLUMBIA VALLEYS DURING 

 THE EPOCH 



Evidence on this question may be obtained at the debouchure 

 of glacial drainage routes into these master valleys. Five of the 

 nine such debouchures will be examined. 



The rock floor of Moses Coulee is fully as low as that of Columbia 

 Valley at the junction of the two. Both contain a great gravel 

 fill here. Columbia River has cut through it, a depth of more than 

 300 feet. There is no such trenching in Lower Moses Coulee, but 

 a well at Appledale penetrates 300 feet of this gravel without en- 

 countering bedrock. 



The two cataracts of The Potholes and Frenchman Springs, 

 which operated in the early part of the epoch, descended nearly 

 the full height of the present Columbia Valley walls there. At 

 The Potholes the glacial cataract can be traced down to less than 

 200 feet above the present Columbia (Fig. 7) . 



Koontz Coulee, 20 miles north of Pasco, is cut in the weak Rin- 

 gold formation. It is 250 feet deep and a mile wide. It is floored 

 with basaltic stream gravel from the scablands farther upstream. 

 Though the Ringold silts extend down to the level of the Columbia 

 at this place, the mouth of the glacial river channel hangs 200 feet 

 above. No cataract could have been maintained here, as was 

 done at The Potholes and Frenchman Springs, and the level of the 

 Columbia of this epoch at this place is thus clearly recorded. 



At the mouth of Palouse River, there are two parallel canyons 

 in the scabland, one containing the river, the other dry. A basaltic 

 butte separates them. It stands nearly in the center of the valley 

 and its summit is between 350 and 400 feet above Snake River. 

 It is a part of the original north wall of Snake River Valley, over 

 which the gigantic cascade tumbled when the glacial flood broke 

 across the preglacial divide from the north. This "Goat Island" 

 testifies to the existence of a Snake River Valley at this place as 

 deep then as now. 



THE GLACIATION 



Because the record of the ice sheet, from which came the streams 

 that made the scablands, is best preserved on the basalt plain about 

 the city of Spokane and along the south side of Spokane River, 



