THE SPOKANE GLACIATIOX 



feet above tide. Using the criterion of tains accnmulation, most of the 

 Brnmheller channels and the existing synclinal river valle}^ date from 

 the Wisconsin glaciation, while the spillway aronnd the east end of 

 Saddle Mountain was formed during the Spokane glaciation. Chan- 

 neled scabland also lies east of the youthful Drumheller channels and at 

 a higher altitude. It apparently belongs to the Spokane spillways. 



It appears^ therefore, that when Spokane waters escaped southward 

 from the Quincy basin there was no noteworthy valley for lower Crab 

 Creek and, as in the Palouse country, the flood spre«id out and found its 

 way among the hills of weaker rock. These hills were removed in the 

 Drumheller tract for a width of more than 10 miles. South of Saddle 

 Mountain the width of the scoured tract is equally great. The flood 

 separated into two parts after passing Frenchman Hills anticline, part 

 continuing south to Esquatzel and Koontz coulees and part turning west 

 in the Beverly syncline. The westward course was eroded more deeply 

 than the southward, so that when, in the succeeding Wisconsin diversion, 

 another flood swept through the Drumheller plexus all of it went west- 

 ward. The striking features of the present plexus, adjusted to the floor 

 of the synclinal route, were then produced. 



Two other outlets for the Quincy basin existed during the Spokane 

 epoch. They are at Frenchman Springs and "The Potholes,'^ two great 

 notches in the wall of Columbia Valley on the western margin of the 

 plateau. Each is an abandoned cataract, to which short channels lead 

 across the western rim of the Quincy basin. "The Potholes" is the best 

 example mapped of a receding waterfall over lava flows which is kno^vn 

 to the writer (flgure 9). The ancient stream spilled over the Columbia 

 cliffs at an altitude of about 1,200 feet above tide and descended at least 

 400 feet over two great rock terraces, each with a scarp of about 200 feet. 

 In the upper cliff is exposed a very conspicuous flow with exceptionally 

 large, well developed, and uniform columns approaching 75 feet in 

 length. The flow which holds up the edge of the loAver rock terrace is 

 more than 100 feet thick and is composed of uniformly small columns 

 from bottom to top. 



The amount of recession in the waterfall is quite unequal in the two 

 rock terraces. In the upper the cataract was double from its beginning, 

 the two parts being nearly equal and receding side by side for nearly two 

 miles. This parallel recession left a great blade of rock a mile and a half 

 long, 1,000 feet wide, and 375 feet in maximum height between them. 

 A huge elongated pothole was left by the recession of each member of 

 the twin upper falls, deepest and widest below the falls. There are great 

 grayel bars in these potholes, especially in the downstream portions. 

 The one below the southern fall is 200 feet thick (figure 10). 



