THE i^POKAXE GLACIATIOX 593 



else lower routes in either direction wonld have taken the discharge. 

 That it was the Spokane ice-sheet is evident from the amount of break- 

 ing down of cliffs since the waters ceased to flow. Searching over the 

 basalt, the wide glacial stream finalh' selected its central portion, now 

 Grand Coulee, for deeper trenching and withdraw from the margins. 

 Only Steamboat Rock records any of the original anastomosis. To what 

 depth the Spokane waters cnt in upper Grand Coulee will be discussed 

 under the subject of Wisconsin glaciation. 



HartUne structural valley. — The monoclinal flexure along which much 

 of lower Grand Coulee is eroded swings toward the east about 2 miles 

 north of Coulee City and extends beyond Almira toward Hellgate. An- 

 other fold, anticlinal in character, lies nearly parallel with it, about 5 

 miles to the south. The tract between is structurally and topographically 

 a valley and contains a gravelly plain approximately 40 square miles in 

 area. The floor of the coulee at Coulee City is only 200 feet below this 

 flat, and the entire descent to the town from the flat is across gravel. 

 Wells on the flat penetrate sand and gravel to comparable depths. 



The drainage of the Hartline gravel plain is largely southward 

 through Deadmans Draw, a tributary of Spring Coulee, 10 miles east of 

 Grand Coulee. Deadmans Draw is but the deepest and most pronounced 

 of a scabland com23lex of abandoned channels, basins, cascades, and falls, 

 identical in character with and very similar in proportions to those on 

 the plateau along the east side of the head of Grand Coulee. Talus de- 

 velopment is the same. Furthermore, the patchy gravel deposits and 

 stranded erratic boulders tell unequivocally of glacial waters escaping 

 southward across the rim of the structural basin. 



East of Deadmans Draw, as far as Wilson Creek, are gently rolling hills 

 with concave lower slopes and deep loessial soils, which have never been 

 touched by invading glacial waters; but between the draw and Grand 

 Coulee to the west, and from Coulee City south to Bacon Station, a dis- 

 tance of 8 miles, is a tract which for wild ruggedness is unsurpassed any- 

 where among the glacial spillways thus far described. The channels are 

 canyons and the knobs are hills 100 to 300 feet high. Stream gravel 

 covers the interchannel hills. The whole area was overrun by the glacial 

 flood out of the Hartline structural valley. Besides this, at least three 

 of the channel canyons lead out of Grand Coulee below the falls, but at 

 about the level of the floor above the falls. It is magnified scabland of 

 the Palouse type. One of these channels leads to Spring Coulee, the 

 others converge to Dry Coulee, a small feature debouching into the 

 Quincy structural basin 5 miles east of the mouth of Grand Coulee. 



Some of these are truly distributary canyons. They mark a distrib- 



XXXIX — Bill. Geol. Soc. Air., Vol. 34. 1022 



