62 2 / HARLEN BRETZ ' 



nor even Niagara falls and gorge itself approach the proportions of 

 some of these scabland tracts and their canyons. From one of 

 these canyons alone' lo cubic miles of basalt was eroded by its 

 glacial stream. 



THE BASALT PLAIN, NORTH OF THE SCABLANDS AND 

 MATURE TOPOGRAPHY 



This physiographic feature extends westward from Spangle 

 for 50 miles along the south side of Spokane River and varies from 

 3 to 12 miles in width. It is determined by the upper surface of 

 the Columbia basalt formation. It is interrupted in places by 

 short valleys tributary to Spokane River, and the different portions 

 are known as prairies.^ This plain is bounded on the south by 

 channeled scabland and maturely eroded loessial hills. The differ- 

 ences between it and adjacent broader scablands are not marked, 

 but the loessial hills are in striking contrast with it. These hills 

 which, elsewhere on the plateau, come right to the edge of Snake 

 and Columbia valleys,^ nowhere overlook the Spokane Valley. 

 They terminate abruptly on the southern margin of this plain. 

 On the plain there is no mature topography and no channeled scab- 

 land. There is no area on the plateau like it. The nearest approach 

 to it is the northern portion of Douglas County, back of the Wis- 

 consin terminal moraine. This narrow plain must be the result of 

 conditions which prevailed no farther south. 



These conditions can be summed up in one word — glaciation. 

 Deposits of till and many striated erratics have been found in every 

 township examined on it. The till is patchy in distribution. No 

 moraine margins the southern edge of the plain and no good moraine 

 ridges occur on it, though here and there is morainic topography. 



The genesis of the plain thus established, the questions of its 

 character before glaciation and the method of its development 

 arise. These are answered clearly when the adjoining mature 

 loessial topography is studied. The larger valleys of this topog- 



' Upper Grand Coulee. 



2 Paradise Prairie, Sunset Prairie, Indian Prairie, Four Mound Prairie, etc. On 

 the north side of Spokane River are Pleasant Prairie and Five ]Mile Prairie, also parts of 

 this plain. 



3 With the exception of northern Douglas County. 



