TOPOGRAPHIC FEATURES DUE TO LANDSLIDES. 489 



fracture.* At the uorthern end of the canon, however, granitic 

 rocks form a portion of its walls, and stand as isolated towerlike 

 masses within it. Some of these towers are capped with horizontal 

 lava sheets. When the lava was poured out it surrounded a granite 

 ridge having the position of the Grande Coulee, but probably not 

 extending as far south as the depression since formed. The weather- 

 ing and removal of the granite gave origin to a trenchlike depression 

 with vertical walls, composed of basalt above and granite below. The 

 more rapid crumbling of the granite led to the breaking away of the 

 jointed basalt resting on it, and the widening of the depression in the 

 manner already noticed. 



From the brief and inadequate description I have given of cer- 

 tain of the more striking features of "Washington and Oregon it will 

 be seen that landslides have modified the topography of the region 

 occupied by the Columbia lava in several ways. There are yet other 

 changes in the geography of that most interesting and instructive 

 land due to the same causes. Chief among them are the obstruc- 

 tions to the streams formed by landslides, and the production of lakes 

 and rapids. At several localities in the upper Columbia masses of 

 rock which have fallen from the cliffs bordering the stream obstruct 

 its course. There are now no lakes along the course of the river due 

 to this cause, but terraces above rocky rapids, show where such water 

 bodies previously existed. 



Perhaps the most interesting fact brought out by the study of 

 landslide topography is that certain broad, nearly level areas, now 

 covered with deep, rich soil, and in the autumn golden with the 

 sheen of ripened grain, owe the minor features in their relief to 

 ancient landslides. The hills, with broadly rounded summits, and 

 the shallow undrained basins between, in such regions are an inherit- 

 ance from a time when long, precipitous escarpments, by their slow 

 recession, left the land covered with a rugged, confused mass of 

 fallen blocks. A review of the facts concerning the minor features 

 in the relief of the broad wheat lands of southeastern Washington,f 

 in the light of the conclusions here presented, leads to the suggestion 

 that some of the ridges and basins of that region may be due to the 

 recession of cliffs produced by stream erosion, ^^[any portions of the 

 deeply decayed surface of the basaltic plateau of southeastern Wash- 

 ington resemble closely the old landslide topography in the valley to 

 the northwest of Lookout Mountain, shown in the accompanying 

 illustration. 



* A Geological Reconnoissance in Central Washington. By Israel C. Russell. United 

 States Geological Survey Bulletin, No. 108, 1893, pp. 90-92. 



f A Reconnoissance in Southeastern Washington. By Israel C. Russell. Water Supply 

 and Irrigation Papers of the United States Geological Survey, No. 4, 1887, pp. 58-69. 



