CHANNELED SCABLANDS OF THE COLUMBIA PLATEAU 629 



the streams once spread completely from side to side and only 

 later became concentrated in the deeper canyons. 



Fig. 8. — A part of Rock Creek (Lincoln County) and its associated scabland. 

 The creek in the main channel is margined by vegetation. The scabland to the left is 

 barren and the irregularity in lighting is due almost entirely to the cliffs which 

 border the shallow anastomosing channels. (Aeroplane Photo by F. H. Frost.) 



ALTITUDES AND GRADIENTS OF THE SCABLAND TRACTS 



The heads of scabland tracts which are open to the basalt plain 

 range in altitude between 2,350 and 2,500 feet above tide. There 

 are six or eight of these, the number depending on just what is 

 considered to constitute an individual scabland tract. At least 

 the following should be recognized as unit tracts. The order in the 

 list is from east to west. 



1. Pine Creek channel. Altitude of head, 2,450 feet A.T. 

 Gradient approximately 25 feet per mile 



2. Cheney-Hooper tract 

 Fom* heads: 



a) Marshall-Spangle, altitude, 2350=^ 

 h) Four Lakes, altitude, ? 



c) Medical Lake, altitude, 2,425 



d) Deep Creek, altitude, 2,3 50 ± 

 Total width of the group is 22 miles 



