642 / HARLEN BRETZ 



Further evidence of the depth of the glacial streams will be 

 presented under the next subject.^ 



DEPOSITS MADE BY THE GLACIAL STREAMS 



The record of Pleistocene glacial streams almost everywhere is 

 one of aggradation. Glacialists commonly think of the subject 

 only in such terms, textbooks discuss it only in that light; it is the 

 orthodox conception. But in the Columbia Plateau exceptional 

 factors controlled. The preglacial valleys in general were small and 

 of relatively high gradient, the volume of the glacial streams was 

 very great, the amount of detritus from the ice was very small, and 

 the rock crossed was either loess or closely and vertically jointed 

 basalt, both of which yielded rapidly to the torrents. The result 

 of these conditions was great deepening of the valleys, and deposits 

 made by the streams are of minor importance. Their character, 

 however, adds to the weight of evidence already presented for the 

 origin of channeled scabland by glacial stream erosion. 



The deposits are almost wholly of gravel. Sand is a minor con- 

 stituent and clayey material is lacking. The gravel and sand are 

 almost wholly of basalt, though all deposits contain fragments of 

 rock foreign to the plateau. The basaltic gravel is not well rounded 

 though most of it is sorted and stratified and indubitably of stream 

 origin. Foreset bedding is common, the direction of dip according 

 with the slope of the scabland tract. In some places, the deposits 

 are composed of very coarse material, with abundant, sub-rounded, 

 basaltic bowlders 3 and 4 feet in diameter. These were originally 

 bowlders of decomposition and were derived from flows with par- 

 ticularly large columns, underlying or in the immediate proximity 

 of the deposit. Where a few erratic bowlders are associated^ the 

 deposit itself might be misinterpreted as a bowldery till. 



The gravel deposits rest on irregularly eroded basalt, essentially 

 a buried scabland surface. Nowhere do they lie on or beneath 

 the loess. Neither the gravels nor the underlying basalt are 



' If these enormous streams all came to the Columbia eventually, should not the 

 great volume be recorded farther down the master stream ? The writer has seen enough 

 to convince him that it is so recorded, and hopes to publish on this subject in the 

 future. 



^ As west of Lantz, for example. 



