512 R. T. CHAMBERLIN AND F. P. SHEPARD 



surface, was placed in the bottom of the compression box. After 

 pushing the end of the sand forward for about i inch, a terrace-like 

 development was produced whose scarp appeared about 5 inches 

 from the pressure block; 2 inches of pushing produced a second ter- 

 race scarp 3 inches farther out, the terrace being correspondingly 

 uplifted; 4 inches of compression developed a third terrace i inch 

 in width. With further pressure the entire mass moved forward. 



The cause of these terrace-like uplifts was investigated in several 

 ways. Straws were stuck into the sand between the pressure block 

 and the zone where the first terrace was expected to begin. Upon 

 applying pressure it was found that those straws which were stuck 

 deeply into the sand were bent over away from the pressure block, 

 while those straws which extended but little beneath the surface 

 remained upright. This difference in behavior suggested that the 

 upper portion of the sand moved forward largely as a unit over the 

 lower portion after the manner of thrust faulting in solid blocks. 



The nature of this deformation was further tested by placing 

 smoked glass plates in the sand at right angles to the pressure block. 

 In this way some idea could be obtained both as to how much of the 

 sand was moved by the terracing action, and also its direction of 

 movement, because the moving sand rubbed against the plates and 

 removed the soot in streaks. The results of this method were not 

 always very satisfactory, but some cases showed clearly that there 

 had been thrust faulting with the dips of the slippage, as indicated 

 by the movement of the sand grains, ranging from 20° to 35°. 



Clay was substituted for the sand and pushed in the same man- 

 ner. Similar terraces also developed in this material. Since clay 

 will stand in slopes better than sand, sections were cut in it with a 

 knife at right angles to the terraces. In this way the actual fault 

 planes could be observed for a short time, till the clay broke off and 

 covered them. The angles of these faults varied from 9 to 29 

 degrees. In general the angle seemed to become lower after faulting 

 had progressed for a little distance. 



This thrust faulting in sand and clay suggests that some of the 

 escarpments found in unconsolidated mantle rock in the basin 

 ranges of Nevada and Utah may have been the result of thrust 

 faulting rather than normal faulting. 



