298 H. E. Gregory — The Rodadero (Cusco, Peru). 



The Rodadero a fault plane. 



Field relations and rock texture indicate that the diorite 

 mass whose striated surface forms the Rodadero is intrusive 

 into sediments of Mesozoic age. However, the smoothed, un- 

 weathered exposures immediately adjoining the limestone out- 

 crops exhibit no modification of texture incident to cooling 

 and no interfingering of igneous and sedimentary rock. In fact 

 the theory advanced by certain observers that the Rodadero 

 represents an ancient cooling surface is quite out of harmony 

 with the Held evidence as well as with the theories of the 

 mechanics of igneous intrusion. The presence of limestone in 

 immediate contact with diorite — a relation which furnishes 

 opportunity for solution along the contact — has doubtless 

 facilitated the stripping of the igneous boss. 



The Rodadero is believed to be a portion of a thrust fault- 

 plane marked by an unusual expression of slickensides. Unlike 

 normal slickensides the striated grooves of the Rodadero occur 

 indiscriminately on flat, convex, concave and undulating sur- 

 faces ; and although generally straight and parallel they may 

 also be seen curving gently around minor knobs. The criteria 

 usually employed to distinguish slickensides from striae of 

 glacial origin are not applicable.* The groups of striae of 

 various orientation merge imperceptibly into one another and 

 are apparently contemporaneous, thus indicating the presence 

 of stresses opposed to the dominant direction of movement. 

 Readjustment of pressures following the major faulting has 

 resulted in the production of minor, normal faults which inter- 

 rupt the continuity of the grooves. These faults, trending 

 roughly 90° and 45° to the prevailing direction of striae, are 

 themselves marked by slickensides and fault breccia and exhibit 

 prominent " drags" (fig. 8). 



*Geikie : Structural and Field Geology, p. 309. 



