QUATERNARY 569 



The general appearance of this formation suggests the Oligocene beds 

 of eastern Wyoming more than any other formation with which the 

 authors are familiar. It was probably built up by sluggish streams, 

 along whose courses there may have been shallow lakes. The surface 

 covered by them is thought to have had considerable relief, which was 

 gradually buried. This could be accomplished by the filling action of 

 wide, shallow, mud-laden rivers, in much the same manner as the work 

 done by the Arkansas and other streams that flow through the Great 

 Plains. 



Quaternary 



The trail crosses no consolidated rocks between a point 22 miles south 

 and 18 miles west of Santa Cruz and the city itself. The surface is 

 occupied by gravels, sands, and clays, which extend to an unknown but 

 comparatively shallow depth. The gravels are for the most part hard 

 fragments from the Totora and Bermejo formations, with some of the 

 faceted pebbles of the tillite beds. The sands fall into two classes — 

 river sands and dune sands. The only difference between the two is in 

 mode of transportation, for the individual grains migrate from one 

 class to the other. These sands are almost pure quartz and probably 

 come principally from the sandstones of the Bermejo formation. The 

 dune sands cover most of the plains around Santa Cruz. In places they 

 form low hummocks and curved ridges, which are now anchored in place 

 by a growth of stunted timber and long grass. Live, migrating dunes 

 are rare. 



The clays cover only a small part of the surface. They are commonly 

 dark brown in color, but some of them have more than a hint of redness, 

 suggesting that they are outcrops of the red clays that, farther to the 

 east, north, and south, surface much of the plains. 



Structure 



Throughout most of the area traversed the rocks have been subjected 

 to strong lateral stress and have yielded to it by folding and breaking, 

 so that they are commonly steeply tilted and their continuity is broken 

 by many faults. The most intense deformation is not in the western- 

 most beds, as might be anticipated if the deforming stress was a thrust 

 from the west, but rather about midway between Cochabamba and Santa 

 Cruz. Near Arani, on the west, the beds are only gently flexed and 

 indicate a force acting from the southwest rather than from the west. 



