5o6 R. T. CHAMBERLIN AND F. P. SHEPARD 



of this model caused the plastic layers to be thrust up over the 

 cement block and produced very complex faulting and folding above 

 it. Another way to test the same idea was to place a block of cement 

 in the sand below a portion of a model. The effect of this block was 

 to cause an especially pronounced fold to occur over the block, in 

 spite of the fact that this was placed on the end opposite from the 

 active pressure. 



Since in nature there undoubtedly are cases where such former 

 islands of resistant crystalline rocks are not exposed at the surface, 

 it would seem probable that complicated dome structures might be 

 explained in this way. R. C. Moore has reported that folding in 

 eastern Kansas has tended to follow an old buried granite ridge, 

 which formed a sort of island in the later Paleozoic sediments.^ In 

 a similar way some of the small ranges of central Montana and 

 Wyoming may be due to causes of this sort, and even the Black Hills 

 of South Dakota may owe their origin in part to this selective defor- 

 mation. 



ALPINE OVERFOLDS 



Several attempts were made to reproduce the overfolds, or nappes 

 de recouvrement, of the Alps by applying rotational stresses to 

 models in which competent layers were placed between incompetent 

 layers, since this seemed to represent the conditions in the Alps, 

 where competent limestone formations are interbedded with less 

 competent shales. These experiments, however, did not succeed in 

 developing Alpine structures in this way, since the resulting over- 

 folds were overturned only to a small degree, and continued pressure 

 only setved to cause folding elsewhere, and to produce a vertical 

 rising of the overturned folds. As it is not easy to see how condi- 

 tions in the Alps could have been more favorable for producing over- 

 folds than these experimental conditions, one may well wonder at 

 the current European interpretation which assigns such tremendous 

 extents to the nappes. But if low-angle overthrust faulting enters 

 largely into the horizontal shifting of material, these nappes take 

 on a quite different aspect. 



'Raymond C. Moore, "Buried Granite in Kansas," Bull. Geol. Soc. Am., Vol. 

 XXXIII (1922), pp. 96-98. 



