Mathews — Structure of the Piedmont Plateau. 149 



surface are usualljr indicative of the character of the under- 

 lying rock. Throughout most of the area, especially in the 

 little-dissected remnants of the old peneplane, the disintegration 

 has extended from one to twenty or thirty feet, according to the 

 character of the rock, and one may find entirely disintegrated 

 masses of clay or sand retaining the original textures of the 

 parent rock. Even much of the material which at first is so hard 

 as to require blasting, breaks down when exposed for some 

 time to the influence of the atmosphere. Disintegration of 

 this type leaves few exposures and renders microscopic studies 

 of many apparently fresh specimens unsatisfactory, but in turn 

 facilitates the areal mapping since the soils and minor features 

 of the disintegrated products are often characteristic of the 

 underlying rock. The plant cover is also occasionally distinc- 

 tive since certain forms of plant life are found limited to the 

 areas underlain by certain rock types or there possess certain 

 peculiarities of development. 



The lack of fresh exposures and the detailed complexity pro- 

 duced by the schistosity and minor plications make it exceed- 

 ingly difficult to determine the true bedding plane, and this is 

 not possible in many instances. It becomes necessary there- 

 fore, to lay relatively less stress on the structural features found 

 in single small exposures and to assign greater importance to 

 the areal distribution and the general structural features deter- 

 mined by it. 



Fossils have been found in only one or two areas, where they 

 are much distorted or damaged and there is little hope that 

 other deposits of better preserved forms will be found ; for 

 only the most hardy forms could withstand the metamorphic 

 changes which the rocks have undergone. 



The types of structure encountered in the region are joints, 

 normal faults (usually of small throw), folds and probable 

 unconformities. The jointing is usually in three fairly well- 

 defined series of joints which cause the rock to break into irreg- 

 ular rhomboids. When well exposed in the banded-gneisses 

 of the Baltimore and other areas, they show little or no move- 

 ments along their planes although now and then a displacement 

 of a few inches may be found along fault planes which are 

 more or less parallel to the planes of jointing. The faulting, 

 if present, is usually obscured by the homogeneous character 

 of the rocks and the lack of well-defined bedding in many of 

 the sedimentary masses where the faulting is exposed. It is 

 accordingly impossible to trace the faults beyond the exposures 

 in which they are found. So far as examined the throw of these 

 faults is slight, ranging from a few inches up to a foot or two, 

 and the general faulting structure partakes of that character- 

 istic for the Jura-triassic beds which overlie the Piedmont on 



