V, 



150 Mathews — Structure of the Piedmont Plateau. 



its western border. Large overthrust faults may perhaps occur 

 in the region, obscured by the similarity of deposits and the 

 vegetation, but no evidence up to the present time has been 

 found warranting the assumption of such overthrust faults 

 unless, perhaps, in the region a few miles west of the Northern 

 Central Kail way, at a point twenty-five miles north of Baltimore. 

 If such a fault occurs here, the plane of the overthrust must be 

 very flat and the extent of the thrust small, since the areal dis- 

 tribution of the phyllites to the north and the gneisses to the 

 south show no appreciable break in their boundaries. It seems 

 more probable to the author that the recurrence of the marble 

 in a series of parallel bands is due to folding or minor faulting 

 than to an overthrust. 



The folding in the rocks of the area is of three types : min- 

 ute crinkling, small u asymmetrical wavy folds, and broad Appa- 



' 



lachian ones in which the adjustment appears to have taken 

 place along the bedding. The accompanying figure indicates 

 the differences as sketched from exposures in the field. The 

 intricacy of the minor folds has been the feature usually noticed 

 by earlier investigators, who have many times overlooked the 

 gentler open folding of the larger folds. In the succeeding 

 discussion the emphasis will be laid upon these larger folds 

 and less attention will be drawn to the minor ones, since the 

 broader rather than the detailed local structure of the Mary- 

 land Piedmont is the problem under discussion. 



The dip observations made on small exposures often relate 

 to the minor folding and are usually quite steep, ranging from 

 40° to 80° with an average of 55° in the exposures. These 

 dips are usually considerably at variance within short dis- 

 tances, although there seems to be a tendency for the expo- 

 sures to be formed on those portions of the minor folds where 

 their dip is in the same direction as that of the major fold, 

 the return dips being usually concealed. The earlier structural 

 interpretations have been based for the most part, in Maryland 

 at least, on the dips of these secondary folds, and it has accord- 

 ingly been customary to regard the region as one of steep mono- 

 clinal or closely compressed overturned folding. The dips of 



