90 THE CAMBRIAN AND ORDOVICIAN DEPOSITS or MARYLAND 



laminated limestones with some layers containing occasional, more or less 

 massive., ledges in which the lamination is obscure. Some of these con- 

 tain numerous poorly preserved fossils. Study of these fossils shows that 

 a large part of the local fauna consists of species previously found in the 

 Beekmantown limestone of the Champlain Valley in New York, Vermont, 

 and southeastern Ontario. In 1910 * the northern extension of these 

 strata was distinguished and separately mapped under the same name by 

 which the formation is known in New York. 



The Frederick Valley in Maryland, east of the Blue Ridge, also con- 

 tains a considerable development of rather pure massive limestone hold- 

 ing Beekmantown fossils. This development of the Beekmantown is 

 discussed in a separate chapter, so that the following description of the 

 stratigraphy applies only to the Appalachian Valley. As a whole, the 

 Beekmantown limestone of Maryland is quite distinct lithologically from 

 the other divisions of the Shenandoah group, although the occurrence of 

 similar beds in most of the formations often causes difficulty in the 

 recognition of isolated outcrops. Its strata are most likely to be confused 

 with the underlying Conococheague limestone, because edgewise con- 

 glomerates are not uncommon in the Beekmantown, in fact in the upper 

 half of the lowest division they are as well developed as in the Conoco- 

 cheague limestone. The characteristic sandy laminated banded, dark 

 blue rock of the latter, excepting one bed, is not developed in the Beek- 

 mantown. The main mass of the Beekmantown formation is of finely 

 laminated, lighter colored and purer rock than occurs in the Conoco- 

 cheague. The successive beds also are more uniform in texture, color and 

 composition. On this account, it is difficult to distinguish the different 

 portions above the basal division, which contains the exception mentioned 

 above in which siliceous banded limestones occur. 



Fortunately there are four fossiliferous zones in the formation with 

 characteristic species in each, which appear frequently enough in the 

 outcrops to obviate some of the difficulties of determination. Several 

 distinct zones in this formation may also be recognized by residual 



1 Chambersburg-Mercersburg folio, U. S. Geological Survey. 



