106 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



outcrops it is often possible to determine the underground stratigraphy 

 by the character of these surface residual products alone. In fact such 

 criteria were alone available over considerable stretches of the rolling 

 agricultural country with few rock exposures, that in this region at least, 

 is characteristic of Beekmantown limestone areas. On this account, 

 although they have been mentioned incidentally and repeatedly in fore- 

 going pages, it seems desirable to give here a connected discussion of the 

 three most important residual products, namely, the siliceous shale frag- 

 ments near the base of the formation, Cryptozoon and platy yellow chert 

 near the middle, and the cauliflower chert at the top. 



The relatively pure limestone of the lower half of the Stonehenge 

 member weathers rapidly and as its surface is not held up by some 

 resistant residual a slight depression in the land surface results. On 

 the other hand, the upper half of the Stonehenge member, with its sandy 

 laminated strata weathering into a protective covering of thin siliceous 

 shale fragments, forms hills corresponding in width to the outcrops and 

 trending in the direction of the strike of its beds. As the Beekmantown 

 everywhere in the Appalachian Valley of Maryland is highly folded, these 

 hills assume the usual northeast-southwest direction of the folds and 

 their development is so marked a feature that by plotting these elongated 

 narrow hills, the upper Stonehenge member can be mapped in areas of 

 few outcrops of the rock itself. When the succession is normal the lower 

 Stonehenge limestone therefore occurs in a slight valley between the low 

 hills of the upper Stonehenge on one side and the higher land on the other 

 side formed by the chert weathered out of the upper part of the Conoco- 

 cheague limestone. This topographic feature, however, is well developed 

 only to the east of the Martinsburg shale belt. West of this belt the 

 siliceous content of the laminated division of the Stonehenge is so much 

 less that it has little effect on the topography. This topographic feature 

 of the Stonehenge in connection with the line of hills next described 

 makes it possible to map the complicated folds involving the Beekman 

 town limestone without much doubt and has greatly aided in deciphering 

 the geologic structure in areas of few outcrops. The Beekmantown rocks 

 next or immediately succeeding following the Stonehenge divisions are 



