96 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



Comparison of these sections brings out several salient differences in 

 the lithology of the two areas. East of the shale belt, the Stonehenge 

 member with its characteristic siliceous banded limestone, is distinct 

 enough to be mapped as a separate unit, but west of that belt the siliceous 

 banding of the lower Beekmantown is not so well developed. However, the 

 same faunas are present in these strata in both areas so that there is no 

 doubt as to the presence of beds corresponding to the Stonehenge member 

 in both. The higher beds in each area also contain similar faunas, but 

 the lithology is somewhat different, limestone predominating in the east 

 and dolomite in the west. Chert in large quantities weathers from certain 

 portions of the dolomite in the western area, but it is not so conspicuously 

 developed in the east. 



The two sections illustrate the lithologic changes occurring in the 

 formation going from the east, where over three-fourths of the formation 

 consists of pure limestone, across the strike to the western side of the 

 Valley where more than half of the strata is more or less highly magnesian. 

 In Appalachian areas still further west, as in central Pennsylvania, the 

 change to magnesian limestone becomes yet more pronounced. 



FAUNAL ZONES. Although the lithologic features of the various por- 

 tions of the Beekmantown limestone vary considerably, the basal member 

 is the only division which can be definitely recognized from the character 

 of its strata. Above this lower division the Stonehenge member three 

 distinct fauna! zones aid in the recognition of their respective horizons. 

 These are in ascending order above the Stonehenge member, the Crypto- 

 zoon steeli zone, the Ceratopea zone and the Turritoma zone. The value 

 of these zones is not local for they have a wide distribution. 



Stonehenge Member. The village of Stonehenge, just east of Cham- 

 bersburg, Pennsylvania, is located on the lower beds of the Beekmantown, 

 which are sufficiently distinct lithologically and faunally from the remain- 

 ing strata of the formation to warrant their separation as a distinct 

 member. This Stonehenge member is composed of massive finely con- 

 glomeratic pure limestone in the lower half and siliceous banded limestone 

 alternating with layers of large edgewise conglomerate in the upper half. 



