MARYLAND GEOLOGICAL SURVEY 153 



As noted before, this area has been selected by Ulrich as a striking 

 example of the instability of the floor of the continent during Middle 

 Ordovician time. The differences between the two areas seem to indicate 

 that the eastern half of the valley received no sedimentation during 

 earliest Chambersburg time, while in the western half 150 feet or more 

 of granular limestone were deposited. During the periods of deposition 

 registered by the Tetradium cellulosum, Echinospherites, and Nidulites 

 beds, there was no great interruption of sedimentation, but following 

 this time the Christiania bed reaching as much as 270 feet in thickness 

 and the Greencastle bed with a maximum of 200 feet were deposited in 

 the eastern half, while the absence of such deposits in the western half 

 indicates a time of emergence. Fig. 17, on page 131, illustrates the un- 

 equal deposition of these several beds in a northeast-southwest line run- 

 ning from northeast of Chambersburg, Pennsylvania, to Strasburg, 

 Virginia. The Caryocystites bed is not present in any of these sections 

 and the entire area belongs to the Chambersburg belt of outcrop. The 

 thinning of the Lowville limestone at the base of the section in southern 

 Pennsylvania, its absence in Maryland and West Virginia, and its reap- 

 pearance in northern Virginia, is the first example of the variation in the 

 formation along the strike. The succeeding beds continue across Mary- 

 land in varying thicknesses with the exception of the Greencastle bed, 

 which disappears in southern Pennsylvania and apparently does not 

 reappear to the south. Whatever the local expression of the Chambersburg 

 limestone may be, the next succeeding beds in all of the sections are 

 the argillaceous limestones and shales of the overlapping Martinsburg 

 shale. 



AGE AND CORRELATION. The Chambersburg limestone, with a thick- 

 ness not exceeding 300 feet in Maryland, is the thinnest formation of 

 the Cambrian-Ordovician section of the state and its surface outcrops 

 are decidedly small in comparison with any of the other formations. 

 East of the shale belt its areas of outcrop are so small as to be negligible, 

 and west of the same belt there are but two narrow bands, only one of 

 which is continuous across the state. From the paleontologic standpoint, 



