DETAILS OF THE CONESTOGA OVERLAP 517 



the base of the formation in the vicinity of Columbia increase in thick- 

 ness westward and in the vicinity of York give rise to prominent low 

 ridges and hills capped by sandstone. 



Southwest of York the overlap relation, which in itself is complicated, 

 is further confused by thrust-faulting. The Conestoga limestone is 

 largely restricted to the southeast side of the valley, but some outliers 

 of the basal sandstones and conglomerates of the formation cap hills 

 near the north border of the valley. Several excellent exposures of the 

 contact of the basal beds of the Conestoga on the Ledger dolomite were 

 seen in the Pennsylvania Eailroad cuts northeast of Spring Grove, where 

 folding has brought the underlying dolomite to the surface. Here con- 

 glomerates of white marble fragments inclosed in a matrix of siliceous 

 argillaceous limestone occur near the base, and slaty limestone beds fill 

 hollows in the underlying Ledger dolomite. A narrow crevice in the 

 dolomite, filled to a depth of about 15 feet with limestone breccia and 

 masses of oolitic limestone, is laid bare in the railroad cut. In the 

 vicinity of Hanover the Conestoga spreads out over the larger part of the 

 valley and is faulted and pverthrust on the Cambrian limestones. South- 

 west of Hanover the Triassic sediments transgress southward, first cover- 

 ing the Cambrian limestones and then the Conestoga limestone, so that 

 beyond Littlestown the limestones are entirely concealed and the York- 

 Hanover limestone valley ends. 



South and southeast of the York-Hanover Vallev there are narrow 

 helts of blue slaty limestone in the Harpers schist which are regarded as 

 infolds of overlapping Conestoga limestone. During a recent survey of 

 Carroll County, Maryland, the junior author has found similar blue 

 limestone along a belt of valleys extending from the State line south of 

 Littlestown to and beyond Union Bridge, Frederick County, Maryland. 

 This blue slaty limestone is believed to be a southern occurrence of the 

 Conestoga formation that overlaps the Harpers schist and is infolded in 

 synclines in the schist. The Conestoga limestone and Harpers schist are 

 similarly associated in a small elongated area southwest of Emmitsburg, 

 in Frederick County, Maryland, where these rocks are locally exposed 

 west of the Triassic belt. 



In Frederick County, Maryland, limestones again emerge from be- 

 neath the cover of Triassic sediments and fonn the Frederick Valley. 

 The Conestoga limestone is not known to be present in this valley, but 

 a somewhat similar limestone, called the Frederick limestone, overlies 

 the Beekmantown limestone, apparently conformably. It contains a 

 small fauna, some forms of which are the same as those in the Conestoga, 

 and the two limestones are probably in part of the same age. 



