62 THE CAMBRIAN AND ORDOVICIAN DEPOSITS OF MARYLAND 



Exposures of this limestone are infrequent because its area of outcrop, 

 lying as it does at the foot of South Mountain, commonly is covered by a 

 thick deposit of mountain wash in addition to its own mantle of soil. 

 This mountain wash is thickest and most widely spread where streams 

 from the mountain enter the valley and form alluvial cones. In Mary- 

 land, however, as mentioned above, this Lower Cambrian limestone is 

 often only moderately folded and this, in connection with other factors, 

 causes its area of outcrop to be much wider than in neighboring states. 

 Overturned folds and faults are not uncommon, but as a rule these are of 

 relatively insignificant proportions so that the usual condition of gently 

 dipping strata is soon resumed. Such a fold with slight faulting is well 

 displayed along the Western Maryland Railway, one mile west of 

 Cavetown. 



LITHOLOGIC CHARACTERS. In the type area, southern Pennsylvania, 

 the Tomstown limestone is described as a formation Composed largely of 

 thin bedded and massive dolomite and limestone with considerable shale 

 interbedded near the base. The rocks are not well exposed in Pennsyl- 

 vania and it is possible that the marbles which are such a conspicuous 

 feature of the formation in Maryland are present also in Pennsylvania. 

 At any rate proceeding southward into Maryland the main mass of the 

 formation consists of white to pinkish shaly marble which upon weather- 

 ing gives rise to yellow and greenish shale-like fragments quite sericitic 

 in nature. The Tomstown limestone is especially well exposed in a belt 

 of outcrop three miles in width southeast of Hagerstown, where the usual 

 drift deposits are not so thick and widely dispersed. In this area ex- 

 posures, particularly of the middle and upper beds of the formation, are 

 numerous. 



The lowest beds of the Tomstown are not well exposed in Maryland, 

 being nearly everywhere buried beneath the mountain wash. However, 

 it is believed that the gray to dark blue massive, rather pure limestone 

 exposed in the large quarry at Cavetown represents some portion of the 

 lower Tomstown, since at this point the Tomstown is faulted against the 

 Waynesboro and the characteristic marbles are not in evidence. The 



