PALEOZOIC FORMATIONS OF MARYLAND 4 I I 



stage of New York near the base of the Upper Silurian, which, 

 in western New York, is composed mainly of red shales. 



Tuscarora formation. — This quartzitic sandstone, which rests 

 conformably upon the Juniata formation, and forms the beautiful 

 flat-topped arch of Wills Mountain is admirably shown on its west- 

 ern side, especially at the entrance to the Narrows, and in the upper 

 part of the high massive cliffs bordering the gorge. It also forms 

 the greater part of the exposed rocks in Evitt's and Tussey's 

 mountains and two small areas along the Baltimore and Ohio 

 Railroad near Potomac Station. The formation is a white to 

 light gray, very hard quartzose sandstone composed of fairly 

 coarse quartz grains cemented by siliceous material. Small 

 pebbles of quartz and also those of yellowish-green hard clay 

 occasionally occur. The lavers are frequently very massive and 

 cross-bedded structure is not infrequent. Its thickness varies 

 from 250 to 300 feet, and it furnishes building stone and ballast. 

 Arthrophycus harlani, a sea-weed, the only fossil found in this 

 sandstone in the county is fairly common on the upper surfaces 

 of the higher beds in the Narrows. 



The Tuscarora formation, named from Tuscarora Mountain in 

 Pennsylvania, was formerly called the Medina or Levant white 

 sandstone (No. IV*;), and represents the upper part of the 

 Medina stage of New York. The Juniata and Tuscarora forma- 

 tions are probably equivalent to the Medina formation of New 

 York, but in western New York, where it is typically represented, 

 especially in the Niagara region, there is not such a definite sepa- 

 ration into a lower divison composed of red shales and sand- 

 stones and an upper one composed of a white quartzose 

 sandstone as in Maryland. In the Niagara region the Medina 

 is composed of the following divisions named in ascending 

 order : Lower Medina, composed of red shales only about the 

 upper 115 feet of which is exposed in outcrop. Upper Medina, 

 composed of seven zones: (i) 25 feet of gray quartzose sand- 

 stone; (2) 25 feet of thin gray shales ; (3) five feet of gray 

 sandstones and sandy shales ; (4) 6 feet of mainly gray argilla- 

 ceous shales which become reddish at the top ; (5) 35 to 40 feet 



