NO. 8 



SMITHSONIAN EXPLORATIONS, I913 



13 



Paleozoic strata of Maryland, to complete a series of memoirs pub- 

 lished by that State. Owing to the l)revity of this account, only a 

 few ])oints in the physiographic history will he noted here. 



Since Carboniferous time western .Maryland has lieen above the 

 sea, and its rocks have accordingly been subjected to a long period of 

 aerial erosion. During Jurassic time, the area remained stationary 

 for so long a period that the surface of the land in the Appalachian 

 province was reduced to a rolling plain. Later uplift raised this 



Fig. 15. — Jurassic (Schooley) peneplain, preserved in the Blue Ridge of 

 Maryland. Photograph by Bassler. 



plain still higher above sea level, and in Maryland only remnants of 

 the old surface are preserved in the flat skyline of the highest moun- 

 tains. This ancient plain, or Schooley peneplain, as it is termed, is 

 well preserved on the top of the Blue Ridge, as shown in figure 15. 



A second great period of erosion occurred in early Tertiary time, 

 the effects of which were chiefly in the Appalachian A'alley proper, 

 where the erosion is indicated by a pronounced plain at an elevation 

 of about 750 feet. This plain was formed only on the softer Paleozoic 

 rocks, and. because of its prominence near Harrisburg, Pennsylvania, 

 is known as the Harrisburg peneplain. Conococheague Creek trav- 

 erses the Harrisburg ])eneplain in Maryland, and has dissected it 



