No. 8 SMITHSONIAN EXPLORATIONS, I9Q13 13 
Paleozoic strata of Maryland, to complete a series of memoirs pub- 
lished by that State. Owing to the brevity of this account, only a 
few points in the physiographic history will be noted here. 
Since Carboniferous time western Maryland has been 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 
Fic. 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 Valley proper, 
where the erosion 1s 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 peneplain in Maryland, and has dissected it 
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