54 The AmertCdH Geohxjist. Jannary. lh&7 
Jersey would seem to exclude it from that period. "Will the 
evidence of Ptreani erosion concur in that exclusion? 
The plastic clays and the Bryn Mawr gravel rest alike upon 
contorted and eroded pre-Cambrian crystallines and upon up- 
turned strata of Cambrian, Ordovician and Newark age. 
They rest upon the baseleveled [)lain of marked heterogeneity. 
Since this plain was baseleveled these two formations covered 
it to a depth varying from one hundred to two hundred and 
fifty or u)f)re I'eet. 
With this past history thus briefly outlined, the present to- 
pography of the region can l>e readily comprehended. 
The ancient Cretaceous baseleveled plain now stands at a 
fairly uniform hight of from four hundred to four hundred 
and fifty feet. This plain is trenched by valleys which have 
been excavated since the last elevation and since the depos- 
ition of Potomac clay and Bryn Mawr gravel. 
The streams are far from reaching old age. Their head 
waters are still cutting back and contesting the watershed 
with streams farther west. The present discussion is con- 
cerned only with the streams and does not include the Schuyl- 
kill river, whicli is a compound and complex decendant of an 
original Permian river. 
The Wissahickon, with its headwaters at a hight of four 
hundred and forty feet above sea-level, near Montgomeryville 
in Montgomery county, flows southwest and southeast across 
the Newark formation, then it turns and flows almost straight 
south, cutting across Cambrian sandstone and a belt of Or- 
dovician limestone two miles wide. It leaves this non-resist- 
ant belt to erode a gorge in hard gneisses and quartziferous 
mica schists. 
At Chelten avenue, Germantown, it receives a tributary and, 
turning abruptly, flows one and three-eighths miles south- 
west into the Schuylkill. Its course covers somewhat over 
twenty miles with a fall of four hundred and twenty feet. 
The gorge of the Wissahickon in the crystalline schists is 
from one hundred and eighty to three hundred and fifty feet 
deep, and the stream is still carving with youthful vigor an 
ancient surface which bears the scars of other and far older 
accents of erosion. 
