Relation of Sfreain.s to Brijn Jfawr G'rorel. — Bafscoui. 55 
The course of tliis youthful stream is directly transverse to the 
geological structure of the plateau and is entirely independent 
of the lithological character of the rocks over which it flows. 
This course admits of but one explanation, namely, that the 
Wissahickon is a superimposed stream, superimposed upon 
the Potomac clays and Bryn Mawr gravel through which it 
has cut its way. The gentle slope seaward of the plateau after 
the Cretaceous elevation determined the direction of the 
stream; Having trenched the clays and gravels, it found the 
hard crystallines beneath, but its course was too well estab- 
lished to be turned aside by the herterogeneous character of 
its newly discovered bed. 
The abrupt turn at Chelten avenue and the subsequent 
course of the stream is evidently controlled by the tributary 
of which the AVissahiekon becomes a part. 
Vallej^ creek is a stream some ten miles long which heads 
in the quartzite ridge forming the western boundary of Ches- 
ter valk\y. It flows northeast over the limestone of Chester 
valley for some seven miles before it turns to the north at an 
angle of one hundred and thirty degrees to flow through a 
gorge cut in Cambrian quartzite rising three hundred and 
twenty and five hundred and ninety feet above the bed of the 
stream. At Valley Forge it empties into the Schuylkill. 
(See plate II.) For the last half mile its bed lies on the New- 
ark formation. Like the Wissahickon, Valley creek is a su- 
perimposed stream. The cover of the Palaeozoic sediments 
was undoubtedly in part the Newark formation as well as the 
Potomac gravels. 
The way in which Valley creek, after following the strike 
of the limestone for some seven miles, leaves that easily eroded 
rock to traverse the hard strata, almost at right angles to its 
former course, would be remarkable under any other explan- 
ation than that of a superimposed course. 
With the location of its bed due to the original slojie of the 
land and fixed by superimposition, tlie subsequent erosion of 
a gorge through the (juartzite, with tliedij), becomes the most 
natural procedure. 
Tlie tendency of obliquely transverse streams, on upturned 
strata of different degrees of hardness, to gradually shift their 
courses until they become rectangular ones, along the strike 
