56 The American Geologist. Jummry, i897 
of the soft bed and across the strike of the hard bed, cannot 
be ascribed to Valley creek as explained by Gilbert.* 
Gulf creek rises in a ridge of mica-schist one-half mile 
uortli of Stratford station at an altitude of four hundred and 
eighty feet. It flows ten degrees north of east along the base 
of this ridge for about four miles when it turns at an angle 
of ninety degrees, cutting across the ridge, which, at this 
point, now stands at a hight of from four hundred to four 
hundred and eiglity feet. After crossing this ridge the creek 
turns again at right angles and, flowing parallel to its origin- 
al course, it empties into the Schuylkill at a level of fifty 
feet. (See plate II.) Gulf ci'eek illustrates readjustment as 
well as superiinposition. The adjustment occurred after the 
stream had cut through the Potomac cover and discovered the 
underlying crystallines of unequal hardness. 
The early stream heading at 1) with two tributaries, A and 
C, soon cut a deeper channel in the limestone which formed 
its bed than its neighbor, Jf, on the other side of the mica- 
schist divide, was able to do in the gneiss in whicli its bed 
must be eroded. 
The tributary ^1, b}' reason of the grade given it by the 
deepening channel of the main stream, was able to cut back 
through the mica-schist, with the dip, and eventually to rob 
M of its headwaters. 
Since the capture of the headwaters of J/, the channel of 
the diverted stream has rapidly deepened with the increased 
declivity and is now separated from the beheaded M by a low 
watershed, while M flows through a valley larger than its 
present volume warrants. A natural ponding on Gulf creek 
resulting from the sudden accession of detrital material 
brought by the tributary, has been utilized for artiflcial dams. 
Similar transverse courses are pursued by Brandywine, 
Naaman, Chester. Ridley, Crum, Darby, Tacony, and Penny- 
pack creeks which drain large areas of the plateau and empty 
into the Delaware between Wilmington, Delaware, and 
Holmesburg. Pennsj'^lvania. If these streams were superim- 
posed upon a cover of Bryn Mawr gravel they have accom- 
plished all their work of erosion since the elevation of land 
which followed the deposit of that gravel. 
^Gilbert, G. K. Geology of the Henry Mountain. Geog. and Geol. 
Survey of the Rocky Mountain Region, p. 130. 
