404 GEOLOGY OF DELAWARE COUNTY. 



towards the river : the average course? of these streams being a little east of 

 south. This slope is rather gradual till we approach within three or four miles 

 of the river, or the marsh lauds bordering it. Here may be observed on the 

 high ground between the creeks, an abrupt fall in the general surface of the 

 country. The line or direction of this sudden change of slope is aearly at right 

 angles with the large creeks of the County, and preserves a general parallelism 

 with the river and marsh lands, at about the distance that has been mentioned. 

 It has been termed a " u-ater shade," and for reasons that will be mentioned here- 

 after, not inappropriately. This '^ water shade" can be traced beyond the limits 

 of the County on either side of it, and though obscured by streams and ravines 

 in many places, it is very distinct in others. Thus it may be seen below Booth's 

 Corner in Bethel; at Mount Hope meeting-house in Aston, where it has the 

 name of "Aston Ridge;" at Harrison's Hill on the Edgmont road; near Hink- 

 son's Corner on the Chester and Providence road ; above West Dale, Spring 

 Hill, and Clifton stations on the West Chester railroad ; between Kellyville and 

 Garrett Ford, crossing the Garrett road near the toll-gate. 



The abruptness of the fall in the land is in a great degree obliterated in 

 the beds of the several creeks that cross it; but the great fall in these streams, 

 at and for some distance above their intersection by the " water shade," un- 

 mistakeably indicate its presence. During ages they have been cutting their 

 channels deeper upwards, thereby constituting a series of rapids, which in an 

 early Geological period of this district of country, were concentrated into one of 

 great abruptness, or even into perpendicular falls. This cutting back in the 

 beds of our streams has been modified by the character of the rocks in ditterent 

 localities. Thus in Chester Creek it has been much greater than in those of 

 Crum or Darby. Still each stream has its rapids at no very great distance from 

 the crossing of the zvater shade, which in every instance is at a point where the 

 stream has acquired its maximum size, thus affording very many of the best 

 sites for manufacturing purposes, for which the County has become noted. 



The " water shade " has served another valuable purpose. The land from its 

 base gradually declines towards the Delaware, leaving nothing to obstruct the 

 view from its summit of several miles of that river, and over a great extent of 

 New Jersey, thus affording a vast number of building sites, unrivaled for health- 

 fulness and the extent of the views they afford. 



The elevation of the sources of the principal streams of the County above 

 tide water has been ascertained with a reasonable degree of accuracy. That of 

 Cobb's Creek is 392 feet, Ithan 399, Darby 440, Crum, 520, and Ridley the same. 

 The elevation of the source of Chester Creek has not been ascertained with the 

 same degree of accuracy, but that of its eastern branch may be safely set down 

 at a somewhat higher figure than Crum or Ridley Creeks. 



The only exception to the general slope of the surface of the County is in 

 the small part of Radnor drained by the Gulf Creek. This stream at first has 

 an easterly course, then suddenly turns to the north, and after having cut its 

 way through the south valley hill, forming a narrow gorge known as the 

 " Gulf," turns again easterly and empties into the Schuylkill. 



The marsh lands bordering the Delaware would be inundated at every high 

 tide, but for the artificial banks that surround them. Within the large scope of 

 marsh between the Schuylkill and Darby Creek, besides the high part of Tini- 

 cum, there are several parcels of land that would not thus be inundated. These 

 were islands before the banks were made. Their number and general figure 

 may be seen on the "Map of Early Settlements" accompanying this work. 



Having thus presented the general physical aspcctof the surface of the County, 

 it will be next in order to consider its Geology proper. The rocks of our County 

 are relatively the lowest, and belong to the earliest formation known to Geologists. 

 To those that are stratified, Mr. Lyell has applied the terms, Ilypogene and Meta- 

 morphic : the former having reference to their position, as being nether-formed, 

 and the latter to their altered structure from subterranean heat. The term 

 Hypogene will apply equally well to any unstratified rocks within our limits. 



Of the magnificent series of deposits entombing the remains of a succes- 

 sion of organized beings, found in other sections of our country, this County 



