The town of Radnor is located Schuylkill River-Delaware River drainage divide between Gulph Creek, which flows to the Schuylkill River, and Ithan Creek, which flows to Darby Creek, which then flows to the Delaware River. The drainage divide elevation at the Radnor saddle location is between 410 and 420 feet while the drainage divide rises to more than 480 feet to the east and to more than 500 feet to the west. This saddle is evidence of a former water flow route across what is today the drainage divide between two major and different river drainage basins.
Figure 1: The Radnor saddle area. United States Geological Survey map digitally presented using National Geographic TOPO software.
Figure 1 illustrates the Radnor saddle area with the saddle being just west of the word “Radnor”. Ithan Creek flows in a south direction at Willowburn (near south edge of figure 1), where it is joined by west and south oriented Hardings Run. South of figure 1 Ithan Creek joins southeast, south, and southeast oriented Darby Creek, which eventually reaches the southwest oriented Delaware River. Gulph Creek flows in an east-northeast direction across the figure 1 north half and north and east of figure 1 turns in a north direction to flow through a deep water gap before continuing to flow in an east-northeast direction to reach the southeast oriented Schuylkill River. The evidence seen in figure 1 suggests headward erosion of the east oriented Gulph Creek valley beheaded a south oriented channel supplying water that eroded of the Ithan Creek valley.
Looking just at the figure 1 the Radnor saddle area evidence could be considered to be a case of simple stream capture where headward erosion of the Gulph Creek valley has beheaded the Ithan Creek valley. But the Radnor saddle is not the only saddle found along the Schuylkill River-Delaware River drainage divide. East of figure 1 another significant saddle is located near Rosemont and close look at the drainage divide reveals still additional saddles. These and other saddles suggest multiple flow channels crossed the present day Schuylkill River-Delaware River drainage divide and are better explained by a southwest oriented anastomosing channel complex that was first captured by headward erosion of the deep Darby Creek valley and its tributary valleys and subsequently captured by headward erosion of the deeper Schuylkill River valley and its tributary valleys. Saddles, such as the Radnor saddle, notched into the Schuylkill River-Delaware River drainage divide provide evidence of massive and prolonged southwest oriented floods that once flowed across what are today the Main Line suburbs.