The stream referred to in this discussion as Tacony Creek, is sometimes known as Tookany Creek in its upper reaches and as Frankford Creek as it approaches the Delaware River. This distinction probably had more significance in the past than it does today as urbanization has buried tributary creeks and otherwise altered the drainage basin. To avoid using three different names for what today appears to be a single drainage route the name Tacony Creek is used here for all stream segments.
Figure 1: Tacony Creek flows in an east direction from location 1 to the Jenkintown Elbow of Capture at location 2 and then enters the south-southeast oriented Tacony Creek Gorge to flow to location 3. The railroad line extending in a northeast direction from from location 2 is in the Meadow Brook Through Valley. The Meadow Brook Through Valley, the northeast oriented Tacony Creek tributary segment at location 4, and the east-northeast oriented Greenwood Avenue valley at location 5 are relics of the southwest oriented flood flow channels that once crossed the region (although on a much higher surface).
Tacony Creek headwaters flow in an east direction through Glenside to the Jenkintown Elbow of Capture where Tacony Creek then turns in a south-southeast direction to flow to Elkins Park and beyond. Between the Jenkintown Elbow of Capture and Elkins Park Tacony Creek flows in a narrow south-southeast oriented valley carved in erosion resistant metamorphic and igneous rocks. Bedrock outcrops along the valley walls are visible from SEPTA trains traveling between Elkins Park and Jenkintown. The narrow south-southeast oriented Tacony Creek valley between the Jenkintown Elbow of Capture and Elkins Park is here referred to as the Tacony Creek Gorge.
The Tacony Creek Gorge has many water gap characteristics as it is eroded across an intensely folded metamorphic and igneous rock ridge exceeding 320 feet in elevation while just north of the gorge entrance the Meadow Brook Through Valley provides a lower elevation route to the south oriented Pennypack Creek valley (to the east). Tacony Creek crosses the 200-foot contour line as it enters the Tacony Creek Gorge and elevations east of the gorge in Jenkintown exceed 320 feet while elevations in Wyncote west of the gorge exceed 360 feet. Today Meadow Brook Through Valley elevations do not exceed 230 feet. If so, then why and how did Tacony Creek erode a 100-foot (or more) deep valley across an erosion resistant ridge of metamorphic and igneous rocks?
To answer the question it is necessary to visualize the region prior to erosion of the Tacony Creek valley and the Meadow Brook Through Valley (and also prior to erosion of the south oriented Pennypack Creek valley to the east). At that time all regional elevations were at least as high as the highest regional elevations today (which exceed 400 feet along the Tacony Creek-Wisahickon Creek drainage divide). Little evidence of that pre-Tacony Creek high-level surface remains, although aligned drainage routes throughout the region suggest massive and prolonged floods moved across the high-level surface in shallow diverging and converging channels to reach the actively eroding Wissahickon Gorge to the west of the present day Tacony Creek drainage basin. If so floodwaters crossed the present day Tacony Gorge location and must have been flowing at elevations greater than 400 feet (in terms of today’s elevations).
The south-southeast oriented Tacony Creek valley eroded headward across this southwest oriented flood flow and in doing so beheaded and reversed flood flow routes to what at that time was the newly eroded Wissahickon Gorge (southwest oriented Wissahickon Gorge tributary valleys were eroded by the southwest oriented flood flow channels prior to their beheading by Tacony Creek valley headward erosion). The Tacony Creek Gorge straightness suggests the Tacony Creek valley may have eroded headward along a fault line or other easily eroded linear geologic feature. Lower elevations to the east of the Tacony Creek Gorge (compared with elevations to the west) are the result of southwest oriented flood flow moving into the newly eroded gorge before being captured by headward erosion of the deep south oriented Pennypack Creek valley to the east.
The Meadow Brook Through Valley was initially eroded when headward erosion of the deep south oriented Tacony Creek valley captured a west-southwest oriented flow channel on the Meadow Brook Through Valley alignment (although the flow was moving at an elevation equivalent to or higher than elevations surrounding the Meadow Brook Through Valley today). Once captured the Meadow Brook Through Valley became a major flow route supplying water to the actively eroding Tacony Creek Gorge. West-southwest oriented flow in the Meadow Brook Through Valley ended when headward erosion of the deep south-oriented Pennypack Creek valley beheaded and reversed the flow. The reversed flow probably captured floodwaters from southwest flow channels further to the north and the captured flow helped erode the Meadow Brook Through Valley floor so as to create the east-northeast oriented Meadow Brook drainage route to Pennypack Creek.