New Garden Township lies near the southeast edge of the Piedmont, a complex band of ancient rocks that have been severely contorted and metamorphosed. Over half of the Township is underlain by the Wissahickon formation, a mixture of gneiss and schist shot through with lenses of darker finer-grained amphibolite and coarser-grained pegmatite. Boulders and ledges of more resistant gneiss are exposed in the White Clay valley, but generally the Wissahickon is deeply weathered and rarely visible at the surface.
Two anticlines (i.e., upward folds of rocks) bracket the Township – these are older than the Wissahickon formation. On the south end, the Yorklyn-Hockessin anticline is marked by the valley of Broad Run and Somerset Lake, which are underlain by Cockeysville marble, and has the lowest point in the Township – 180 feet above sea level. The ridge east of Somerset Lake at the center of the anticline is Baltimore gneiss, the oldest rock in the area.
At the Township’s northern end, the larger Avondale anticline has a similar pattern of Cockeysville marble wrapping around a core of Baltimore gneiss, but it also has a band of Setters quartzite between the marble and the gneiss. The low Baltimore Pike corridor and the Red Clay north of Avondale, underlain by Cockeysville marble, abruptly gives way to Toughkenamon Hill, a ridge of resistant Setters quartzite. The highest ground in the Township (483 ft.), northeast of Toughkenamon, lies on Baltimore gneiss.
Roughly six square miles or 3/8 of the Township lies in the Red Clay Creek watershed, with the rest drained by the East Branch of White Clay Creek. The drainage divide follows Newark Road from the north end to Route 41 and then Route 41 southeastward. Egypt Run and Broad Run feed into the East Branch of White Clay Creek, which meanders 2.7 miles through the southwest corner of the Township, much of it in a deeply incised, steep, canyon-like valley. Bucktoe Creek and the Toughkenamon branch are the largest tributaries of the west branch of Red Clay Creek, which cuts across the far northeast corner of the Township for about 1/3 of a mile. Near the edge of the Piedmont these streams have a relatively steep gradient, and the available water power was one of the driving forces in the early industrialization of the area.
Refer to Map 2 for more information regarding the Township‘s watersheds and geology.
The influence of geology on plant distribution and communities has been muted in New Garden Township by the long history and high level of agricultural, industrial, and residential land use. The airport has a dry bank community that reflects the underlying Setters quartzite. A few pockets of wetland or edge species associated with Cockeysville marble survive, and the area near Landenberg has a few of the uncommon rich woods species that are characteristic of the lower White Clay valley. In general, most of the remaining native communities have lost many of their most specialized species and are characterized by generalists and aliens.