Appalachian Piedmont

Biotite Tonalite

Fine- to medium-grained, equigranular biotite tonalite usually occurring as rounded boulders. Tonalites are leucocratic (15 to 25% modal mafic minerals), light gray to buff on fresh surfaces, and locally contain mafic enclaves with reddish rims, the result of iron hydroxide staining. Possibly intrusive into the Perkins Run Gabbronorite Suite.

Outcrop Bc44-f: The Tatnall Preschool Grounds

The Tatnall Preschool Grounds contain many light-colored, coarse-grained, igneous-looking rocks (Barley Mill Gneiss) with mafic enclosures. These mafic enclosures make up only a small part of the rock. They may either be random in slope or they are elongated. When the Upper School and Preschool were built in the 1970s and 1980s, a lot of rock was removed from the foundations. The rock is either scattered around as large boulders in the landscaping, or in the back of the athletic fields in a large dump. The rocks in the dump show examples of mafic rock (greenish in color), part mafic and part pegmatite, and granitic rock.

Outcrop Cc12-c: The Red Clay Creek Edge

Along the edge of the Red Clay Creek exists a large outcrop that extends out into the stream. This rock is part of the Wissahickon Formation, with pelitic facies, ½" elongated sillimanite nodules, and disharmonic folds. The compositional layering of this rock is 1/8 – ½" of biotite rich layers alternated with fine-grained psammitic layers (not quartz-feldspar layers). Some of these layers are sheared (shear zones). The sillimanite nodules, pegmatite pods, and shear zones in this rock are all parallel to fold axes. The axial plane of these folds is 20 degrees east of north, plunges 42 degrees northeast, and dips 90 degrees. Within this large outcrop are several 2-3' layers of “rock that rings” (when hit) and are folded with petitic gneiss. This pelitic gneiss shows more intense folding while the rest of the rock is gently folded. The “rock that rings” is also peppered with small lavender garnets.

Outcrop Cb42-c: Windy Hills Bridge Outcrop

Considered one of Delaware's most famous Piedmont outcrops, the Windy Hills Bridge outcrop is composed of mafic and felsic gneiss of the Windy Hills Gneiss. Much of the layering in the outcrop is regular and is 8 to 10" thick. At the contact between these layers there is evidence of partial melting. In terms of mineralogy, this rock contains mainly hornblende, plagioclase, quartz, biotite and epidote. This outcrop shows tight folds that plunge steeply 70-90 degrees to the northeast and southwest. The gneiss is cut by a long lens of pegmatite, which intruded after the folding and metamorphosing that yielded the gneiss.

There is also an interesting layer of cobble just above the bedrock in this area presumed to be the contact with the Coastal Plain sediments. These newer outcrops to the southwest display a 4-10" pelitic layer which becomes more extremely magmatic, with 1" leucosomes and ½" mafic selvages.

Overall, strikes of foliations of the mafic and felsic layering in these outcrops are 70-75 degrees east of north and the dips are a steep 80-85 degrees to the southeast, or almost vertical.

Outcrop Cb15-c: The Confluence Quarry at North Pointe

Just northwest of the confluence of Mill Creek and an unnamed tributary is an abandoned quarry. This quarry sits off the greenway trail, across from a ruined foundation wall, and displays samples of black, coarse-grained, foliated amphibolite. The unnamed tributary and Mill Creek are choked with amphibolite rocks and boulders. The amphibolite here weathers with a rusty rind and has a foliation strike of 10 degrees east of north and an obviously steep to vertical dip.

Outcrop Ca44-d2: The Christianstead Subdivision

Outcrops between Hidden Valley Drive and Farmhouse Road. The Christianstead subdivision is underlain by interlayered mafic and felsic gneiss with large pegmatites. The felsic gneiss, in the northwestern half of this subdivision, is deformed granodiorite, seen as massive igneous layers with only rare crinkle folding. There are a few “bright eye” textures on the west end of this subdivision, which is all underlain by granodiorite.