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.

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.

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.

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.

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.

In Claymont, DE, the intersection of Marvel Avenue and Parkside Boulevard occurs at Lesher Park, which contains Perkins Run, a creek West of Harvey Road. In the streambed of this creek is an outcrop of Perkins Run Gabbro, which is part of the Arden Plutonic Supersuite. The gabbro displays joints that are oriented 10 degrees west of north. Along these joints, veins of charnockite (orthopyroxene-bearing granite of the Ardentown Granitic Suite) can be found.

Located in Ardentown are a few silicic boulders just on the northwestern side of the railroad bridge that crosses the South Branch of Naaman Creek. These boulders are part of the Ardentown Granitic Suite. Some have very large (several cm) feldspar phenocrysts. Some display contacts between granitic rock and quartz-rich rock, which is probably metasedimentary rock due to the granular nature of quartz.

On the south side of Chestnut Hill is an outcrop of very large boulders in the woods of Hanby Park near Arden, DE. This area of the park seems to be the site of an old quarry. The rocks here are very similar to the rocks found down the South Branch of Naaman Creek (Ardentown and Ardencroft) as they both share the same fine-grained, mafic properties with traces of coarse-grained charnockite.

Running through Knollwood Park in Claymont, DE is the South Branch of Naaman Creek. This stream is laden with fairly mafic, medium to coarse-grained charnockite. Some of the charnockite samples here may be mylonitic. A few boulders contain xenoliths as well. Other gabbro boulders display charnockite veins in a gradational zone over about 1-2 meters.

In the patch of woods north of the upper parking lot in Brandywine Creek State Park, there are large outcrops of amphibolite. The outcrops are rounded from exfoliation, and are black with few structural features. The mafic hornblende grains are elongated parallel to a few thin felsic bands. This lineation strikes east-west and dips to the north. These boulders are located on the northwest facing slope of the valley and are probably a paraglacial feature left over from a colder period in Delaware's geologic past.

In the valley of the South Branch of Naaman Creek, through Ardentown, is a group of charnockite boulders and fine-grained mafic rock (probably amphibolitized gabbroid). The mafic rock is mostly non-megacrystic with some coarse-grained and equigranular charnockite. On the ground around the boulders are small pieces that contain a clear example of a contact between coarse-grained and fine-grained rock types.

Found in the creek bed and flood plain, the large boulders in Shellpot Creek are excellent examples of Bringhurst Gabbro. The gabbro is very coarse-grained with crystals up to 2" long; however, variations in the grain size exist over a scale of a few inches. While observing this rock closely, one can occasionally find grains of orthopyroxene (possibly bronzite) up to 4" long. Some of the boulders have grains of olivine surrounded by double coronas of orthopyroxene, spinel, and hornblende.

The Rockford Park boulders can be found just beyond the Rockford Tower on the slope facing the Brandywine Creek. Some areas of the Rockford Park Gneiss actually display some banding of felsic gneiss and mafic gneiss which are interlayered on a scale of 4" to 2'. This banding strikes 30 degrees east of north and dips 60 degrees to the northwest. The mafic layers are boudinaged and broken, some of which are weathered away into a prominent relief. Between some layers, the rock is intruded by a coarse-grained and apparently undeformed gabbro.

Located in Wilmington, DE, the Cliffs of Alapocas Woods are opposite the old Bancroft Mills across the Brandywine Creek. Along the creek you will find large exposures of Brandywine Blue Gneiss. Compared to other outcrops in the Piedmont of Delaware, the rock examples here are massive. When observed closely, the felsic gneiss displays a medium grain size. Most of early Wilmington was built from the stone from these quarries. These impressive rock features are enjoyed by local rock climbers as well as many who use the Northern Delaware Greenway.

Prime examples of Iron Hill Gabbro can be found in the area surrounding Chestnut Hill at Rittenhouse Park. The gabbro here is considered coarse to very coarse grained. Boulders of Iron Hill Gabbro are located on the northeast facing slope southwest of the Christina Creek. This gabbro boulder field is probably a paraglacial feature left over from ice age times deep in Delaware’s geologic past.

The Setters Formation is located in southeast Avondale, PA. Huge slabs of rock have been exposed by a gravel company that has been removing the hillside quarrying for quartzite to sell as building stone and grinding pelitic rock into gravel and stone. These slabs have a foliation with a strike of 45 degrees East of North and a southeastern dip off of the Avondale Anticline. They also display quartzite, schist, and pods of pegmatite, containing large garnets (1-2 in. diameter) and schorl tourmaline, that appear to be “sweated out of schist.” A dramatic contrast in rich type-shelf facies reflects beach sand and bogs or inlets.

Wissahickon gneisses and amphibolites are exposed in the railroad cut near Yorklyn. Here the rocks are unusual because the layering is accentuated by the presence of fault gouge between the layers. Fault gouge forms as movement along a fault in hard, brittle rocks crushes and grinds the rocks into a powder. Gouge was a term used by miners because they could easily "gouge" it out of the rock. Here the gouge "weathered out" leaving deep indentations that emphasize the layering and the tilt, which is to the southeast at an angle of about 45 degrees.

The Mt. Cuba Picnic Grove provides an opportunity to look at the gneisses and amphibolites of the Wissahickon Formation. The large boulders of gneiss lying beside the steps are peppered with dark-red garnets and elongated nodules of dull-white sillimanite. These sillimanite nodules (1/4" to 3/4" long) are abundant in the gneisses at Mt. Cuba and are an interesting feature of these highly metamorphosed sedimentary rocks. Alternating layers of gneisses and amphibolites crop out on the east side of the track. The gneisses show some typical upright folds and fractures. Contacts between the layers trend northeast, parallel to the regional trend of the Appalachians.