Outcrop Be23-g: Charnockite Boulders in the South Branch of Naaman Creek

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.

Outcrop Bd21-a: Boulder Field at Brandywine Creek State Park

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.

Outcrop Be22-k: Charnockite Boulders at Ardentown

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.

Outcrop Bd44-b: Bringhurst Gabbro boulders in Shellpot Creek

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.

Outcrop Bd41-b: Rockford Park Gneiss Boulders at Rockford Park

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.

Outcrop Bd42-e: The Cliffs of Alapocas Woods

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.

Outcrop Da15-h: The Paraglacial Boulder Feature of Chestnut Hill

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.

Outcrop Ba14-a: The Setters Formation at Avondale Quarry

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.

Outcrop Bb25-c: The Yorklyn Railroad Cut

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.

Outcrop Bc32-a: The Mt. Cuba Picnic Grove

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.