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Site content related to keyword: "Appalachian Piedmont"

Outcrop Be21-e: Hanby Park Quarry

Rock Outcrop Be21-e: Hanby Park Quarry

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.

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

Rock 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

Rock 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

Rock 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

Rock 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

Rock 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

Rock 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

Rock 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.

Geologic History of the Delaware Piedmont

Fig A. Cross section of eastern North America as it may have looked 543 million years ago, active volcano is offshore.

The Delaware Piedmont is but a small part of the Appalachian Mountain system that extends from Georgia to Newfoundland. This mountain system is the result of tectonic activity that took place during the Paleozoic era, between 543 and 245 million years ago. Since that time, the mountains have been continuously eroding, and their deep roots slowly rising in compensation as the overlying rocks are removed. It is surprising to find that although the Delaware Piedmont has passed through the whole series of tectonic events that formed the Appalachians, the mineralogy and structures preserved in Delaware were formed by the early event that occurred between 470 and 440 million years ago, called the Taconic orogeny.

Outcrop Ba14-a: The Setters Formation at Avondale Quarry

Rock 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

Rock 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

Rock 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.

Outcrop Bc32-b: The Mt. Cuba Railroad Cut

Rock Outcrop Bc32-b: The Mt. Cuba Railroad Cut

The Mt. Cuba railroad cut is narrow and deep, and much of the rock is covered with dirt and soot from the train. The rocks are interlayered gneisses and amphibolites, with gneisses predominating in the south end of the cut and amphibolites in the north end. Folding is well developed, but the angle of the sunlight as it shines on the walls of the cut will determine which of the folds will be the easiest to see. A 4" amphibolite layer outlines the fold in this part of the outcrop. Wonderful examples of the effects of rock type on folding styles can be seen in the cut and in many of the rocks piled north of the cut.

Map of Selected Piedmont Outcrops

Map and image gallery of selected rock outcrops in the Delaware Piedmont.

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Outcrop Cc12-a: The Cave at Brandywine Springs

Rock Outcrop Cc12-a: The Cave at Brandywine Springs

Approximately 100 yards east of the tracks is one of the largest outcrops in the park. Here along the hillside, a thick layer of crinkle-folded, yellow-weathering gneiss overlies a layer of garnet-bearing quartzite and amphibolite. At the contact between the quartzite and the schist, a large piece of the quartzite has fallen out creating a small cave. Maybe Indians used this cave, but it is not very inviting. If you hit the black rocks with a hammer they will ring. Look for the tiny lavender garnets in the quartzite.

IS4 Domestic Water Systems

Domestic Water Systems

Thousands of homeowners in Delaware currently rely on individual wells and water systems to provide water. In addition, hundreds of new wells and systems are constructed each year to provide water for those not served by public water systems. Methods used to construct water wells in Delaware are discussed in DGS Information Series No. 2 (Domestic Water Well
Construction). Domestic water systems are described herein.

IS3 Ground Water in Delaware

Ground Water in Delaware

Because of its "renewability" water is unique among earth resources that sustain and enhance life. No other mineral resource that we extract on a long-term and continuous basis can be counted on for at least some degree of replenishment within a human lifetime. This attribute allows a great deal of flexibility in management of the resource. In Delaware local rainfall, approximately 40" to 44" per year, renews part or all of our water supply on a regular basis. However, not all of the rain that falls is available for use. From this total rainfall must be subtracted the water that evaporates (about 20"/ year), the amount that is used by plants (about 3"/year), and the amount that runs overland to surface streams during storms (about 4"-5"/year). The remainder, approximately 13" to 15" is Delaware's bank of water for the year. This water is stored in a system of ground-water reservoirs, or aquifers, that underlie most of the State. Not only do these ground-water reservoirs provide water to wells but they also maintain the flow in surface streams during times of no rainfall. Streamflow between rainfall events is nothing more than the discharge of
excess ground water.

IS2 Domestic Water Well Construction

Domestic Water Well Construction

The storage and movement of ground water depends on the types of rocks and associated
interconnected spaces in which the water occurs. The Piedmont Province in northernmost
Delaware is underlain by crystalline rocks. Because of the massiveness and hardness of such
rocks, they yield little or no interstitial water to wells. Water is stored in and moves through fractures, cracks, and solution cavities. The amount of water available depends on the number and size of openings, and the degree to which they are interconnected. Wells drilled in the Piedmont range from 100 to 400 feet in depth and yields are highly variable over very short distances.

In the Coastal Plain, the rest of the State, ground water is stored and transmitted in spaces between adjacent rock particles. As much as 30 percent of the rock mass may be saturated. Unconsolidated rocks are analogous to a bathtub filled with sand into which water is poured. The Coastal Plain consists of sandy water-bearing units referred to as aquifers interlayered between non-water-bearing units. Wells constructed for domestic use range in depth from 15 feet to 500 feet. Yields are generally much greater than those obtained from the crystalline rocks of the Piedmont. In general, minimum well yields of 3 to 5 gallons per minute are adequate for most domestic water supply systems.

Catalog of Delaware Earthquakes Spreadsheet

Catalog of Delaware Earthquakes Spreadsheet

The occurrences of earthquakes in northern Delaware and adjacent areas of Pennsylvania, Maryland, and New Jersey are well documented by both historical and instrumental records. Over 550 earthquakes have been documented within 150 miles of Delaware since 1677. One of the earliest known events occurred in 1737 and was felt in Philadelphia and surrounding areas. The largest known event in Delaware occurred in the Wilmington area in 1871 with an intensity of VII (Modified Mercalli Scale). The second largest event occurred in the Delaware area in 1973 (magnitude 3.8 and maximum Modified Mercalli Intensity of V-VI). The epicenter for this event was placed in or near the Delaware River. Sixty-nine earthquakes have been documented or suspected in Delaware since 1871.

Baltimore Gneiss

Pbgn

Granitic gneiss with swirling leucosomes and irregular biotite-rich restite layers is the dominant lithology and constitutes approximately 75 to 80 percent of the exposed rocks. The remaining 20 to 25 percent comprises hornblende-biotite gneiss, amphibolite with or without pyroxene, and pegmatite. Granitic gneiss is composed of quartz, plagioclase, biotite, and microcline. Minor and accessory minerals are garnet, muscovite, magnetite, ilmenite, sphene, apatite, and zircon. The hornblende gneiss contains plagioclase, quartz, hornblende, and biotite with/without orthopyroxene. Accessory minerals are garnet, muscovite, clinozoisite, perthitic orthoclase, iron-titanium oxides, sphene, and apatite. Amphibolites are composed of subequal amounts of hornblende and plagioclase with minor quartz, biotite, clinopyroxene, and orthopyroxene.