Appalachian Piedmont

Geologic History of the Delaware Piedmont

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

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.

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.

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

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

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.

IS2 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