William "Sandy" Schenck lead a field trip through the Delaware Piedmont for the A.I. duPont High School Earth Science Class. The trip made use of the Wilmington-Western Railroad and everyone rode the railroad's "Doodle Bug." Activities included up close examinations of rock and mineral features and even "Panning for Garnets" at Brandywine Springs Park.
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.
This report accompanies a new map that revises the original bedrock geologic maps of the Delaware Piedmont compiled by Woodruff and Thompson and published by the Delaware Geological Survey (DGS) in 1972 and 1975. Combined detailed mapping, petrography, geochemistry, and U-Pb geochronology have allowed us to redefine two rock units and formally recognize eleven new units. A section of the Pennsylvania Piedmont is included on the new map to show the entire extent of the Mill Creek Nappe and the Arden Plutonic Supersuite.
GeoAdventures are designed to allow the reader to learn about a particular geologic point of interest in Delaware’s Piedmont province and then take a short field trip to that area. Want to know more about the Wilmington blue rock or Brandywine blue granite? Take the Wilmington Blue Rock GeoAdventure and go see just what the blue rock looks like.
In 1977, the Delaware General Assembly, acting on a proposal by the Delaware Mineralogical Society, established sillimanite as the Delaware State Mineral. This act recognizes the geological and mineralogical significance of the large masses of this mineral found as boulders at Brandywine Springs, an occurrence that was recognized as important in the 6th (1892) edition of Dana's System of Mineralogy. The Brandywine Springs boulders are remarkable for their size and purity. The sillimanite has a fibrous texture reminiscent of wood and could potentially be cut into cabochon gems showing a chatoyant ("cat's eye") effect. Sillimanite is not mined as an ore or raw material in Delaware.