Richard N. Benson

Figure 1The Pencader Historical Area (PHA) is located within the innermost portion of the Atlantic Coastal Plain of Delaware. The most prominent topographic features, however, are Chestnut and Iron hills plus Sandy Brae in Delaware and Gray's Hill in adjacent Cecil County, Maryland. These features that arise from the plain are outliers of the Piedmont Province that are surrounded by the unconsolidated sedimentary rocks of the Coastal Plain (Fig. 1). The elevation of the top of Iron Hill is over 340 feet above sea level, which is more than 250 feet above the general level of the adjacent plain.

The boundary between the Piedmont and the Coastal Plain is marked by the Fall Line, named this because falls and rapids on the major tidally influenced rivers of navigation occur on the hard crystalline rocks of the Piedmont where they meet the Coastal Plain sediments. The major East Coast cities of the U.S. began as colonial settlements along the Fall Line as the falls and rapids prevented further navigation on the rivers and provided water power for mills.

The major through-going stream of the PHA is the Christina River which changes course abruptly from southsoutheastward-flowing in the Piedmont to northeasterly after crossing the Fall Line. This is characteristic of all the streams draining the Delaware Piedmont.

The generalized geologic map of Figure 2 shows the distribution of the rock types found in northern Delaware. The Piedmont rocks are metamorphic and igneous rocks comprising the eroded remnants or roots of ancient mountains formed during the first of three mountain-building events called orogenies that culminated in the Appalachian Mountains. The rocks are referred to as crystalline rocks because they are composed of the crystals of minerals that have grown together to interlock with one another. Igneous rocks crystallized from a molten state, either a magma or lava. Rocks that solidified from a magma deep underground cooled slowly so the mineral crystals grew large enough so they can be seen by the naked eye. Such rocks are described as coarse-grained, and the rock bodies they comprise are called plutons. In contrast, volcanic igneous rocks solidified more rapidly from magmas that reached the earth's surface as lava; therefore, they are fine-grained so most of the crystals of the rock are too small to be seen without magnification. Metamorphic rocks are "changed rocks," rocks that originally were sedimentary and volcanic or plutonic igneous rocks that changed through heat and pressure without being completely melted. Heat, pressure, and hot fluids deep within the Earth during mountain-building or from magmas are the processes of metamorphism.

Figure 3
Figure 3
Figure 3

Figure 4

The cross section of Figure 4 shows the exposed and buried Piedmont rock bodies of the PHA. They are part of the Wilmington Complex which consists of numerous metamorphosed sedimentary, volcanic, and plutonic rock units that comprise the eroded remnants of the ancient volcanic arc. The bedrock of Chestnut and Iron hills and the other Piedmont outliers of the Coastal Plain consists of the black to very dark green, coarse- to very coarse-grained igneous rock pluton mapped as the Iron Hill Gabbro. Pyroxene and plagioclase feldspar are the two major groups of minerals that crystallized from the magma along with lesser amounts of olivine. The resulting rock was later extensively altered by hydrothermal solutions. Chemical weathering has leached away much of the gabbro and related rocks exposed at the surface to produce yellowbrown rocks with a "honeycomb" texture. The product of weathering is a layer of iron oxides mixed with jasper to form a material called gossan. Native Americans made artifacts out of the jasper. During colonial times and later, the iron oxides were mined for their iron, thus the name Iron Hill.

Next to the Iron Hill Gabbro and underlying Newark and area to the west is the Christianstead Metapluton, a coarsegrained gneiss (banded metamorphic rock) that originally was a pluton consisting of minerals that identify the igneous rock granodiorite. Predominant minerals are quartz, plagioclase and microcline feldspars, and lesser amounts of biotite mica. It is also part of the Wimington Complex volcanic arc. Because the area of contact between the Iron Hill Gabbro and the Christianstead Metapluton is covered by Coastal Plain sediments, the relationship between the two plutonic rock bodies is unknown There is some evidence to suggest that the Iron Hill Gabbro magma intruded the Christianstead rock body. Another possibility is that there is a fault between them, movement along which brought the two together. South of Iron Hill but buried by Coastal Plain sediments are other rocks that may be similar to metamorphosed volcanic rocks of the Wilmington Complex exposed elsewhere in the Piedmont nearby. Although water wells drilled in this area have penetrated these rocks, no data exist on their composition.

The final orogeny that constructed the young Appalachian Mountains is called the Alleghanian Orogeny near the end of the Paleozoic Era between about 330 and 250 million years ago. This was part of the world-wide convergence of all the continental tectonic plates that culminated in one supercontinent called Pangea. In North America, later during the Triassic Period an extensive rift zone developed that created fault-bounded sedimentary basins similar to the East African rift basins of today. During the Jurassic Period about 175 million years ago, Pangea began to break apart along this rift zone and Europe and Africa began to drift apart from North America, creating the growing North Atlantic Ocean basin between. Today, the Red Sea is an example of a new ocean basin being formed as two continental plates are moving away from each other.

After separation, the North American continental margin began to subside (sink) to accommodate a great thickness of sediments derived from erosion of the Appalachian land mass. The thickest accumulations are in sedimentary basins offshore, as much as 50,000 feet thick beneath the continental shelf off the Middle Atlantic states. The Atlantic Coastal Plain is the surface of this sedimentary mass that is above sea level. The maximum thickness of the seaward-thickening wedge of Coastal Plain sediments is about 10,000 feet at the DelawareMaryland border at the coast (Fig. 5).

As the Atlantic continental margin continued to subside, younger sediments began to lap onto the eroded crystalline rock platform until the eroded Appalachians including the Piedmont were partially buried. The Coastal Plain sediments reflect periods of sea level rise and fall and periods of high contrasted with low rates of sediment influx.

The PHA at one time was completely buried by sedimentary rocks that later were removed by erosion. What is left today is the river sands and floodplain muds of the Potomac Formation (Fig. 4) that were deposited on the eroded Piedmont surface during middle Cretaceous time about 100 million years ago. Iron and Chestnut hills were buried and later exhumed by erosion of the overlying Potomac and younger formations and now are outliers of Piedmont rock within the Atlantic Coastal Plain. Remnants of the Potomac Formation occur on Chestnut Hill adjacent to 1-95 at about 200 feet elevation. In Cecil County, Maryland, the basal layers of the Potomac Formation are at elevations as high as about 400 feet.

The latest chapter recorded by the rocks of the PHA is of the ice ages of the Quaternary Epoch (the last 1.5 million years). The sands and gravels of the Columbia Formation, now partially eroded by modern processes, were deposited over much of the eroded surface of the older Coastal Plain and Piedmont rocks. The sediments are the debris derived from melting continental glaciers that had advanced as far south as Long Island and northern New Jersey and Pennsylvania about 440,000 years ago. Evidence of cold climates in Delaware are from pollen that indicate boreal forest and even as frigid as taiga environments. In Rittenhouse Park, boulder fields of Iron Hill Gabbro attest to the extreme frost action that pried loose those rocks during glacial times.

The modern streams formed the present topography developed on all the rocks described in this report. This is what greeted the first human occupants of the Pencader Historical Area.

Plank, M.O., and Schenck, W.S., 1998, Delaware Piedmont geology including a guide to the rocks of the Red Clay Valley: Delaware Geological Survey Special Publication No. 20, 64 p.
Schenck, W.S., Plank, M.O., and Srogi, LeeAnn, 2000, Bedrock geologic map of the Piedmont of Delaware and adjacent Pennsylvania: Delaware Geological Survey Geologic Map Series, No. 10.