The Geology of Delaware

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Common Rocks and Minerals of the Delaware Piedmont

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The Red Clay Creek has flowed through the rolling hills of northern Delaware for many thousands of years, cutting a deep valley into the old deformed rocks of the Appalachian Piedmont. The Red Clay valley contains many of the common rocks found throughout the Delaware Piedmont.

Igneous Rocks

Igneous rocks are those that form by the crystallization of a hot molten liquid called magma or lava. We can see igneous rocks form today where lava erupts from volcanoes and cools to form solid rock. If it was not for volcanoes, it might be difficult to convince anyone that rocks can form from molten lava. Igneous rocks that form on the Earth’s surface are called volcanic rocks or extrusive igneous rocks.

Not all molten rock rises from deep within the Earth to erupt in a volcano. Sometimes the molten rock, or magma, does not reach the surface, but is held in big underground chambers where it slowly solidifies to form intrusive igneous rocks. We can see this type of igneous rock only where erosion has removed the overlying rocks.

Extrusive and intrusive igneous rocks can be distinguished by the size of their mineral grains. If the individual crystals are too small to be seen without magnification, the rock is fine-grained and probably extrusive. If you can easily differentiate the grains, it is considered coarse-grained and intrusive. Extrusive rocks are fine-grained because lava cools quickly and large grains do not have time to form. Intrusive rocks cool slowly deep inside the Earth and have time to grow large mineral grains.

The igneous rocks exposed in the Red Clay Valley are mostly coarse-grained, intrusive rocks that are named granites, granitic pegmatites, diorites, and gabbros. These rocks form in large masses usually without the layering that is characteristic of sedimentary and metamorphic rocks.

Common Igneous Rocks of the Red Clay Valley
Rock Type Description
Basalt A fine-grained, dark-colored, extrusive igneous rock that forms
by the crystallization of lava flows. Most basalt flows in the Red Clay Valley have been metamorphosed to amphibolites and are now composed of plagioclase, pyroxene, and amphibole.
Granite A coarse-grained, light-colored rock composed of quartz and two feldspars (plagioclase and orthoclase), with lesser amounts of mica or amphibole.
Gabbro A coarse-grained rock composed of greenish-white feldspar (mostly plagioclase) and pyroxene. Gabbro is usually very dark in color. It is the intrusive equivalent of basalt.
Pegmatite An igneous rock with very large (usually > one inch), well-formed crystals. A granitic pegmatite has the mineralogy of a granite and abnormally large grains, whereas a gabbroic pegmatite has the mineralogy of a gabbro and very large grains.
Diorite A coarse, uniformly grained rock composed of a feldspar and less than 50% amphibole or pyroxene. A quartz diorite has the composition of a diorite plus quartz and biotite, whereas a granodiorite has the composition of a diorite plus quartz and two feldspars.

Metamorphic Rocks

Metamorphic rocks are sedimentary or igneous rocks that have been changed. These changes usually occur deep within the Earth, by processes we cannot observe; however, we do know that under the lithosphere the mantle is a slowly churning reservoir of fiery hot rock. Thus, when rocks are deeply buried, they are heated from the reservoir below and squeezed from above by the overlying rocks. At these high temperatures and pressures, some minerals will become unstable and change into new minerals. For example, clay will change into mica, mica plus quartz will change into sillimanite, and chlorite will change into garnet. The mineral changes that occur in solid rocks as they are heated and deeply buried are known as metamorphism.

Common metamorphic rocks are slate, schist, gneiss, quartzite, marble, and amphibolite. The dominant rocks in the Delaware Piedmont are gneisses and amphibolites, rocks that were highly metamorphosed by heating deep within a subduction zone.

Common Metamorphic Rocks of the Red Clay Valley
Rock Type Description
Gneiss A course-grained rock commonly having imperfect, but prominent light-dark layering. In the Delaware Piedmont the light layers are composed of feldspars and quartz and the dark layers of mica, garnet, sillimanite, amphiboles, and pyroxenes.Gneisses are formed by the high-grade metamorphism of either igneous or sedimentary rocks.
Schist A sharply layered, commonly crinkle-folded rock, that can easily split into flakes or slabs due to a well developed parallelism of platy minerals such as micas or amphiboles. Schists commonly form by the medium-grade metamorphism of igneous and sedimentary rocks.
Amphibolite A rock composed primarily of amphibole and feldspar. The amphibole grains are commonly elongated with long axes parallel. In the Delaware Piedmont most amphibolites are formed by the metamorphism of igneous rocks.
Serpentinite A greenish-yellow, greasy soft rock composed essentially of the mineral serpentine. It may be soft enough to carve with a pocketknife. Serpentenites are formed by the metamorphism of ultramafic (iron-magnesium rich) rocks. Ultramafics originate deep in oceanic crust and occur on land only as slivers of rock that have been thrust faulted onto the continental margin.
Quartzite A massive rock composed essentially of interlocking quartz grains. Quartzites are formed by metamorphism of sand or sandstone.
Vein Quartz A rock composed of sutured quartz crystals that formed by precipitation from a solution or melt. In the Piedmont vein quartz commonly fills ancient fractures.
Marble A massive, coarse-grained sparkling blue-white rock composed mostly of calcite and/or dolomite. Marble forms by the metamorphism of limestone.

Sedimentary Rocks

Sedimentary rocks are made up of the debris from weathering and erosion of rocks, from chemical precipitates, or from the remains of living things. Most sedimentary rocks are formed from particles of older rocks that are carried by rivers and streams to lakes or oceans where they are deposited, deeply buried, and then consolidated into solid rock. They cover most of the ocean floor and three-quarters of the land. The most common solid sedimentary rocks are shale, sandstone, conglomerate, and limestone.

The only sedimentary rocks in Delaware’s Piedmont are modern sediments (sand, silt, and clay) that are being eroded, transported, and deposited in the local streams as the rocks within the watersheds weather and erode. The Piedmont has been a source of sediment that is deposited elsewhere, and has been for a long part of geologic time.



At some time almost everyone has picked up and examined a rock. It may have been round and smooth and you liked the way it felt; it may have been just the right size to skip across a pond; or it may have been beautiful or unusual. Whatever your reason for picking up a rock, we hope you observed that it was made up of many small individual grains. These small grains are minerals. Most common everyday rocks, such as granite, slate, or gneiss, are made up of several different minerals, but it is possible for a rock, such as quartzite, to be composed of only one mineral. The dictionary broadly defines a mineral as a naturally occurring solid with a definite chemical composition and an ordered (crystalline) atomic arrangement.

Minerals can form in many ways, such as crystallization from a lava or magma, by recrystallization when a rock is heated or compressed, or by precipitation from water. Usually new minerals crystallize in a medium where they are competing for space with other minerals that are forming at the same time, and they end up as a maze of interlocking grains. However, if the minerals are allowed to crystallize without competition, such as in water or molten magma, the minerals will crystallize into geometric shapes that are strikingly beautiful and often valued by collectors. There are thousands of different minerals that form in the Earth, but only a few are found in the Red Clay Valley.

Common Minerals of the Red Clay Valley
Rock Type Description
Quartz A glassy, transparent to translucent mineral that breaks and fractures like glass. Its color is usually white to gray. Quartz is present in almost all Piedmont rocks.
Feldspar In weathered rocks or granitic pegmatites, feldspars occur as milky white or pink porcelain-like minerals that often break into rectangular shapes with shiny flat surfaces. In fresh, unweathered amphilbolites or gneisses, the feldspars are glassy and transparent. In the 18th and 19th centuries, feldspar was quarried in the Red Clay Valley for use in porcelain, china, and glazes. Orthoclase and plagioclase are two types of feldspar found in the Delaware Piedmont.
Mica A mineral with perfect basal cleavage that easily separates into sheets. The varieties are black biotite, white muscovite, bronze phlogopite, and green chlorite. Micas are common in all Piedmont rocks except the high-grade gneisses of the Wilmington Complex.
Garnet Most Piedmont garnets are a dark-red, iron-rich variety called almandine. They usually occur as 12-sided crystals that vary in size from crystals so small they can be seen only under a microscope to crystals of an inch or more across. Garnets are considered semi-precious stones, but in the highly deformed rocks of the Piedmont they are usually fractured and not suitable for jewelery. Garnet is also used as an abrasive.
Sillimanite Sillimanite, or fibrolite as it is commonly called, occurs as aggregates of thin fibers, nodules, or veins. Its color is either gray blue or dull white. It is a high-grade metamorphic mineral that occurs in the gneisses and granitic pegmatites. Sillimanite is the Delaware State Mineral.
Calcite and Dolomite The major minerals in marble. In the Delaware Piedmont they occur in the Cockeysville Marble as blue-white, coarsely crystalline interlocking grains. Years ago the marble was quarried, converted into quick lime, and used as a soil conditioner.
Serpentine A secondary mineral that forms by the alteration of magnesium-rich minerals. Serpentines are always shades of green, they are soft, and have a slightly soapy or greasy feel
Amphibole A large family of minerals. In the Delaware Piedmont, they are usually black or dark green. Amphiboles usually have one good cleavage that will sparkle on fresh surfaces. Arock containing around 50% or more amphibole is called an amphibolite.
Pryoxene A group of dark minerals that are common in the Piedmont rocks. They usually occur as interlocking grains in the highest-grade gneisses, amphibolites, and gabbros.