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Exploring the Fossils of Delaware USA

The U.S State of Delaware may well be the second-smallest state by area in North America but its geodiversity dates back over one billion years to the Grenville orogeny. Known for the Horseshoe Crab a living fossil the state has the Belemnite an extinct mollusc related to squid as the official state fossil and in 2022 designated the Dryptosauridae a relative to the Tyrannosaurus rex as the state dinosaur.

Fossilised Belemnite Rostra
Fossilised Belemnite Rostra - Image by Yuelan

Brief History of Delaware


Delaware was one of the 13 original states of the New England Colonies including New Hampshire, Massachusetts, Rhode Island, New York, New Jersey, Pennsylvania, Connecticut, Maryland, Virginia, North Carolina, South Carolina and Georgia. The first European settlers of Delaware were from Sweden who established the colony of New Sweden at Fort Christina in present-day Wilmington in 1638.



Delaware was nicknamed “The Diamond State” because Thomas Jefferson referred to it as a “jewel among the states” due to its prime location on the Eastern Seaboard. Also, the state is known as the "Blue Hen State" a nickname given to Delaware after the fighting Blue Hen Cocks that was carried with the Delaware Revolutionary War Soldiers.


Delaware’s Living Fossil


In the shallow sandy coastal waters of Delaware Bay, an estuary outlet of the Delaware River is the habitat and Spring spawning grounds for the Horseshoe Crab (Limulus polyphemus) known as a “living fossil” dating back over 400 million.


Horseshoe Crab (Limulus polyphemus) on the shore of Delaware Bay
Horseshoe Crab (Limulus polyphemus) a living fossil on the shore of Delaware Bay

During full moons, new moons, and high tides in May and June Horseshoe Crabs converge in the waters of Delaware Bay especially the protected area of Mispillion Harbour to breed along with migrating birds such as Red Knots.


There are four species of Horseshoe Crab and interestingly they are not true crabs or even crustaceans but are more closely related to arachnids like spiders, ticks and scorpions. The oldest known Horseshoe Crab species (Lunataspis aurora) is estimated to be nearly 450 million years old.



This living fossil has proved to be tremendously important in medicine as its bright blue blood has invaluable antibacterial properties and its immune cells can protect against harmful bacteria. The blood of the Horseshoe Crab has been instrumental in the formulation of many vaccines for over the last 50 years and more recently the battle against COVID-19.


Delaware Fossils and Dinosaurs


Delaware is home to a variety of fossils that are found in the sedimentary rock formations of the state's Atlantic Coastal Plain. These fossils provide important insights into the region's geological history and its past as a shallow marine environment.


Both the Chesapeake and Delaware Canal's are good areas for hunting Upper Cretaceous Period fossils. The sedimentary rocks of these areas show that there were two periods of transgression-regression in sea level that has influenced the diversity of the fossils found.


The fossils include a large clam, oyster, the small echinoid an armoured sea urchin called Boletechinus delawareicus and also "steinkerns". The term steinkern is derived from the German for 'stone core' and are formed when the shell of gastropod or bivalve fills with mud that later hardens leaving a cast once the shell has dissolved.



Along with the teeth, vertebrae and coprolites of sharks and their relatives, the skates and rays one of the more common fossils found is the belemnite. In July 1996 the belemnite species Belemnitella americana was officially named as the official fossil of Delaware.


The dinosaur fossils of Delaware are found in the marine sediments of the Marshalltown and Merchantville Formations excavated from the Delaware and Chesapeake Canals having been originally washed out into Delaware Basin. Fossil fragments of hadrosaurs ‘duck-billed dinosaurs’, Ornithomimidae "bird-mimics" theropod dinosaurs and marine reptiles have been found in Delaware.


The Delaware State Dinosaur is the large 7.5 meter long Dryptosauridae (drip-toh-sor-us) a bird-like predator related to the Tyrannosaurus rex whose bones have been found in the Chesapeake and Delaware Canal. The Dryptosauridae was chosen over the hadrosaur because Hadrosaurus foulkii had already been designated by New Jersey as its official state dinosaur.


The Dryptosauridae lived along the shoreline hunting shore birds. A reconstruction of a dryptosaurid skeleton is on display in the Delaware Museum of Nature and Science and New Jersey State Museum.


Dryptosaurus aquilunguis mounted skeletons, New Jersey State Museum
Dryptosaurus aquilunguis mounted skeletons, New Jersey State Museum

Delaware’s Geodiversity


Delaware's geodiversity is a reflection of its location on the East Coast of the United States within the two physiographic provinces of the Appalachian Piedmont (meaning ‘foothills’) and the larger area of the Atlantic Coastal Plain. Delaware is generally known for its relatively low and flat topography and the absence of fossils on the Appalachian Piedmont.


Only New Castle County in Delaware is located within the Appalachian Piedmont that stretches along the eastern edge of the Appalachian Mountains. The area includes the state’s highest point at Ebright Azimuth at 137 meters on Ebright Road north of Namaans Road, east of route 202 near the Pennsylvania border. Because of the absence of fossils in the Appalachian Piedmont the complexity and age of the geology has been determined from radiometric dating.



The oldest rocks in Delaware pre-date the formation of the Appalachians and date back to the Grenville orogeny a mountain building event named after the village of Grenville in Québec and occurred between 1.3 - 1.0 billion years ago. This event was a result of subduction of the Iapetus Ocean by the collision of Earth’s two of three early continents or cratons named Laurentia and Amazonia.


This orogeny played a crucial role in the formation of the Grenville Province, a vast geological region encompassing parts of North America, including portions of Canada and the north-eastern United States. The collision and compression of these landmasses resulted in the thickening of the Earth's crust and the uplift of mountain ranges.


The combination of compression and heat led to widespread metamorphism of rocks. The Grenville orogeny is considered one of the major orogenic events in Earth's history and a precursor to the start of the Appalachian orogeny during the Precambrian over 500 million years ago and the formation of the Appalachian Piedmont. The rocks of this area are a complex mix of schist, gneiss, slate, and marble metamorphic rocks.


The Appalachian orogeny and the formation of the Appalachian Mountains would continue during the Middle Ordovician with the Taconic orogeny approximately 472 million years ago through to the Acadian orogeny during the Middle to Late Devonian 390 million to 370 million years ago and lastly the Late Carboniferous to Permian Alleghenian orogeny 300 million to 250 million years ago.


The Appalachian orogeny and formation of the Appalachian Mountains that have played a crucial role in shaping the eastern United States and influenced the geology, topography, and natural resources of the region.


The Delaware Fall Line geologically marks the transition both in elevation and rock type between the harder and more ancient rocks of the Appalachian Piedmont and the lower lying porous sediments of the Atlantic Coastal Plain.


The boundary of the Delaware Fall line is often noticeable for its waterfalls and rapids along rivers and streams that can be seen on various waterways in the region including the Brandywine River and the Christina River. The Delaware Fall line historically is where early settlers founded the future cities of Richmond, Washington, Baltimore, Wilmington, and Philadelphia.


Waterfall from the Delaware Fall Line - Image by Aaditya Bhatt
Waterfall from the Delaware Fall Line - Image by Aaditya Bhatt

The Atlantic Coastal Plain of Delaware is made up of sediments of silt, sand, and gravel that have been eroded off the Appalachian Piedmont and adjacent Appalachian Mountains. At the coastline the sediments can be up to 3,000 meters in depth and significantly increases in depth offshore on the continental shelf. Underlying the sediments of the Atlantic Coastal Plain of Delaware is consolidated rock known as the ‘basement’ probably a subsurface extension of the Appalachian Piedmont.


On top of all of Atlantic Coastal Plain of Delaware is a thin layer sand and gravel that was carried into Delaware by glacial outwash from the melt-water pouring off the glacier.


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