Crow's Nest Natural Area Preserve - Geologic History of Crow's Nest

Geologic History of Crow's Nest

The mid-Atlantic Ridge started to form in the early Jurassic period (175 million years ago), breaking apart the super continent Pangaea and beginning the expansion of the modern Atlantic Ocean. It has widened steadily to its present size. This rifting event separated North America from Africa, and the area known today as Virginia became the trailing edge of the newly-formed North American continent. The Jurassic Period is found in the Mesozoic Era, or the Age of the Dinosaurs. The Mesozoic Era spanned a length of time from 251 million years ago to 65.5 million years ago.

In the early Eocene Epoch (55.8 million years ago) the Coastal Plain of Virginia was completely underwater. Sediment accumulating beneath this sea eventually would become the Crow's Nest of today. The Paleocene and the Eocene epochs are found in the Cenozoic Era, or the age of the mammals. The Cenozoic Era spanned a length of time from 65.5 million years ago to the present.

During the Eocene, the Atlantic Ocean was considerably narrower than it is today, making migration relatively easy for marine animals. The climate of Virginia during this period was considerably warmer than it is today. According to extensive records, the Earth experienced its warmest interval of the past 65 million years during the early Eocene, and Virginia was probably much warmer than central and southern Florida today. The remains of large alligators and turtles have been found at this latitude in Virginia.

The calcareous soil strata within Potomac Creek, the Aquia Formation, is 60 million years old. This formation is found within a larger outcrop belt known as the Pamunkey Group, a sequence of Lower Tertiary (Paleocene-Eocene, 55.8 million years ago) sands and clays that formed in shallow marine environments beneath the western margin of the Atlantic Ocean. The Aquia Formation is composed of marine sediments that are dark green to gray-green, argillaceous, highly glauconitic, well sorted, fine-to medium-grained sand with shell beds up to 100 feet (30 m) in depth.