Plenty of geological fun at Cornwallis’ Cave
Deborah Painter (USA)
Cornwallis’ Cave, a feature along the bluffs overlooking the York River in historic Yorktown, Virginia in the USA, is not a real cave and may not even have sheltered British General Charles Cornwallis during the final weeks of the American War of Independence. The National Park Service, which oversees the feature, has little historical evidence that Cornwallis ever used it as a meeting place or as shelter. He probably used a bunker located elsewhere along the river.
It is one of the United States’ best-known man-made ‘caves’ and, though composed of Pliocene epoch coquina – a type of sandstone composed mainly of fossil shells – it is unrelated to actual karst features in the area. This feature is a cultural resource that contains holes carved in the stone cave walls for wooden beams to enable storage of supplies during the later American Civil War and is part of the Colonial National Historical Park encompassing many hectares.
The cultural history
Cornwallis’ Cave is approximately 12.19m in length. It has been sealed off partially by the National Park Service and one can only enter approximately a meter into the cave and view its interior through a wrought iron gate. Were it not for the historic value of the feature and its proximity to the site of testing of mid-nineteenth century hot air balloon warfare, the ‘cave’ might have been levelled long ago. Thankfully, it has not. It is rich in fictional lore, including its reputation for ghosts. A regular night-time ‘ghost tour’ of Yorktown includes a stop at the cave. My friend David Hawk and I saw and heard no ghosts during a recent daytime visit to the cave. However, we were haunted by the spectacle of litter tossed within its dark recesses.
Cornwallis’ Cave is located within the bluff at the southern shoreline of the York River in Virginia, on the opposite bank of Gloucester Point. Fig. 1 illustrates the location of Yorktown and its relation to the final battles between the British and Franco-American forces.
Ironically, there are genuine karst features nearby in York County and in the city of Newport News. These are known as the Grafton Sinkhole Complex, an increasingly uncommon feature now protected partially under United States (Federal) Clean Water Act laws and some Commonwealth of Virginia regulations. They are small basins, often no more than six or seven meters in width and two to three meters in depth, that draw down in dry periods, exposing substrate by the end of the growing season in Virginia’s coastal region. There is usually a shallow organic layer overlying silt or clay loam. The sinkhole ponds are karst features that formed through dissolution of carbonate shell marl deposits of the Miocene/Pliocene epochs (mostly, the Bacons Castle Formation).
Cornwallis’ Cave can be very easily reached on foot. No climbing or caving is needed, nor is it permitted. It is not a site where potentially hazardous geo-adventure is to be had. However, this should not be a detriment to one’s enjoyment of it, especially if one is short on time, is in the area, and wants a geologically interesting locale that will not take more than an hour or two to explore. Literally a few metres from “Riverwalk”, a pedestrian stretch of Water Street at the historic themed beachfront, the cave is located at the face of a bluff managed by the National Park Service and is part of the Colonial National Historical Park. The feature’s coordinates are 37 degrees 14’ 6.92” N, 76 degrees, 30’ 18.70” W.
A free, small, two-level parking garage is near and after a walk of approximately 0.6km, one is there. Figs. 2 and 3 are maps of the Riverwalk and related sites.
A trolley bus sometimes makes runs (Fig. 4). Immediately west of the cave is a small historic home and outbuildings, identified as “An Archer House” (Fig. 5). This site is believed to have been one of the homes of former Virginia senator Thomas Archer, Sr and was built by either him or his father, Abraham Archer, prior to 1750.
No admission price is needed to see the cave, since no admission is needed to visit any of the attractions at the York River waterfront (Figs. 6 and 7). During the COVID-19 pandemic, some restrictions were in effect at the beachfront at the time of our visit in late May of 2020. No public use of the sandy beach was allowed and only a few restaurants with outdoor seating were open. Tours of the interiors of historic buildings were not being conducted.
It is unclear whether the cave was excavated before the American Revolution, but it is believed that munitions may have been stored here at that time to protect them from the elements. The tobacco and slave ship port had been in existence since the late 1600s and featured a church, taverns, a medical shop and a few homes during the Colonial period.
Some residents are reported to have sought shelter in Cornwallis’ Cave during the bombardments of the 1781 siege. The siege at Yorktown was the deciding battle of the American Revolution. Cut off from the ocean by French ships in Chesapeake Bay, Lord Cornwallis fought the American and French in the Bay, returned to Yorktown, and cut the trees to build earthworks to surround the port. Cornwallis crossed the York River with some men and, being informed by British arriving in a small boat that British ships were en route, felt it was advantageous to remain at Yorktown.
However, dysentery ravaged the town. The British engaged the French and American forces for months. Overwhelmed by the Franco-American numbers, both from the Bay and an October bombardment from the land side, Cornwallis found his troubles compounded by a savage storm, possibly a late-season hurricane. It kept him from crossing the York to Gloucester Point a second time. General Cornwallis surrendered to American General George Washington and French General Rochambeau on 19 October 1781. There would still be skirmishes and engagements here and there in the Colonies, but this surrender marked the official end to the war. By 1848, “Cornwallis’ Cave” was the name given to the cave and it became a commercial attraction.
In fact, the small town played a vital role over an 87-year span in two wars, the American Revolution and the American Civil War. The shoreline area was used by the Confederate troops to store munitions and serve them as a general staging area during the early 1860s. Five square depressions were dug in the outer wall of the Cave to insert timbers covered with planks (Fig. 8). These were then covered up in clay to deny any Union gunboats a view of the cave, employed as a place to store gunpowder.
Confederate General Magruder, and later General Johnston, built up the Confederate defences. The Library of Congress possesses several good photographs of this beachfront operation and of the cave. Brigadier General Fitz-John Porter was in charge of siege operations for the Union army. Along with Professor Thaddeus Lowe, he made balloon observation flights in the Constitution and the Intrepid over Confederate lines in April and May 1862, at the same elevation of shrapnel from ground artillery. The men and the balloon were not hit, and Professor Lowe continued his flights. The Confederates had their own hydrogen hot air balloons for gathering intelligence concerning the Union troops. The balloons were large targets, but amazingly, none of them were ever struck.
The Siege at Yorktown was long, but, around midnight on 3 May 1862, the Confederate heavy guns ceased firing, were spiked and left behind by the retreating army. However, the Confederates had left behind a new weapon of war — land mines, which claimed the lives of several Union soldiers, months after hostilities had ceased. The Union army continued the use of the cave as a powder magazine.
After the war, Cornwallis’ Cave was privately owned and used as a potato storage area. Tours were given for ten cents during the Jamestown Exposition of 1907 that celebrated the 300th anniversary of the founding of Jamestown in Virginia. In 1930, the National Park Service acquired a large portion of the land near Yorktown and established the Colonial National Historical Park.
The geology
It is time to delve into the natural history of the cave now we have looked at its cultural history. The Yorktown Battlefield, where the decisive battle of the Revolution was fought, is landward of the bluff where the cave is located. Its geologic units are the upper Pliocene Bacons Castle and the Pleistocene Windsor Formation. The cave is part of the lower Pliocene epoch Yorktown Formation, which is one of the predominant formations throughout the eastern Virginia area. The shoreline erosion has exposed this older Formation. The Yorktown Formation consists of shelly layers and sands, and dates between four and five million years BCE.
The Atlantic reached the Fall Line that extends north to south in Virginia, bisecting the city of Richmond. The Fall Line is the demarcation between the Piedmont and the Coastal Plain in Virginia. It is where metamorphic and igneous bedrock meets the sandy outwash plain. The Atlantic waters of the early Pliocene were warmer than today. Scaphopods, corals, sponges, marine worms, pelecypods (Pecten clintonius), gastropods (Turritella alticostata), crustaceans and varieties of barnacles are common invertebrate fossils. The cave owes its structural integrity to being composed of cross-bedded coquina, an indurated biofragmental sandstone that has been cemented by salt spray from the York River. Ninety percent of the coquina is carbonate, and the rest is clay minerals, quartz and iron oxides (Fig. 9).
Elsewhere along the bluffs are silty sand facies and shelly sand facies distinguished by bluish grey quartzose sand. The area of Cornwallis’ Cave sets itself apart from other facies by the large-scale cross-stratification. The cross beds dip to the northwest at 24 degrees (Fig. 10). Some trace fossils of crustacean burrows cut through the beds (Fig. 11). These are important in understanding the inclined beds and how they were deposited.
Similar deposits have been discovered by geologists W B Rogers, J R Bowman and others in bore holes to the southwest of the cave, within the Colonial National Historical Park, as well as in the Chuckatuck area of Suffolk, approximately 53km to the south.
The question you may be asking yourself now is: Why are the cross beds at Cornwallis’ Cave inclined? Were they not laid horizontally in the Atlantic Ocean? There are two hypotheses:
- One is that the layers of shelly material accumulated horizontally and then were tilted by tectonic activity. Much of Colonial National Historic Park is located on a subsiding side of a rotating fault-bounded block (Johnson et al, 2001).
- Another hypothesis presents the idea that the strata were deposited on an incline. On modern submarine shoals, one may see an incline of nearly 30 degrees. At Cornwallis’ Cave, the fossil burrows seen are trace fossils left by decapod crustaceans (Ophiomorpha) of several species. These burrows are nearly vertical. Since the animals made the burrows when the sediments were laid, this is good evidence that the strata were deposited on a slope. The burrows would be tilted along with the strata if tectonic activity had inclined the sediment strata in this portion of the Yorktown Formation.
More protective measures have been needed for some time at Yorktown and nearby. Hurricane Isabel and Tropical Storm Isaias are recent storms that caused more erosion, mainly due to fallen trees at the bluff. Breakwaters were constructed years ago against natural forces of longshore transport, but have not stopped the relentless and inevitable pounding of the beachfront (Fig. 12).
The Virginia Institute of Marine Science (visible at the foot of the York River Bridge in the distance in Fig. 13) is finalising a plan to protect the estuarine York River shoreline.
About the author
Deborah Painter is an ecologist and general environmental scientist specialising in defence facility and industrial development planning to minimise deleterious environmental impacts. She lives in the United States.
References
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