About 35 million years ago, a "bolide" (meteor or comet) about 1-3 miles wide slammed into the earth at a location that is now the southern tip of the Eastern Shore. The impact cracked the crust of the earth as deep as 7 miles. The bolide also blasted a crater 85 miles wide, creating a flash of evaporating ocean water and a plume of ejected bedrock that may have risen in a towering cloud 30 miles high.1
When the bolide hit, there was no Chesapeake Bay or Eastern Shore. The sediments that form the Delmarva Peninsula were deposited much later, and the Chesapeake Bay itself did not form until after the Wisconsin glaciation ice sheet melted 18,000 years ago. The impact location 35 million years ago was in the Atlantic Ocean off the eastern edge of Virginia, when sea levels were higher and the shoreline was roughly the path of I-95 today.
As described in a story in the Richmond Times Dispatch:2
Over the last 35 million years, erosion has deposited sediments on top of the crater, and shifts in the path of the Susquehanna River have formed the peninsula of the Eastern Shore. Today, the impact crater is buried under 1,500 feet of gravel, sand, silt, and clay, with the center of the crater underneath the Delmarva peninsula.3
The zone of weakened rock may have shaped the direction of the Susquehanna River and James River as they carved their channels to the Atlantic Ocean, ultimately affecting the location of the Chesapeake Bay. As noted by the Chesapeake Bay Impact Crater geologists:4
However, the bolide did NOT blast out the current basin of the Chesapeake Bay; the Bay is NOT simply water filling a hole carved by a bolide. The impact was 35 million years ago, while the bay formed much more recently - over just the last 10,000 or so years, as sea levels rose and flooded the valleys of the Susquehanna and James rivers. The location of those river channels (and the modern Chesapeake Bay) appears to be affected by the much-older depression created initially by the bolide, followed by subsidence for the last 35 million years. Even today, sediments deposited in the crater continue to subside and maintain a continuous low spot in the crust of the earth.
Far more than the earth's surface above the crater, groundwater today is affected dramatically by the ancient impact. The U.S. Geological Survey drilled a test hole 2,699 feet deep into the impact crater near Cape Charles, Virginia, during May/June 2004. Ground-water salinity in the well was saltier than sea water, reaching 40 parts per thousand.5 (Sea water is roughly 35 parts of salt per thousand parts of water.)
Between September 2005 and May 2006, the Chesapeake Bay Impact Structure Deep Drilling Project drilled again into the crater at a location about 4 miles north of the town of Cape Charles, at Eyreville Farm. The geologists drilled through post-impact sediments (those less than 35 million years old), then through the "breccia" (shattered rock created when the bolide smashed through the water column and into the ocean bottom), and finally reached the crystalline basement rocks more than one mile below the surface.
The deepest of three wells dug there reached 1,766 meters (5,794 feet, over a mile deep into the ground). They ran into granite rock at a shallow depth, greatly complicating what they expected to be drilling through soft sediments of the Coastal Plain. After deciding that the hole was too shallow to have struck bedrock below the sediments, they kept drilling. After drilling through 900 feet of granite, they broke through it and encountered the expected breccia, the broken pieces of bedrock mixed with sediments.6
It appears the bolide tossed a big chunk of granite into the air at impact, 35 million years ago. The geologists had the misfortune to pick the location where that boulder had landed for their drilling project.
how the impact fractured the bedrock
Source: USGS Fact Sheet 049-98:
The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution
The dramatic disruption of the bedrock layers "entrained" hypersaline water in some places underneath what is now the Eastern Shore and Hampton Roads. As the debris fell back from the impact, it sealed off aquifers where the heat of the impact had evaporated some of the seawater, trapping extra-salty pools of water underground. The "inland salt-water wedge" of unusually salty groundwater in the crater basin has a thin lens of freshwater on top, in the sediments deposited during the time after the impact.
aquifers that could supply groundwater in Newport News area
Source: City of Poquoson Comprehensive Plan 2008-2028
The Hampton Roads region is limited to extracting a limited amount of fresh water from the more-recent sediments deposited over the last 35 million years. The disruption of the bedrock layers has greatly complicated efforts of Newport News to find a reliable supply of fresh drinking water. The city plans for substantial growth in demand for fresh water, but is unable to extract a sufficient supply from groundwater. The City of Poquoson gets its fresh water from Newport News, and notes in their Comprehensive Plan how the Lower, Middle, and Upper Potomac aquifers were affected:7
The James River runs right next to Newport News, so there is plenty of water - but at that location in Tidewater, the river is brackish. It is too salty to drink without expensive treatment. As a result, Newport News tried to build a surface reservoir in King William County, damming Cohoke Creek and (since the creek's watershed is so small) pumping fresh water from the Mattaponi River into that reservoir. Ultimately, after great political debate and multiple lawsuits, the proposed King William Reservoir was blocked by the Federal government.
The bolide that impacted 35 million years ago affected just the eastern edge of Virginia. In the last million or so years, meteor or comet fragments could have created Lake Drummond and other crater-like features on the Coastal Plain of southeastern Virginia.
A different bolide that struct 65 million years ago created the Chicxulub Crater on the Yucatan Peninsula of Mexico. That larger bolide had a world-wide effect, triggering the extinction of dinosaurs and the end of the Cretaceous Period.
a bolide struck over 300 million years ago near modern-day Cumberland Gap, and the town of Middlesboro (Kentucky) developed in that crater
Source: US Geological Survey (USGS), Geologic map of the Middlesboro and part of the Bristol 30 x 60 minute quadrangles, southeastern Kentucky (2004)
deep geologic structure of eastern Virginia
Source: US Geological Survey (USGS) - Professional Paper 1680,
A Surficial Hydrogeologic Framework for the Mid-Atlantic Coastal Plain (Plate 2)
Chesapeake Bay Geology and Sea Level Rise
Groundwater in Virginia
King William Reservoir