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 Most debris fell back into the crater, but some "tektites" (tiny glass beads) and "shocked quartz" (which has unique physical properties identifiable with an optical microscope) were scattered as far as the Continental Shelf off the coast of New Jersey.
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. 35 million years ago, the impact location was in the ocean off the eastern edge of Virginia, 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, as the Appalachian Mountains have eroded and the Coastal Plain sediments extended eastward, the impact crater was buried under 1,500 feet of gravel, sand, silt, and clay.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 granitic 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 900 feet of granite, they broke through the granite and encountered the expected breccia, or broken pieces of bedrock mixed with sediments.6 It appears the bolide tossed a big chunk of granite into the air at impact, and the geologists had the misfortune to pick that location for their drilling project.
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.
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 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.
References1. "Chesapeake Bay Crater Offers Clues to Ancient Cataclysm," National Geographic, November 13, 2001, http://news.nationalgeographic.com/news/2001/11/1113_chesapeakcrater.html (last checked August 20, 2011)
2. "Drill explores blast: Research seeks insight into explosion that carved huge crater under the Chesapeake," Richmond Times Dispatch, September 8, 2005, (no longer available online)
3. "Geologic Columns for the ICDP-USGS Eyreville A and C cores, Chesapeake Bay impact structure: Postimpact sediments, 444 to 0 m depth," in The ICDP-USGS Deep Drilling Project in the Cheesapeake Bay Impact Structure: Results from the Eyreville Core Holes, Special Paper 458, The Geological Society of America, 2009
4. "The Chesapeake Bay Bolide Impact: A New View of Coastal Plain Evolution," US Geological Survey Fact Sheet 049-98, 1998, http://marine.er.usgs.gov/fact-sheets/fs49-98/ (last checked August 390, 2012)
5."Site report for USGS test holes drilled at Cape Charles, Northampton County, Virginia, in 2004," Open File Report 2007–1094, p.12
6. "Chesapeake Bay Impact Structure Deep Drilling Project Completes Coring," Scientific Drilling, No. 3, September 2006, http://rockbox.rutgers.edu/p38-41_Chesapeake.pdf (last checked September 8, 2008)
7. "City of Poquoson Comprehensive Plan 2008-2028," CHAPTER 5- NATURAL RESOURCES ELEMENT, Section III: Ground Water Sub-element, p.5-42, http://www.ci.poquoson.va.us/sites/default/files/CompPlan03.22.10_0.pdf (last checked January 1, 2013)