Glaciers and Ice Ages in Virginia

the ice sheet stopped in Pennsylvania 18,000 years ago at the peak of the Wisconsin glaciation
the ice sheet stopped in Pennsylvania 18,000 years ago at the peak of the Wisconsin glaciation
Source: ESRI, ArcGIS Online

A series of "ice ages" started 2.6 million years ago. The ice ages could have been triggered by the closing of the Panama Seaway and alterations in the salinity in different parts of the oceans, changes in the earth's orbit, large-scale changes in ocean currents and the East Asian Monsoon, and/or lower carbon dioxide levels in the atmosphere caused by the weathering of the Himalayas.

Four times major ice sheets grew southward across North America, then retreated. The last, the Wisconsin stage, started about 75,000 years ago. That continental ice sheet has been retreating for the last 18,000 years.1

The Laurentide Ice Sheet grew during the Wisconsin stage. It stopped north of the Ohio River and the Pennsylvania Turnpike, and at modern New York City. During the furthest extent of the ice that ended 18,000 years ago, Virginia was never buried underneath a thick layer of ice.

18,000 years ago, a glacial ice sheet stretched south into what is now Pennsylvania
18,000 years ago, a glacial ice sheet stretched south into what is now Pennsylvania
Source: Dr. Ron Blakey, Paleogeography and Geologic History of North America

There may have been a small mountaintop glacier on Mount Rodgers, the highest peak in Virginia at 5,729 feet, but no evidence of glaciation remains. Peaks in Virginia may have been covered with snow during the winter, but wind and sun removed the snow before it was compressed to form glaciers. Mount Mitchell in North Carolina is 6,684 feet high, and in the 1970's there was a claim of glacial scratches on the bedrock of that peak. However, later investigation revealed that cables used in logging operations had carved the grooves.2

Virginia still experienced significant impacts from the cold climate in the last Ice Age. The Laurentide Ice Sheet altered the location of the Atlantic Ocean shoreline. At the start of the last glacial period 125,000 years ago, sea level was about 15' higher than today. Sea level dropped 400' as water was trapped in the ice sheet. The Continental Shelf was exposed, and the shoreline was as much as 75 miles east of modern Virginia Beach.

The winds, rains, and temperature of Virginia's paleoclimate were affected significantly by the presence of the Laurentide Ice Sheet. Cold dry winds blasted south, bring only 25" of moisture annually compared to over 40" today. The wind may have created Carolina bays on the sandy Coastal Plain, including perghaps the first stage of Lake Drummond in the Great Dismal Swamp. The pocosins of today in the Hampton Roads area, the wetland bogs with woody shrubs growing on sandy peat soil, may owe their origin to an ice sheet far away in Pennsylvania.

Paleo-Indians sheltered on the sandy hills along the Nottoway River at Cactus Hill and Blueberry Hill to avoid the winds. The earliest humans in Virginia preferred north-facing slopes even though the sun was warmer on south-facing slopes, because the winds swept away the biting insects in the river valley. The options for where to stay on hunting and gathering trips increased as the climate warmed, except during the Younger Dryas.

In the Younger Dryas period 14,500-11,500 years ago, temperatures dropped again to near-glacial conditions. Ocean currents bringing warm water from the tropics along the Atlantic Ocean coastline may have been disrupted by the shift of cold meltwater that had been flowing from the edge of the ice sheet down the Mississippi River. Melting of the ice may have opened up a path through the St. Lawrence River valley, blocking the currents that were bringing warmth to Virginia.3

The ice was 2,000' thick on top of modern Manhattan Island, taller than the Freedom Tower. The edge of the ice sheet stayed at the same location long enough for meltwater to deposit a massive amount of debris scraped up by the glacier, forming Long Island. As the ice sheet melted and then retreated, much of that glacial sand was swept down the Hudson and Susquehanna rivers to form te Eastern Shore.4

The weight on the ice sheet pressed down on the bedrock, depressing the land into southern Pennsylvania. The land further south flexed in response to the pressure and bent upwards.

As a result, at the peak of the Ice Age the land along Virginia's coastline was raised up. Sea level was already as much as 400 meters lower because so much water was trapped in the glaciers, and the eastern coastline of Virginia was as much as 40 miles east of the current location. The flexing earth raised the Virginia coastline even higher and extended the Coastal Plain even further to the east.

Today, the ice sheet has retreated far north and the depressed bedrock in Pennsylvania is rebounding upwards. The area along the current Atlantic Ocean shoreline, the crust that was uplifted in response to the nearby ice, now is sinking back down. That adjustment, combined with global warming, is causing faster-than-average sea level rise along Virginia's Atlantic Ocean shoreline and in the Chesapeake Bay.5

In the Valley and Ridge province, blocks of stone cracked loose from the bedrock on the sandstone ridges, slid downhill, and created piles of boulders in a talus slope below the quartzite bedrock. When the climate warmed, the hillsides stabilized and the forests returned. Trees grew below the talus pile and above their eroding bedrock source, but not on the barren boulders themselves.

The freezing and thawing cycle was less intense, however, and few new rocks were added to the talus in the last 10,000 or so years. The pile of talus rock slid downhill under the force of gravity, but new talus was no longer added regularly on the uphill edge of the talus pile. Over centuries, a gap developed between the source bedrock and the talus pile, and trees grew in that gap. The isolated block fields - below the trees and now separated from the layer of sandstone that eroded to form the original talus pile - stand out against the forested mountainsides today, remnants of an ancient climate.6

"Block fields" are still visible on the western slope of Massanutten Mountain. During the last glacial period, cold winds blew across the mountains and little vegetation could grow there. Cracks in the sandstone bedrock widened as the moisture in them froze. The ice froze and melted repeatedly, creating The Channels on Clinch Mountain at the border of Russell and Washington counties.7

the ice sheet that stopped in Pennsylvania affected the creation of The Channels over 200 miles away
the ice sheet that stopped in Pennsylvania affected the creation of The Channels over 200 miles away
Source: ESRI, ArcGIS Online

Glaciers did not carve the landscape in southwestern Virginia, but the Ice Age climate (as well as earthquakes) may have been a factor in creating the largest known landslides in eastern North America. For 20 miles, land has slumped along the eastern slope of Sinking Creek Mountain west of modern-day Blacksburg.

When the climate was colder, vegetation was less and erosion was evidently higher. Soil washed away at the base of rock layers on the mountain, and earthquakes may have triggered a series of massive landslides until 10,000 year ago.8

Icebergs did float past the coastline of Virginia about 31,000 years ago. A pulse of cold water escaped from the edge of the ice sheet near Hudson Bay. Barriers of ice which had dammed up lakes melted during different short-term warming periods and a flood of water/ice was released, similar to the way the Channeled Scablands in the State of Washington were created. Another possibility is that the high rate of icebergs calving off the front of the ice sheet was triggered by intermittent advances and retreats of the ice sheet. Those shifts could have been caused by warming of the ocean at the ice sheet edge, due to disruptions in the flow patterns of the Atlantic meridional overturning circulation (AMOC) and North Atlantic deep water (NADW).

Six "Heinrich Events" during the Wisconsin stage have been identified by layers of ice rafted debris. On an average of every 5,000–6,000 years, these Heinrich events lasted about 500 years. The first Paleo-Indians camping on Virginia's Atlantic Ocean shoreline would have seen icebergs stranded and floating offshore 18,000-15,000 years ago during Heinrich Event 1.

The current of cold water which flowed south was powerful enough to counter the north-flowing Gulf Stream, and carry glaciers as far as the Florida Keys. There are 700 lines today that scour the bottom of the Atlantic Ocean off the East Coast, showing where the current dragged glaciers across the seafloor.

Today the scour lines are 557 feet-1,246 feet deep, but the glaciers did not have to be large enough for their keel to reach those depths. At the time the glaciers flowed south, water levels were roughly 400 feet lower. The icebergs dug trenches into the sediments as much as 60 feet deep. That depth suggests some icebergs plowed into ridges on the ocean bottom and became stuck in place, then melted partially and floated away.9

The ice did not etch out the Chesapeake Bay, but expansion and melting of the ice sheet over 400,000 years did cause its formation. The climate warmed and cooled in stages. During warm periods, rivers and ocean currents brought sediments south from Pennsylvania and New York. During periods when the climate cooled again, the sea level dropped and islands of deposited sediments were exposed on the ocean surface.

The energy of water flowing in the Susquehanna River diminished and sea level dropped in the cold periods. Instead of eroding the islands, the Susquehanna River shifted further south to reach the Atlantic Ocean.

Cycles of rising and falling sea levels created a peninsula of land separating the Susquehanna River from the ocean, today's Eastern Shore. Sea level has risen since the end of the last cold stage of the Ice Age, flooding the southern end of the Susquehanna River and forcing Native Americans to move their campsites 40 miles or more to the west as the Continental Shelf was gradually flooded. The drowned Susquehanna River Valley, the Chesapeake Bay, reached its current form about 3,000 years ago.10

Warmer, wetter climate led to a transformation of the pattern of plants and animals in Virginia. Species such as spruce and fir trees that prospered in cold, dry settings disappeared, or managed to survive primarily on the mountain ridges. The Eastern deciduous forest expanded from refugia in stream valleys, Oaks, hickories, and other broad-leaved plants expanded into the lands once occupied by a:11

...park-like forest of spruce, fir, jack pine, alder, and birch spaced far apart, with a carpet of grasses and scattered woody shrubs underneath.,/dd>

Chesapeake Bay Geology and Sea Level Rise

Climate of Virginia

How Will the Environment of Virginia Change?

Virginia and the "Snowball Earth/Popsicle Planet" Theory

Virginia Ecosystems

The Channels was created when joints and fractures in the sandstone at the top of Clinch Mountain were widened by the freezing and thawing of ice during the Pleistocene The Channels was created when joints and fractures in the sandstone at the top of Clinch Mountain were widened by the freezing and thawing of ice during the Pleistocene
"The Channels" was created when joints and fractures in the sandstone at the top of Clinch Mountain were widened by the freezing and thawing of ice during the Pleistocene

Links

References

1. "Glacial Geology - Ice Ages in the Geological Record," Indiana Geological and Water Survey, https://igws.indiana.edu/MarionCounty/IceAgeGeoRecord.cfm; "New theory on cause of ice age 2.6 million years ago," Science Daily, June 27, 2014, https://www.sciencedaily.com/releases/2014/06/140627094404.htm; "The history of ice on Earth," New Scientist, May 24, 2010, https://www.newscientist.com/article/dn18949-the-history-of-ice-on-earth/; Y. J. Wang,, H. Cheng, R. L. Edwards, Z. S. An, J. Y. Wu, C.-C. Shen, J. A. Dorale, "A High-Resolution Absolute-Dated Late Pleistocene Monsoon Record from Hulu Cave, China," Science, Volume 294, Issue 5550 (December 14, 2001), http://doi.org/10.1126/science.1064618 (last checked June 8, 2018)
2. John B. McKeon, John T. Hack, Wayne L. Newell, James O. Berkland and Loren A. Raymond, "North Carolina Glacier: Evidence Disputed," Science, New Series, Volume 184, Number 4132 (April 5, 1974), p.89, http://www.jstor.org/stable/1737466 (last checked April 19, 2017)
3. "Virginia Wasn't Recognizable During Ice Age," Daily Press, December 31, 2000, http://articles.dailypress.com/2000-12-31/features/0012280150_1_ice-sheets-ice-age-massive-ice; Michael F. Johnson, "Cactus Hill, Rubis-Pearsall and Blueberry Hill: one is an accident; two is a coincidence; three is a pattern - predicting "old dirt" in the Nottoway river valley of Southeastern Virginia, U.S.A.," PhD thesis, University of Exeter, 2012, p.60, https://ore.exeter.ac.uk/repository/handle/10871/9541; "Sea Level Rise, After the Ice Melted and Today," National Aeronautics and Space Administration, January 2007, https://www.giss.nasa.gov/research/briefs/gornitz_09/; Kurt M. Cuffey, Shawn J. Marshall, "Substantial contribution to sea-level rise during the last interglacial from the Greenland ice sheet," Nature, Volume 404 (April 6, 2000), http://doi.org/10.1038/35007053; "The Younger Dryas," National Oceanic and Atmospheric Administration (NOAA), https://www.ncdc.noaa.gov/abrupt-climate-change/The%20Younger%20Dryas (last checked September 19, 2021)
4. "How the Ice Age Shaped New York," New York Times, June 5, 2018, https://www.nytimes.com/2018/06/05/science/how-the-ice-age-shaped-new-york.html; "What is a pocosin?," National Oceanic and Atmospheric Administration (NOAA), https://oceanservice.noaa.gov/facts/pocosin.html (last checked September 19, 2021)
5. "Glacial Rebound: The Not So Solid Earth," National Aeronautics and Space Administration (NASA), August 26, 2015, https://www.nasa.gov/feature/goddard/glacial-rebound-the-not-so-solid-earth; "Sinking Atlantic Coastline Meets Rapidly Rising Seas," Scientific American, April 14, 2016, https://www.scientificamerican.com/article/sinking-atlantic-coastline-meets-rapidly-rising-seas/ (last checked June 8, 2018)
6. "Glimpses of the Ice Age from I-81," US Geological Survey, 1998, http://pubs.usgs.gov/gip/i81/i81.pdf (last checked February 19, 2015)
7. "Channels State Forest," Virginia Department of Forestry, http://www.dof.virginia.gov/infopubs/_state-forests/CHSF-2013-05_pub.pdf (last checked June 8, 2018)
8. "The Mountain that Moved - Ancient Giant Landslides," US Geological Survey, 2000, http://pubs.usgs.gov/gip/mountain/mountain.pdf (last checked June 8, 2018)
9. "Icebergs as tall as Eiffel Tower once flowed past Outer Banks to Florida, study finds," Miami Herald, June 28, 2021, https://www.miamiherald.com/news/state/florida/article252231868.html; Alan Condron, Jenna C. Hill, "Timing of iceberg scours and massive ice-rafting events in the subtropical North Atlantic," Nature Communications, Volume 12, Article 3668 (June 16, 2021), https://doi.org/10.1038/s41467-021-23924-0; "Heinrich Event," Science Direct, https://www.sciencedirect.com/topics/earth-and-planetary-sciences/heinrich-event; J. Alvarez-Solas, M. Montoya, C. Ritz, G. Ramstein, S. Charbit, C. Dumas, K. Nisancioglu, T. Dokken, and A. Ganopolski, "Heinrich event 1: an example of dynamical ice-sheet reaction to oceanic changes," Climate of the Past, Volume 7, 1297–1306 (2011), https://doi.org/10.5194/cp-7-1297-2011 (last checked June 30, 2021)
10. C. H. Hobbs, "Geological history of Chesapeake Bay, USA," Quaternary Science Reviews, Volume 23, Issues 5-6, (March 2004), p.649, http://dx.doi.org/10.1016/j.quascirev.2003.08.003; "Bay Geology," Chesapeake Bay Program, https://www.chesapeakebay.net/discover/ecosystem/bay_geology (last checked September 19, 2021)
11. "Virginia Wasn't Recognizable During Ice Age," Daily Press, December 31, 2000, http://articles.dailypress.com/2000-12-31/features/0012280150_1_ice-sheets-ice-age-massive-ice (last checked September 19, 2021)

glacial advance
glacial advance... to Pittsburgh, but not into modern-day Virginia
Source: US Geological Survey, National Atlas


Rocks and Ridges - The Geology of Virginia
Virginia Places