Geology of Cumberland Gap

bedrock at Cumberland Gap consists of sedimentary layers, disrupted by thrust faulting and even a meteorite impact
bedrock at Cumberland Gap consists of sedimentary layers, disrupted by thrust faulting and even a meteorite impact
bedrock at Cumberland Gap consists of sedimentary layers, disrupted by thrust faulting and even a meteorite impact
Source: Kentucky Geological Survey, Geology of Cumberland Gap National Historical Park

Cumberland Gap is a crack in Cumberland Mountain. The bottom of the gap, at 1600 feet in elevation, is 400 feet lower than the ridge top to the south, including the summit of Tri-State Peak.1 "7.5-minute topographic map for Middlesboro South, KY-TN-VA," US Geological Survey (USGS), 2016, https://prd-tnm.s3.amazonaws.com/StagedProducts/Maps/USTopo/PDF/KY/KY_Middlesboro_South_20160324_TM_geo.pdf (last checked May 8, 2018)

The bedrock consists of sedimentary layers deposited between 416-300 million years ago, when the area was at the edge of the continental craton. Limestones accumulated below the Sauk Sea and Tippecanoe Sea, while shales and sandstones were deposited in foreland basins during the Taconic and Neo-Acadian orogenies. The flat-lying ediments were thrust westward and tilted 40° to the northwest during the Alleghanian Orogeny.

In the course of the trusting and tilting, the Pine Mountain Thrust Sheet cracked. The Rocky Face Fault weakened the bedrock, and erosion along the fault has created gaps through the sandstone ridges that form Cumberland Mountain and Pine Mountain. Cumberland Mountain created a 100-mile long barrier, but those low spots in the Allegheny Front opened a "vital gateway to the west during the early expansion of the United States."1 "Cumberland Gap National Historical Park: geologic resources inventory report," National Park Service, Natural Resource Report NPS/NRSS/GRD/NRRó2011/458, September 2011, p.v, pp.1-2, https://nature.nps.gov/geology/inventory_embed/publications/reports/cuga_gri_rpt_body_print.pdf (last checked May 6, 2018)

a digital elevation model (DEM) reveals that Cumberland Gap and Pineville Gap are features of the Rocky Face Fault
a digital elevation model (DEM) reveals that Cumberland Gap and Pineville Gap are features of the Rocky Face Fault
Source: Kentucky Geological Survey, Field Guide to the Middlesboro and Flynn Creek Impact Structures (Figure 3)

The Pine Mountain Thrust Sheet (also known as the Cumberland Overthrust Sheet) is a block of sedimentary rock about 4,000 feet thick and roughly 125 miles on each side. It was broken free, pushed up and over adjacent rocks, then forced westward and rotated during the collision between tectonic plates called the Alleghanian Orogeny.

Each of the four sides is marked by faults, where the block of crust was cracked free and displaced. The leading edge on the northwest is now known as the Pine Mountain Thrust Fault. The southwestern edge moved 11 miles, while the northeastern end moved 4 miles. The block is folded in the middle, with once-flat sedimentary layers bent down to form the Middlesboro Syncline.2 Keith A. Milam, Jonathan C. Evenick, Bill Deane (editors), "Field Guide to the Middlesboro and Flynn Creek Impact Structures," Impact Field Studies Group, 2005, p.10, p.14, https://web.archive.org/web/20140106031520/http://web.eps.utk.edu/~faculty/tennmaps/lectures/MiddlesboroGuide.pdf (last checked May 7, 2018)

Many such blocks of rock were cracked free, then pushed on top of the same sedimentary layers that had once been next-door neighbors. (If you cut a ham and cheese sandwich and slide one half sideways and on top of the other, you can create the equivalent of a thrust sheet.)

Gondwana (northwest Africa) and Laurentia (North America) merged to create the supercontinent Pangaea. Gondwana slid up and over the Avalon and Taconic terranes that had previously been accreted to the edge of Laurentia in the Taconic and Neo-Acadian Orogenies, cracking them free and sliding them west over the limestones, shales, and sandstones that had accumulated in the Sauk, Tippecanoe, and Kaskaskia seas. Rock formations that had once been adjacent to each other as horizontal neighbors ended up being cracked and stacked vertically, and tilted as well.

Gondwana compressed the continent like a musician squeezing an accordion, and "Africa" ended up on top of "North America." The edge of Africa may have sat on top of North America as far west as modern-day Kentucky, 300 million years ago.

Those rocks have eroded away. What was once part of Gondwana, pushed up and over the edge of Laurentia, has been washed downstream and blown downwind to create younger sedimentary layers in the Mississippi Valley and on the Coastal Plain.

The front edge of the thrust sheet is the Pine Mountain Thrust Fault, at the base of Pine Mountain. It may mark where deformation from continental collsion finally stopped, as the energy of the tectonic collision was dissipated by folding and faulting of sedimentary layers in the Valley and Ridge province.

The Pine Mountain thrust sheet was bent, twisted, and cracked as it was pushed up and slid for miles across the top of rocks that had once been adjacent. Erosion has stripped away overlying formations. The sandstone-rich Lee Formation has been slower to erode, and today forms the ridges of Pine Mountain and Cumberland Mountain.

Cutting through the mountains is an ancient crack in the thrust sheet, the Rocky Face Fault. Differential erosion has carved the Pineville and Cumberland gaps where that fault weakened the bedrock. The Cumberland River still flows through Pineville Gap. Cumberland Gap is now being eroded on the west side by Little Yellow Creek, and on the east by Gap Creek.3 "Cumberland Gap National Historical Park: geologic resources inventory report," National Park Service, Natural Resource Report NPS/NRSS/GRD/NRRó2011/458, September 2011, p.2, https://nature.nps.gov/geology/inventory_embed/publications/reports/cuga_gri_rpt_body_print.pdf; "The Late Paleozoic Alleghanian Orogeny," The Geological Evolution of Virginia and the Mid-Atlantic Region, http://csmgeo.csm.jmu.edu/geollab/vageol/vahist/K-LatPal.html; Keith A. Milam, Jonathan C. Evenick, Bill Deane (editors), "Field Guide to the Middlesboro and Flynn Creek Impact Structures," Impact Field Studies Group, 2005, p.14, https://web.archive.org/web/20140106031520/http://web.eps.utk.edu/~faculty/tennmaps/lectures/MiddlesboroGuide.pdf (last checked May 6, 2018)

Cumberland Mountain is part of the Pine Mountain thrust sheet
Cumberland Mountain is part of the Pine Mountain thrust sheet
Source: Kentucky Geological Survey, Geology of Cumberland Gap National Historical Park

Cumberland Gap is located east of the Middlesboro impact structure and the edge of Pine Mountain Overthrust Fault
Cumberland Gap is located east of the Middlesboro impact structure and the edge of Pine Mountain Overthrust Fault
Source: Kentucky Geological Survey, Geologic map of the Middlesboro and part of the Bristol 30 x 60 minute quadrangles, southeastern Kentucky

Cumberland Gap

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Cumberland Gap is located east of the Middlesboro impact structure and the edge of Pine Mountain Overthrust Fault
Cumberland Gap is located east of the Middlesboro impact structure and the edge of Pine Mountain Overthrust Fault
Source: National Park Service, Digital Geologic Map of Cumberland Gap National Historical Park and Vicinity, Kentucky, Tennessee and Virginia

Cumberland Mountain is formed from a slower-to-erode sandstone conglomerate, the Lee Formation
Cumberland Mountain is formed from a slower-to-erode sandstone conglomerate, the Lee Formation

the Lee Formation is a conglomerate
the Lee Formation is a conglomerate

Links

References

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Rocks and Ridges - The Geology of Virginia
Virginia Places