Question: How long is the extension of the blue ridge mountain range that Nichol's Gap Road was built over?
Context: South Mountain is the northern extension of the Blue Ridge Mountain range in Maryland and Pennsylvania.  From the Potomac River near Knoxville, Maryland, in the south, to Dillsburg, Pennsylvania, in the north, the 70 mi range separates the Hagerstown and Cumberland valleys from the Piedmont regions of the two states.  The Appalachian National Scenic Trail follows the crest of the mountain through Maryland and a portion of Pennsylvania. Paris Mountain is a peak of Blue Ridge Mountain on the border of Loudoun County and Clarke County in Virginia.  The 1926 ft peak, which is located just north of Ashby's Gap and the town of Paris, for which it is named, represents the highest elevation in both counties.  The peak is accessible by Blue Ridge Mountain Road from the gap.  The Appalachian Trail is crosses the western slope of the peak. Spicers Gap Road is a heritage-listed road at Spicers Gap Road Conservation Park (in the Main Range National Park), Tregony, Southern Downs Region, Queensland, Australia.  It was built from 1859 to 1865.  It is also known as Spicers Gap Road Conservation Park and Spicers Peak Road.  It was added to the Queensland Heritage Register on 23 July 1999. Snickers Gap, originally William's Gap, is a wind gap in the Blue Ridge Mountain on the border of Loudoun County and Clarke County in Virginia.  The gap is traversed by Virginia State Route 7.  The Appalachian Trail also passes across the gap.  Bear's Den and Raven Rocks are adjacent to the gap. The Nichol's Gap Road was a central Pennsylvania highway established in the 18th century near Maryland extending westward from the Black's Gap Road "just west of Little Conewago Creek" at the "Crofs Keys" stand of James Black.  The road was past both the Rock Creek Church and 1761 Samuel Gettys tavern where Gettysburg would be surveyed in 1786.  The highway was built over the South Mountain (Maryland and Pennsylvania) via Nichol's Gap ( ) and down the Devils Racecourse into the Cumberland Valley, allowing access to the 1762 Hagerstown, Maryland.  Called the "Hagerstown Road" during the Battle of Gettysburg, parts of the road are now designated (east-to-west): U. S. Route 30 in Pennsylvania, Pennsylvania Route 116 (Fairfield Road to Fairfield, Pennsylvania), Iron Springs Road, Gum Springs Road, and Old Route 16 (the summit section through Nichol's Gap--"Fairfield Gap" during the American Civil War—no longer has a roadway.) The Blue Ridge Mountains are a physiographic province of the larger Appalachian Mountains range.  This province consists of northern and southern physiographic regions, which divide near the Roanoke River gap.  The mountain range is located in the eastern United States, starting at its southernmost portion in Georgia, then ending northward in Pennsylvania.  To the west of the Blue Ridge, between it and the bulk of the Appalachians, lies the Great Appalachian Valley, bordered on the west by the Ridge and Valley province of the Appalachian range. Elk Ridge is a mountain ridge of the Blue Ridge Mountains in Maryland.  The ridge is located to the west of South Mountain and runs roughly parallel to it from Rohrersville, in the north, to the Potomac River across from Harpers Ferry, West Virginia, in the south.  Across the Potomac the ridge continues as Blue Ridge Mountain in Virginia and West Virginia.  The southern end of the ridge, which is part of Harpers Ferry National Historical Park, is known as Maryland Heights. Mountain City is an incorporated town in Rabun County, Georgia, United States.  The population was 829 at the 2000 census.  The town straddles the Eastern Continental Divide in a deep gap in the Blue Ridge Mountain front.  The gap allows U.S. Route 441 to cross the range at an elevation of 2168 feet without the significant grade required by roadways at most other mountain passes along the Georgia Blue Ridge. Keyes Gap or Keyes' Gap is a wind gap in the Blue Ridge Mountain on the border of Loudoun County, Virginia and Jefferson County, West Virginia.  The gap is traversed by Virginia State Route 9/West Virginia Route 9.  The Appalachian Trail also crosses the gap. The Blue Ridge Tunnel (also known as the Crozet Tunnel) is a historic railroad tunnel built during the construction of the Blue Ridge Railroad in the 1850s.  The tunnel was the westernmost and longest of four tunnels engineered by Claudius Crozet to cross the Blue Ridge Mountains at Rockfish Gap in central Virginia.  At 4237 ft in length, the tunnel was the longest tunnel in the United States at the time of its completion in 1858.  The tunnel was used by the Virginia Central Railroad from its opening to 1868, when the line was reorganized as the Chesapeake and Ohio Railroad (renamed Chesapeake and Ohio Railway in 1878).  The Chesapeake and Ohio routed trains through the tunnel until it was abandoned and replaced by a new tunnel in 1944.  The new tunnel was named the "Blue Ridge Tunnel" as well, although the original tunnel still remains abandoned nearby.  The old Blue Ridge Tunnel has since been named a Historic Civil Engineering Landmark.
Answer: 70 mi