Hillsboro Gap, also known as the Gap in the Short Hill is a water gap

Water gap

A water gap is an opening or notch which flowing water has carved through a mountain range. Water gaps often offer a practical route for road and rail transport to cross mountain ridges.- Geology :...

 in the Short Hill Mountain

Short Hill Mountain

Short Hill Mountain is a mountain ridge of the Blue Ridge Mountains in northwest Loudoun County, Virginia.- Geography :Short Hill rises sharply from the Piedmont countryside of the Loudoun Valley, northwest of Purcellville, as a low-lying ridge barely over above the surrounding area...

 formed by the North Fork of the Catoctin Creek

Catoctin Creek (Virginia)

Catoctin Creek is a tributary of the Potomac River in Loudoun County, Virginia, with a watershed of . Agricultural lands make up 67 percent and forests 30 percent of Catoctin Creek's watershed...

 in Loudoun County, Virginia

Loudoun County, Virginia

Loudoun County is a county located in the Commonwealth of Virginia, and is part of the Washington Metropolitan Area. As of the 2010 U.S. Census, the county is estimated to be home to 312,311 people, an 84 percent increase over the 2000 figure of 169,599. That increase makes the county the fourth...

. The gap derives its name from the town of Hillsboro, which is nestled in the gap. Virginia State Route 9

Virginia State Route 9

Virginia State Route 9 is a primary state highway in the U.S. state of Virginia. Known as Charles Town Pike, the state highway runs from the West Virginia state line near Mechanicsville, where the highway continues west as West Virginia Route 9 , east to SR 7 and SR 7 Business near Paeonian...

 passes through the gap in the town.

In colonial times the main road between Alexandria

Alexandria, Virginia

Alexandria is an independent city in the Commonwealth of Virginia. As of 2009, the city had a total population of 139,966. Located along the Western bank of the Potomac River, Alexandria is approximately six miles south of downtown Washington, D.C.Like the rest of northern Virginia, as well as...

 and Winchester

Winchester, Virginia

Winchester is an independent city located in the northwestern portion of the Commonwealth of Virginia in the USA. The city's population was 26,203 according to the 2010 Census...

, Vestal's Gap Road, passed through the gap. That road eventually became the Charles Town Pike and modern day Route 9. The gap poses a substantial barrier to the widening of the congested road as the historic town completely fills the gap and as such Route 9 would have to be routed around or over the Short Hill Mountain to be widened.