Cross Mountain is the highest peak in the

Bear Pond Mountains The Bear Pond Mountains are a sub-range in the Appalachian Mountains, that straddle Pennsylvania and Maryland in the United States. These mountains are a part of the Ridge and Valley Appalachians and reach their highest point at Cross Mountain ...

, a sub range of the

Appalachian Mountains The Appalachian Mountains, often called the Appalachians, (french: Appalaches), are a system of mountains in eastern to northeastern North America. The Appalachians first formed roughly 480 million years ago during the Ordovician Period. They ...

. This grouping of mountains straddle the

Maryland Maryland ( ) is a state in the Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware and the Atlantic Ocean to ...

and

Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...

border. Cross Mountain and Hearthstone Mountain which is on the Maryland side, form a unique geologic formation known as the "Punchbowl". This feature was created by the weathered shales of the

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

age in the center of a south-plunging

anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...

, having been eroded to expose a large amphitheater like feature (punchbowl). Cross and Hearthstone Mountain are made of hard resistant

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tec ...

of the

Tuscarora Formation The Silurian Tuscarora Formation — also known as Tuscarora Sandstone or Tuscarora Quartzite — is a mapped bedrock unit in Pennsylvania, Maryland, West Virginia, and Virginia, USA. Description The Tuscarora is a thin- to thick-bedded fine-gr ...

of the Silurian age. Cross Mountain is also the site of a deactivated microwave relay station, that was used during the Cold War. The unit was encased in a 103 ft. tower, and known as the "Cannonball" site 2. The site was closed in 1977. Access to the summit is still restricted.

References

Alan R. Geyer (1979) "''Outstanding Geologic Features of Pennsylvania''",

Geological Survey of Pennsylvania Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other E ...

* Albert LaFrance, ''Secret Cold War Landscape'', Presidential Emergency Facilities Microwave Radio Relay Station Site 2 - "Cannonball" atop Cross Mountain, near Shimpstown, PA., Cold War webring, last update June 12, 1999. {{coord, 39.72829, -77.99176, region:US-PA_type:mountain, display=title Mountains of Pennsylvania Landforms of Franklin County, Pennsylvania