Guide to Shenandoah National Park and Skyline Drive

The geologic story of Shenandoah National Park began 1 billion years ago. Molten magma, miles beneath the earth's surface, slowly solidified to become the "basement rock," or core, of what we know today as the Blue Ridge Mountains. For the next 500 million years, erosion and the uplifting of the earth's crust exposed the granitic basement rock. You can see the granite component of this rock in Old Rag Mountain and its surroundings.

Blackrock Summit
Photo taken by Matthew Singer

Over millions of years, the uplift of the earth's crust formed deep cracks in the granite, which led to volcanic activity. For centuries, molten basaltic lava burst forth and poured over the land, forming a smooth, flat plain called the "Catoctin Formation." (The bedrock underlying Big Meadows was formed by at least 12 of these lava flows which, collectively, are approximately 1,800 feet thick.) The cooling and contraction of the lava produced a network of polygonal cracks, or columnar jointing.

When continental drift (the separation of the Americas from the continents of Europe and Africa) began, about 600 million years ago, it formed a broad, shallow depression from Alabama to Newfoundland. Then, for 300 million years, an ancient sea flooded the area that is now the Appalachian Mountains. Layers of water-borne sediments accumulated on the ocean floor, followed by limestone sediments composed of fossilized marine animals and shells. The weight and resultant heat caused by the sediments eventually compressed the two layers into metamorphic rock.

As a result of the eons-old shifting of the earth's tectonic (or crustal) plates, North America and Africa collided. This re-elevated and fractured the sea floor, causing the older, underlying layer of metamorphic rock to tilt upward and slide over the younger layer creating a towering mountain range that we call the Appalachians. The process occurred so slowly that even if geologists had been present, they would not have known that it was occurring. The new Appalachians probably looked more like the present Hima-layas than the rounded mountains we see today.

The powerful forces of wind, water and frost have worn away the Appalachians for nearly 250 million years. These forces continue to create and refine the spectacular scenery of Shenandoah National Park. Water runoff, as demonstrated by the "500-year" rainstorm of June 1995, has carved the mountains' distinctive alternating pattern of ridges and valleys. As you explore the park, look for signs of its geologic history and for how wind, water, frost and ice continue to sculpt the land.

Reference: History of Shenandoah