The park is located in the Blue RidgeMountain section of the Appalachian Mountain Range. The geological history of the park begins 550 million years ago when this area was covered by a shallow sea. Deposition of sediments such as sand, clay, and limestone, began at this time. When the continent of Africa collided with the continent of North America about 360 million years ago, the Appalachian Mountains rose. Normal compaction along with the heat and pressure generated by this collision changed these sediments into the quartzite (sand), phyllite (clay), and limestone (fossil shells and mud) rock types found in the mountains here today.
As the Appalachians rose, the sea evaporated and the Potomac River cut through the rock, eventually forming the water gap between MarylandHeights and LoudounHeights. This is considered by many to be the most prominent geological feature in the park. While the Potomac River was cutting through the gap, the Appalachians, which were once taller than the Rocky Mountains are today, were being worn down by rock, wind, rain, and ice. After this erosion, only the roots of the Appalachian Mountains were left. Water running off of the mountains began collecting at their base, forming what is now the ShenandoahRiver. This river flows along the base of the Blue Ridge Mountains until reaching Harpers Ferry, where it joins with the Potomac River and flows east towards the Chesapeake Bay.