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.