The escarpment is capped by relatively hard, resistant rocks of the Silurian-age Lockport Group (chiefly dolostones and limestones), which are underlain by less resistant rocks (shales and limestones, like the fossiliferous Rochester Shale).

The geology of Niagara Falls is primarily composed of a thick layer of Lockport Dolostone (a type of limestone-like rock rich in magnesium) overlying softer layers of Rochester Shale. This specific layering is what creates the "falls" effect. The Lockport Dolostone acts as a "caprock"—it is extremely hard and resistant to erosion, forming the sturdy lip over which the water plunges. Below it, the softer shale and sandstone are easily eroded by the "plunge pool" and the swirling mist at the bottom of the falls. As the softer lower layers wear away, the heavy dolostone caprock eventually loses its support and breaks off in large chunks, causing the falls to slowly "retreat" upstream toward Lake Erie. This process is known as undercutting. Geologists estimate that the falls have moved about 7 miles south over the last 12,000 years. To slow this natural erosion, engineers have diverted much of the water's flow through hydroelectric turbines, reducing the sheer force that would otherwise cause the rock to crumble at a much faster rate.

Niagara Falls is primarily composed of dolomite (a type of limestone) and shale. The top layer, which forms the caprock of the falls, is a hard, erosion-resistant dolomite known as the Lockport Formation. Beneath this layer lies softer shale, specifically the Rochester Shale and Queenston Shale, which erodes more easily. This difference in erosion rates between the hard dolomite and the softer shale causes the falls to retreat upstream over time. The Niagara River cuts through these rock layers as it flows from Lake Erie to Lake Ontario.