The Horseshoe Falls, the largest of the three falls that comprise Niagara Falls, derives its curved shape from the underlying geological structure. It was shaped over time through the processes of erosion.
When water falls over a cliff, it erodes the rocks at the base of the falls more quickly than the top of the falls. This results in the creation of a deep pool called a plunge pool and a gradually receding cliff face, giving it a curve over time.
Furthermore, the bedrock beneath Niagara Falls consists of two types of rocks: hard, resistant rocks (such as limestone and dolostone) and softer, less resistant rocks (such as shale). The harder rocks can withstand the force of the falling water, whereas the softer rocks get eroded more quickly. This process contributes to the horseshoe-shaped curvature of the falls.
Also, the falls continue to move backwards due to ongoing erosion, thus continuing to alter the curve of the falls over the years. As water continues to erode the river bed, the shape of the Horseshoe Falls will change, making the site a continuously evolving natural wonder.
Moreover, ice blockages in the Niagara River, frost weathering and rockfalls also contribute to the curved shape of the falls. These factors, coupled with thousands of years of natural erosion, have carved out the iconic, rounded horseshoe shape that we recognize today.