What is formed when a river flows from an area of harder rock to an area of softer rock?

When a river flows from an area of harder rock to an area of softer rock, it typically begins to erode the softer rock more quickly, leading to the formation of features such as waterfalls or rapids. This transition can create a distinct change in the river's gradient and flow characteristics. Over time, the erosion of the softer rock can also result in the formation of valleys or gorges as the river carves its path through the landscape.