The drop formed when a river flows from harder rock to softer rock is called a waterfall.

Here's how it works:

* Harder rock resists erosion: The harder rock erodes more slowly, creating a steep cliff.

* Softer rock erodes faster: The softer rock erodes quickly, creating a drop.

* Water plunges over the drop: The river flows over the edge of the cliff, creating the waterfall.

Over time, the waterfall will continue to retreat upstream as the softer rock is eroded away.