Everyone knows that a waterfall is a place where a river or creek flows over a vertical drop-off, but did you know that there is a geologic reason why they form? A waterfall, like Cedar Falls pictured above, forms where a hard, resistant rock such as sandstone overlies a soft, easily eroded rock like shale. The difference in the rate each rock type weathers is what creates the waterfall.
When a stream passes over a single rock type, it erodes it evenly, carving a channel with a gradual slope. However, when a stream’s course passes from a hard to a soft bedrock, it scours the soft rock at a faster rate. As the supporting soft rock is eroded, the overlying harder rock progressively collapses, creating a vertical bluff over which the stream flows. As this process continues an ever taller waterfall develops, and the location of the waterfall gradually migrates upstream.
Because we know how landforms such as waterfalls form, geologists can use tools, like aerial photographs and satellite images, to predict what kind of rock will be in an area before ever going there.