Stream Superimposition

Stop 1 of 11 (Site 1c)

Grus - The granite that you see in these photographs is in state of decay. Soil once covered the granite rock, keeping water in contact with the granite minerals for weeks and months at a time, even in a desert. The net result turns hard granite into bits and pieces of sand called grus. You see the grus in this state of decay, because the overlying soil has been eroded by flowing water.

Superimposition - The water flowing across the granitic rock in both pictures runs counter to intuition. Why would water decide to flow right across this bedrock knob? Why wouldn't the tiny channel flow around the bedrock?

The making of grus and the superposition of a tiny channel across the bedrock are connected by a three-step process.

First, this location was buried in soil. The soil cover helped weather the bedrock into grus.
Second, the soil was eroded by water flowing in a small channel. The channel didn't know that bedrock was right underneath the surface when it started to erode (incise) the channel. The channel was just flowing on and eroding the sand.
Third, as the soil eroded away and the channel incised deeper, a small ravine cut through the bedrock. In the middle picture, you can see that the drainage could have easily gone around bedrock by flowing to the nearby drainage to the left. It did not shift. Instead, the small channel stayed where it was and eroded deeper into the weathered bedrock

Drainages that cross bedrock , even though alternate routes would seem to make more sense, are termed transverse drainages (or drainage anomalies). This particular type of transverse drainage is superimposition — where the drainage forms on a cover of weak material (in this case soil) and is superimposed across the underlying bedrock.

A much larger example of superimposition can be seen below at Red Rock Canyon, eastern California. This is a famous site for Hollywood movies. The original structure of titled rocks was covered by stream deposits (former cover). Then, the streams incised into the former cover of deposits and eroded channels into the titled layers. Most of the dry channels seen in the aerial photograph below are transverse to the structure of the titled rocks. [The scale of the image is about 2 miles from left to right.]