Gravel Monkey, Indeed!

AggMan Staff | Published on June 7, 2012

Geological deposits such as alluvial fans are complex landforms that require more than a primate to identify and excavate.

 

I do not spend much time watching television, but there is one sitcom that my wife, Pam, and I especially enjoy — The Big Bang Theory. The show revolves around four geeks from Cal Tech — three of them are physicists and one is an engineer. Sheldon, a theoretical physicist, has great contempt for any other science, and periodically demeans the university’s geology department. In one show, he had the audacity to refer to the geologists as gravel monkeys.

Hey, I’ve been monkeying around with gravel for 40 years! Believe me when I say that gravel is not as simple as Sheldon thinks. I would like to grab him by his scrawny neck and drag him out to Death Valley to see a geological landform that is typical of gravel deposits in California — an alluvial fan.

Coalescing alluvial fans in Death Valley, Calif. The light-colored areas on the fans are the youngest gravels located along the present washes. The older gravels are stained dark with desert varnish. Telescope Peak, with an altitude of 11,049 feet is seen in the upper center of the photo. The foreground altitude is 260 feet below sea level.

Although rainfall is scarce in desert valleys, precipitation is greater in the mountains surrounding them. Occasionally, torrential thunderstorms flood the mountain canyons, and the raging water picks up the rock detritus in its path — everything from giant boulders to the smallest sediment. When the floodwaters reach the valley floor, the suddenly reduced carrying capacity of the water causes it to drop the sediment, forming an alluvial fan.

As its name implies, an alluvial fan has a fan shape that radiates outward from the fan apex — the point where an alluvial fan exits the mountains. The fan surface is crossed by braided channels that are entrenched near the apex. The channels become shallower down-fan until they reach the intersection point, which is where the channels are no longer entrenched. From that point on, the flood waters flow unconfined across the fan surface.

During the high-volume flows, heavy, large boulders and gravel are transported and deposited near the fan apex. At the intersection point, water flows out across the surface of the fan as sheet flow, forming lobes of smaller boulders, cobbles, and sand. These deposits are called sieves because most of the fine-grained material has been winnowed away by water flowing through the “sieve.” Most of the fine-grained material is deposited near the distal end (the downstream end) of the fan.

During low-volume flows, the stream cannot move boulders. Only lighter sand and silt will be transported and deposited close to the mountain front on top of the previously deposited boulders and cobbles.

Over time, different types of rocks may be eroded in the mountain canyons, so deposits in one part of the fan may be quite different from those in another. In addition, each flood that washes down the canyon will most likely take a new path over the fan, thus moving the locus of deposition across the fan. Numerous braided stream channels pile one atop another, gradually building up the alluvial fan, sometimes 200 feet or more thick. Multiple fans sometimes coalesce in the valley.

As you can see, an alluvial fan is a very complex landform. Gravel monkeys can help identify the various depositional environments of the fan, thus making it easier to excavate and process the gravel.

Maybe Sheldon will finally see the value of geology after a day in Death Valley. If not, maybe he can hitch a ride on a Large Hadron Collider.

 

Bill Langer is a research geologist who spent 41 years with the U.S. Geological Survey. He can be reached at Bill_Langer@hotmail.com

 

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