“All we are is dust in the wind, dude.” -Ted


As I was out riding my bike this morning, this week’s geojournal prompt was heavy on my mind:

Go outside and find evidence of sediment transportation.

If I time were my friend, I’d go explore Great Sand Dunes National Park, which is only a couple hours south of me. I’ve always wanted to go and haven’t made it there yet.

Photo credit: Christian Collins on VisualHunt.com / CC BY-SA
Photo credit: Christian Collins on VisualHunt.com / CC BY-SA

But time has been my nemesis this semester, so I pulled my thoughts closer to home. Very close. In fact, right in front of my slick road bike tires. FYI, roadies hate gravel.

Questions of the Week:

  • What type/size of sediments are being deposited here?

The sediments deposited are a combination of silt and sand, with a few small pebbles thrown in for good measure.

  • How about the sorting? Would you expect the grains to be well sorted, poorly sorted?

The dirt is layered; fine sediment has settled on the bottom, with gravel in the middle and pebbles on top. I’m not sure if this is considered well sorted, but gravel on the side of paved roads always seems to be layered like this.

  • Well rounded or angular? 

Overall, the dirt is angular, and a sharp edge can work into the tire and pop a tube, resulting in a flat. That’s one reason roadies hate gravel so much.

  • Is this a high energy or low energy environment?

This is a low energy environment most of the time. When thunderstorms are present, the game changes.

  • Did you notice sedimentary structures being formed (ripple marks, cross bedding, dunes, etc.) in this area?

Not far from the picture of the asphalt is a hill that shows evidence of rain gullies. When the thunderstorms dump too much rain too fast to be absorbed by the ground, it flows down the slope and onto the street, bringing the sediment and gravel with it.

  • What type of sedimentary rock would be formed if this area were to immediately compact and cement into a rock?

My answer: Conglomerate. “Conglomerate is a coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm in diameter.” (thank you, Google). Doesn’t this type of rock practically look like pavement already?

Photo credit: Grand Canyon NPS on Visual hunt / CC BY

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