Omu’s Journal: Cobble rock

by Tom Elder
Uintah Trails Working Group

Cobble rock.

Not the park. I am more interested in the term, “cobble rock.” Here’s my definition:

“A cobble is a roughly rounded stone, made of a fairly hard rock type, that has been deposited in a high-energy environment, normally by violently moving water.”

You are welcome.

The park at the intersection of Main and Vernal has archways of cobble rock cemented togeth-er. It is not a preferred building material precisely because cobbles tend to be too round to work with, sliding and rolling off each other. You need a lot of concrete to get them to stay in one place. Much easier to stack flat stones on top of each other, you may not even need to ce-ment them.

A cobble is rounded because the sharp edges have been knocked off by the bashing it receives from the rock’s immediate environment. A good cobble has to be hard, because a soft rock like shale would be quickly pulverized into fragments by the environment.

Igneous rocks like granite and basalt make good cobbles, as well as metamorphic rocks like gneiss and quartzite. Sedimentary rocks are often too soft to survive this rough treatment, but limestones and hard sandstones can become cobbles.

The high-energy environment that formed them is sometimes a seashore, where the power of the wave action sweeps away all smaller sediments such as mud or sand.

Such seashores may be covered in sand during the “quiet” season, usually summer, but the sand gets washed away in the stormy season, normally winter. The cobbles are then exposed again.

As the rocks shift and roll under the wave action of winter storms, they become rounded. Then the sand is deposited on top again when things quiet down. Of course, some seashores are AL-WAYS sandy, and some are ALWAYS stony and without sand. The world is various.

Our local cobble rock has been tumbled down the streams that begin high in the Uintas. Their sharp points break off against neighboring rocks as they make their episodic journey during high-water events.

They fill the dry streambed of Ashley Creek. Their color is most often reddish. The majority are composed of the hardest rock we have locally, the Uinta Mountain Group that forms the core of the namesake mountain range.

When you look up at Marsh Peak/“Baldy,” or at the canyon walls on either side of Flaming Gorge Dam, you are looking at Uinta Mountain Group.

In the 80s, I was told that this rock is metamorphic, but since then the geology nerds have de-cided that it is just a very hard sandstone, courtesy of the long and deep burial and extreme pressures it experienced. Science marches on.

Even though Ashley Creek flows out from that core across many other formations, most of the other rock types that wind up in the creek will not travel very far downstream before they are broken into bits. The hard sandstone of the Uinta Mountain Group, however, survives this rough treatment much better.

Such cobbles get more and more rounded the farther they roll downstream, on their way to be-coming nearly indestructible red cannonballs.

The closest bedrock source for these cobbles is halfway up Ashley Gorge, so the ones I see un-der the bridge on 1500 West have travelled at least 12 miles, bouncing and bashing their way downstream during high water. (I sometimes wonder how many years a particular cobble has been traveling, thousands? Tens of thousands?).

In high flows such as we are likely to see this spring, you will hear them clacking against each other like giant marbles.