Driving by on Highway 40 west of Hermosa, you might not notice the rust-colored outcropping of rock in the ditch.

But it might be fracking sand, a key ingredient of controversial method of oil-and-gas recovery that is driving energy expansion in neighboring states.

It might not be fracking sand, too. Only time and tests will tell.

Tom Marshall and Mark Fahrenbach are the ones to figure that out. As scientists for the South Dakota Geological Survey, they're hunting these days for suitable fracking-sand deposits across parts of the Black Hills.

Suitability is serious business when it comes to the finding the correct sand. It must be tough enough when injected in hydraulic fracturing to hold open underground seams, but porous enough to allow oil or gas to flow through and be recovered.

On Monday, the geologists viewed the sandstone outcropping near Hermosa with more than a typical traveler's eye. They stopped, looked and chipped off some samples, carefully recording location and characteristics before heading southeast toward Fairburn and Hot Springs to collect more samples.

Marshall liked what he saw and heard from the first reddish stone he scraped with a small rock pick.

"Oh yeah, that's a nice sand," he said. "Listen, you can hear it: scrunch, crunch. This is a nice outcropping."

Whether it is suitable fracking sand remains to be seen; tests by the state for size, roundness and strength of sand particles will show that.

In coming weeks, Marshall and Fahrenbach will continue a state project to take samples from at least 180 locations in six geological formation across the Black Hills. They are searching for fracking sands to serve the oil-and-gas industry in North Dakota, Wyoming and elsewhere.

Fracking is a controversial method of oil and gas recovery that hasn't yet found a home in South Dakota, which has limited oil fields in northwest counties. But the process leads to disputes about environmental damage where it is used.

The mining of fracking sands, as it is done in states like Minnesota and Wisconsin, is also controversial process because of its dust, noise and potential environmental impacts.

Critics are already gearing up to oppose fracking-sand development in western South Dakota. Meanwhile, at least two companies have staked claims or begun exploration for the right sands in the Black Hills.

Marshall and Fahrenbach don't consider themselves part of that fight. They are state scientists doing the kind of survey work their jobs often involve. They picked up Monday where they left off in December in the survey of sandstone formations in the Black Hills.

The Geological Survey began the work last year in anticipation of fracking-sand exploration by private companies. They plan it to produce an inventory of deposits that could be suitable for mining and offer that information on a state website.

Marshall and Fahrenbach are working the southern hills this week, checking maps of geological formations, traveling rural roads and scanning the landscape for tell-tale outcroppings.

They are looking for sands that were shaped, moved and deposited by the shifting of an ancient sea the covered the region 500 million to 550 million years ago. The same reddish formation that juts out from the grass along Highway 40 reaches down into Colorado and the Red Rocks area.

The sandstone deposits are exposed here through the upheaval that formed the Black Hills. Over hundreds of millions of years, those beach sands turned into sandstone.

"This was essentially sand on a beach," Fahrendorf said. "And it it will end up just like that if it's used for fracking."

The Geological Survey will evaluate the samples using standards by the American Petroleum Institute. Different shapes or sizes of particles, or those that crush too easily, will not work.

"That's why it's so important that they be the same size and shape," Fahrenbach said. "It's putting a bunch of billiard balls or BBs together. There's always some space in-between."

That space is essential to allow oil and gas to flow and be collected.

"There's a whole lot of factors that go into a fracking sand," Marshall said. "You can't just use any sand you find."

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