The following review is from Lois Tilton in the Internet Review of Science Fiction:

Everyone knows that when doors and roads suddenly appear where there were no such doors and roads a moment ago, they usually lead to trouble. But geologist Elias Ullstrom has been offered a new job in Agate County, somewhere in the scablands of Eastern Washington, so that is the road he takes. His job is a mineral survey of the canyons belonging to the Awattamapum tribe, but the local game warden has a warning for him.

"There are things out in this country that are dangerous. If you stick near the trails and follow the instructions the county gave you, you should be fine. But, if you start screwing around playing National Geographic explorer up in those back canyons, you're asking for trouble. Stay on the map, man. Listen to your instincts, if something doesn't seem right then get the hell away from it, comprende?"

Unfortunately, Elias gets carried away by his discoveries.

What Elias discovers is authentically scary and weird, and it is by no means obvious whether he will escape or not. The characters are genuine, and the setting is both strange and real, with the strange stuff made quite credible by the author.

RECOMMENDED (end of review)

From the author:

Geologist Elias Ullstrom should have heeded the game warden's warning but couldn't resist the siren call of Agate County's time-lost mysteries. Now he is hunted, pursued through deadly desert canyons by a prehistoric evil, a monstrous foe hidden for untold millennium: The Riders of the Three-Toed Horse!

Originally published in Jim Baen's Universe magazine, February 2009. The novelette has also been adapted into a radio play and was chosen for the 2009 Locus Recommended Reading List in the Novelettes category.