If you're hiking across the landscape at Arches National Park, following rock cairns across the slickrock, how would you know when to turn right or left? The answer lies within the pages of Cairns: Messengers in Stone, a wonderful little book that traces the history of these piles of rock.

The answer -- look for a small rock on either the right or left side of the cairn; it indicates which way to turn -- is, like the stones that go into creation of a cairn, just one of the building blocks that David B. Williams hands us.

A former national park ranger in Arches and elsewhere in the system, Mr. Williams also happens to be a geologist by training and an inquisitive natural history writer by vocation who knows how to tell a story.

"... unlike other animals, we need artificial enhancements to aid our navigation. We can't sniff our way back like a salmon, track the earth's magnetic field as monarch butterflies do, or detect polarized light, a trait that allows some birds to migrate thousands of miles. Seasoned travelers do, of course, make use of many clues to find their way -- such as noting the topography, tracking the movement of the sun and stars, and observing where plants grow -- but not at the level of, say, a homing pigeon.

"I worry though that new technology may render our use of natural clues and cairns obsolete. More and more people now carry GPS units in the backcountry, so much so that the classic urban 'hunch' of a person bent slightly, head titled down, staring at a screen, has become a common trailside feature. Are these people relying too much on technology and not enough on the subtle details around them? Will they no longer look for that reassuring cairn letting them know that others have been there before?"

That Mr. Williams' college years were spent studying geology is, if you weren't aware of that aspect of his background, quickly discerned as he delves not only into the different types of rocks that can be found on the ground -- sandstones, limestones, granites, plutonic rocks, methamorphics -- but which are best for building cairns. And he also explores the environmental factors that can turn cairns into rubble -- freeze-thaw cycles will slowly breakdown sandstone, for instance, while salt air can also steadily disintegrate rocks.

Funny thing is, if not for an editor's suggestion, Mr. Williams might not have written the book.

“I had certainly noticed them on the trail, so once I dove in I realized it was far more interesting than I initially thought they would be," he said from his Seattle home. "I had noticed them all over the place and never given them much thought."

Indeed, during his days as a seasonal ranger at Arches National Park he spent more than a little time taking down cairns the park hadn't set up.

"I did a lot of destroying them as a park ranger, which is a very common. I talked to park rangers all over the place and they all have the same stories. It was a very common practice to do, mostly because there was concern for ecological issues, but also making sure that people didn’t get lost, or taken to areas that were beyond their abilities," said Mr. Williams. "And certainly in the desert, where it’s so fragile, one of your goals is to keep the people on the maintained trails.”

While Acadia National Park is heavily forested, it has more than a few cairns, particularly on the summit of Cadillac Mountain.

“There are lots of cairns there," he said. "People were building so many at the summit that they’re really doing environmental damage."

With hopes of dissuading the cairn constructors, the park put signs up explaining the need for officially placed cairns, and the problem illegally erected ones posed to the landscape and easily waylaid hikers.

While the signs initially helped, 10 years later rangers reported there was no significant change in behavior: "People are building them all over the place. Particularly these summit cairns, these sort of cities, as I call them, cities of cairns, where there will be dozens of them," the author recounted.

Now, one man's, or woman's, cairn, might not be another's. It all comes down to how you define a cairn. While piles of rocks to mark trails are certainly cairns, Mr. Williams pointed out that piles of rocks used by prehistoric hunters to trap or drive wildlife to their demise also are considered to be cairns.

"The basic definition is a heap of stones. The analogy I’ve been using of late has been that a cairn is sort of like the Supreme Court’s definition of pornography: you know it when you see it," explained Mr. Williams. "I think we also in this country generally think of them as trail markers, but I try to make the point that they have a much broader usage."

One of the most fascinating sets of cairns, to Mr. Williams, exists in Þingvellir (Thingvellir) National Park in Iceland. It's here where you find "one of the few spots on Earth where the (tectonic) plates come to the surface. You have the North American plate and you have the European plate, and they come to the surface at Iceland," the author said. "And as you go to Thingvellir, you walk across this valley, it’s about 3 or 4 miles wide, and you walk from North America across the plate boundary, into Europe. And it’s a cairned route all across the valley, and I thought that was really cool."

Cairns: Messengers in Stone is a really cool book, whether you enjoy connecting the rock piles like dots on your hike, are curious about the various architectural styles of cairns, are fascinated by geology, or simply intrigued by rock pile lore the world over.