"He knelt down and gingerly tilted one of the rocks out of its bed in the soil. We all gasped as we saw the sharp line around its girth. The pale bottom was not merely light-colored soil caked onto the black rock; the rock itself was clearly pale below and black above. This was desert varnish, he said, and it formed only when the rock was exposed to the wind, rain, and sun."

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In one shot we are flying toward the mountainous lunar horizon. It seems close. Twenty miles? Two hundred? I can't gauge distances in this alien land. But what rivets me to the scene is the light. It rakes low across the slopes and ridges with savage clarity, and I momentarily relive a flight through the Pinacate volcanic field in Mexico, when bold evening sunlight skimmed crater and cone as if it could hardly be bothered to stop on its way past our planet.

In another photograph, taken at ground level, a four-wheeled lunar rover is parked in the middle distance, its empty seats open to the black sky. The background swells with voluptuous hills, but my eyes go to the tracks. There are tracks everywhere. I can see the tracks of the ambling person who is taking this picture. The other astronaut is nearby poking at a boulder, and his trail looks like he's been on foot for some time. The rover's metal mesh tires have furrowed the lunar soil in the foreground, and I realize with a jolt that the twin tracks are the most familiar thing in the whole scene: 240,000 miles from home and we still drive to the back country.

At twelve, in the heyday of the Apollo program, I knew a lot about the moon. I knew there were concerns about the depth and viscosity of dust on the lunar surface. I knew there was no weather, and that whatever we put there would last pretty much forever. To my young imagination, it seemed a noble achievement to leave a mark of any sort on the moon. Thirty years later, I just stared at all the tracks and wondered. How long before they will be fenced off to protect them from hordes of visitors to the future Apollo Lunar Heritage Site?

In western Arizona, southeastern California, and northwestern Sonora, there are places where the desert floor is very old -- nothing like the age of the moon's surface, but remarkably ancient for a world with wind, rain, and life. They are sometimes called Pleistocene surfaces, which is an especially evocative phrase if you hold it in your mind while you walk across one. The gravel crunching underfoot could well have rested there since the ice ages. Centuries of wind have swept away loose grains from between the walnut-sized stones, which are set in their soil beds with the delicate cement of rare showers. The pebbles themselves tend to be quite dark in color, sometimes almost black. This patina is the result of microbes working on minerals in the rock, and is the badge of long exposure to the sky. If you pluck the pebble from its socket of dirt and turn it over in your fingers, you will be startled by its nearly white underside -- and by the similarly light patch of soil it leaves exposed in its bed. The overall effect is of a natural mosaic, as if every piece were carefully placed to fit with its neighbors and the light color of the grout was chosen to set off the dark shards.

The desert near Tucson where I live is too lush and lively to sustain such inert surfaces. Ants, rodents, spring flowers and drenching rains keep the soil constantly on the move. But when I fly west across the desert the vast dry flats open up. Vegetation retreats into the drainages, which are almost always dry, but are closer to underground moisture. The land between the watercourses becomes nearly barren of plants. These areas grow wider and more level as I fly deeper into the desert. At some point the drone of the airplane begins to spawn silly daydreams ... how easily I could land the plane on one of these natural clearings, or ride a motorcycle for miles without so much as a trail to follow. This is when I know I am in the land of the earth figures; I am only the most recent human mind to imagine a use for these immense empty canvases.

Known variously as geoglyphs, intaglios, ground paintings, gravel pictographs, and giant effigies, these figures are inscribed in the darkened gravels of the desert pavement to reveal the light-colored soil underneath. There are geometric designs, such as lines, circles, and spirals. There are also more realistic human and animal forms. In some places where larger rocks are plentiful, rock alignments are used instead of scraping.

The mood of mystery about them is powerful. They titillate our deepest urges to make meaning. Each visitor is free to intuit, divine, or deduce their purpose. No explanation can really be wrong, since none are known to be correct. Even professionals who study them don't have much hard information about why, when, or by whom they were made. But beware: they may ultimately be a kind of Rorschach test, in which our speculations say more about us than about the figures.

When I first saw them, I was in the company of one of the leading researchers of earth figures. On a brilliant but cool January morning we hiked along cliffs of sheer volcanic rock the color of aged rust, stopping every so often to study a new spread of petroglyphs. He told us of bringing Indians here to learn their interpretations of the images, and as the minutes passed, his captivating exegesis brought me to the edge of dreamtime. Lines were no longer just lines. They had become the horizons of other worlds and pathways between realms.

Then we made our way through a cleft in the rocks to the mesa top above, and a moment later we stood on what he said was a prehistoric foot trail. Despite the lingering spell from the numinous rock art below, a muted scoff rose up within me. This was too much. How could anyone possibly know such a thing about this simple path? I've been around livestock and wildlife trails all my life, and this didn't look any different. But I decided to be patient and open. We followed a jog in the trail to the edge of the mesa where black stones lay in a cluster. The Indians had told him that this was a teaching station along an initiation pathway. He knelt down and gingerly tilted one of the rocks out of its bed in the soil. We all gasped as we saw the sharp line around its girth. The pale bottom was not merely light-colored soil caked onto the black rock; the rock itself was clearly pale below and black above. This was desert varnish, he said, and it formed only when the rock was exposed to the wind, rain, and sun. The longer the exposure, the thicker and darker the varnish. It hasn't yet yielded absolute dates, but it does give valuable clues for relative ages. He set the rock back in its hole. We looked around us with new eyes. Here and there across the mesa a spot of white signaled an overturned rock, and we began to understand that this landscape would brook no fakery.

link to image of summit pathway

Walking lightly, we followed the trail to the end of the mesa and descended a slope. At the bottom our guide paused and turned to look behind us. We saw immediately what caught his gaze. The soccerball-sized stones that were strewn randomly across the flank of a nearby mesa had been cleaved by a broad pathway leading straight up the side of the hill. The stones were deep brown, and had obviously rolled down from the volcanic rock that capped the mesa. But the sediments on which the caprock lay -- the bulk of the mesa's mass-- were the color of Caucasian skin. The clearing was unmistakably intentional. This was a fine example of a summit pathway, we were told. They've been observed on low hills such as this throughout the region, and although they usually proceed directly from the base toward the summit, there don't seem to be any other features associated with them; they don't go anywhere. The desert varnish suggests they are quite old.

link to image of volcanic mesa pathway

Half a mile away, we stood on another volcanic mesa overlooking the river channel. This one was covered with a fine black gravel, and I dreaded the thought of being here in any season but winter. Before us was another prehistoric foot trail, this one shaped like an extremely elongated racetrack. It had been apparently formed by tramping the gravel into soil underneath. The gentle swale of the pathway was pressed nearly smooth, and several of us took our shoes off to try it with bare feet. Again, the Indians had a story: in annual gatherings here, quarreling groups selected their best runners and settled their disputes with the outcome of footraces.

In the distance farther back from the river, spindly rows of stones littered the barren desert floor. I fought down my disbelief as we heard more tales of symbolism and ritual. How much can one read into such spare signage? Yet I had to admit that there were perceptible geometries here, and none of them showed any hint of modernity in either form or mode of creation. The area was devoid of vehicle tracks.

link to rattlesnake and other images

Later I visited other sites with more recognizable figures. These had clearly been formed by scraping off the desert pavement, and the little mounds of gravel at the ends of limbs gave me a pang of human connection, like finding a fingerprint in ancient mortar. Some of these sites are surround by fences. Many lesser known figures, however, are protected only by their remoteness and the conscience of the occasional visitor.

link to fisherman and "modern geoglyphs"

I am fortunate to be able to fly over them because I can study and enjoy them without rolling over so much as a single pebble. And from the air, they take on new life. If unfenced, they seem to be part of the whole landscape around them, and I find myself examining the nearby topography to learn something more about why they're in this place. Fenced, they become a singularity. Our footprints press as close as possible, adding a bold contemporary element to the composition -- the lasso of isolation. Our prosperity makes such measures necessary if these prehistoric expressions are to survive the storm of our civilization. But in an environment that most of us would consider unlivable without the comforts afforded by electricity and petroleum, the earth figures challenge us to imagine relatedness and to allow for mystery. Although the facts of their origins may forever elude us, they remind us that our own impulse to mark the world with our purposes is ancient, powerful, and enduring.