FINE DISTURBANCES; ON THE BORDER

JEFF TIETZ. The New Yorker.

United States Border Patrol operations above the Texas stretch of the Rio Grande often begin with a single tracker on foot, staring at the earth. In the Border Patrol, tracking is called cutting sign. "Cutting" is looking; "sign" is evidence. No technology is involved. Trackers look for tread designs printed in the soil and any incidental turbulence from a footfall or moving body. They notice the scuff insignia of milling hesitation at a fence and the sudden absence of spiderwebs between mesquite branches and the lugubrious residue of leaked moisture at the base of broken cactus spines. (The dry time is a stopwatch.) The best trackers know whether scatterings of limestone pebbles have come off human feet or deer hooves. They particularly value shininess--a foot compresses the earth in one direction, which makes it shine, and wind quickly unsettles this uniformity, so high-shining groups are irresistible.

Low-light cameras and night-vision goggles and thermal-imaging scopes and seismic sensors are useful along the river, but in the brushland north of it pretty much the only thing to do is follow illegal immigrants on foot. Nearly every southern Border Patrol station maintains a network of footprint traps called drags--twelve-foot-wide swaths of dirt that are combed every day with bolted-together tractor tires ball-hitched to the back of an S.U.V. Agents monitor drags endlessly and follow foreign prints into the brush.

One morning just after dawn, I was out with an agent named Mike McCarson. He was driving a customized Ford F-250 down a ranch road, cutting a drag that ran alongside it. The Rio Grande was close; the sky was unusually congested. The drag and the ranch road extended out of sight through mesquite and prickly-pear cactus and purple sage and huisache and whitethorn, all semi-aridly dwarfish: the horizon was visible everywhere. Driving about eight m.p.h., McCarson leaned out the window a bit, his hinged sideview mirror folded flush with the door.

McCarson is forty-three; he has been tracking illegal immigrants for eighteen years. He works out of the Border Patrol's Brackettville station, which is about twenty miles northeast of the Rio Grande and thirty miles east of the town of Del Rio. Brackettville's hundred and twenty agents are responsible for a twenty-five-hundred-square-mile rectangle of mesquite flat and limestone breaks. No more than forty agents are ever in the field at once--one per ninety square miles.

Federal guidelines recommend that undocumented aliens be apprehended at least five times before they're charged with illegal entry and held for trial, but Brackettville's agents rarely detain people unless they've been caught more than fifteen times. On any given trip, an illegal immigrant's odds of eluding Brackettville's defenses are extremely good. Immigrants can angle in from anywhere along a fifty-mile stretch of the Rio Grande, and overpowering numbers of people often cross the river simultaneously. Brackettville doesn't have the personnel to track more than five groups at once.

"It's just the luck of the draw which ones we chase," McCarson told me as we drove. "We generally don't even work groups as small as three, four people, or groups that happen to have crossed real early the night before."

I spent many days with McCarson, cutting drags and trails and meandering through mesquite in the F-250. He is a big, sauntering, freewheelingly mouthy guy who says "tars" for "towers" and "boo-coo" for "beaucoup," a word he uses a lot, sometimes as a noun ("There's boo-coos of trails through there"). McCarson instinctively registers the happenstance, ambient comedy of ordinary life, sometimes with a falsetto, arc-of-joy cackle. He likes to stop for lunch on hillsides and in old Seminole cemeteries, and he can be precipitately melodramatic. ("Now, the seasoned journeyman agent will see the totality of circumstances in his area and know precisely what's normal activity and what's not.") McCarson is married but doesn't have kids, which may partly explain his widely entrepreneurial imagination: he owns a scuba business on Lake Amistad; he's a partner in a San Angelo real-estate venture; he's a successful self-taught stock picker.

On the drag, we were seeing a lot of overlapping cloven-hoof sign, like combinations of the alphabet's last four letters. Agents have found sign of people crawling across drags on their hands and knees (long, chutelike impressions), and sign of people tiptoeing across drags (shoe-tip abbreviations), and sign of people walking across drags backward in socks (blunted images, kicked soil revealing the true direction of travel). Someone once walked across a drag on his hands (profound, torqued handprints); the man was followed from the drag to a railroad siding, where the prints transformed themselves into elongate gouges in the embankment stones: running hard, he'd jumped a freight. Sometimes, illegal immigrants will survey drags until they find a particularly rocky stretch and cross there, stepping only on stable rocks, as if they were fording a stream. If unyielding terrain frames the rocks, they leave almost no sign.

Ideally, a tracker will find fresh prints on a drag, follow them until he can establish a line of travel, and radio in the group's drag coordinates, its heading, and its tread profile. Then another agent will cut the group on a drag some miles north or northeast--all the sheltering towns and safe houses and freight tracks and pickup points lie that way--and either catch it or radio in a revised line, allowing a third agent to leap up to the group. In practice, it's often impossible to categorically identify a group, establish a line, or get a forward cut.

Because the surest way to identify a group is by its distinct assembly of tread patterns, trackers use a lexicon of soles: super chevron, racetrack, wishbone eight ball, propeller, Tetris, basket weave, Kmart special, rifle sights, Flintstones, hourglass, running W, matchstick. When I was out with McCarson, the radio regularly issued sole descriptions: a "diamond-within-a-diamond heel with an instep logo," an "island running W with a slash in the middle and a fine line," a "beetle pincer in the toe." The increasing complexity of sneaker-sole designs frequently creates neologisms. Regions, and sometimes stations, have their own dialects: a sole dominated by lugs in the shape of a Z is a "zebar" in Brackettville and a "zorro" in Del Rio.

We cut a blank stretch for a while. "It's gonna be a little quiet," McCarson said. "We haven't had much moon." The moon is a prime instrument of navigation, because it illuminates without stark exposure. (Illegal immigrants also use the sun, stars, and diverse landmarks.) A few minutes later, McCarson stopped the truck. "Well, we got some crossers," he said.

The prints were shallow and had been scrambled by superimposition; it took McCarson about thirty seconds to disentangle the tread designs and stride lengths and foot sizes. I couldn't find a discrete print. "Group of five," he said: running W, matchstick, heavy-lug boot, fine wire mesh, star-in-the-heel waffle. "They were here last night. They're fresh, but they're not smokin'-hot fresh. When the ground is retaining moisture like this, and you don't have much wind, the sign can hold its shape for a long time. But I see a little better color in our prints than in theirs." Foot pressure, concentrating moisture in the soil, darkens prints; the clock of evaporation steadily lightens them. I couldn't see any color difference. "They could be anytime last night," McCarson said. "And we've had the correct amount of moisture over the correct amount of time to make the ground about like concrete, so they're not leavin' much sign."

If drag prints aren't decisive, trackers examine a group's sign along the first stretch of trail, wary of the corrosive or stabilizing effects of weather and terrain--hilltop wind withers prints; damp arroyos embalm them--and noticing things like insect crossings and preservative soil composition and raindrop cratering. Every left-behind object is a potential timekeeper. There are rare, unambiguous tokens: jettisoned cans of beans that ants haven't yet noticed, sunlit Kleenex still clammy with mucus, fresh bread crusts on a hot, clear day. But most often agents encounter noncommittal objects--desiccated bread crusts in full sun which could be two or ten hours old.

Swinging the shepherd's cane he uses as a tracking aid, McCarson started cutting. Just beyond the drag was a strip of mesquite scrub, and then freight tracks on a high gravel embankment. In the scrub, the prints disappeared abruptly, as if the group had been choppered out. But McCarson read a carnival of sign through the scrub and followed a sequential displacement of gravel up the embankment. He pointed to a railroad tie and kept going. Possessed by the live trail, he couldn't pause to explain.

I sat down and studied the tie. It looked like every other tie. I began shifting position relative to the obfuscated sun. Eventually, three stacked Ws of discoloration, collectively the size of a half-dollar, shimmied into view.

On the other side of the tracks, McCarson was standing in front of a fence, methodically locating the crossing point with his cane. Beyond the fence, the mesquite gave way to scrofulous pastureland. The wind picked up and a stop-start drizzle began. Any moderately heavy rain melts sign, toughens vegetation, and hardens the arid earth. McCarson walked along the edge of the scattershot grasses for a while, hesitant, and then accelerated.

He was mainly scrutinizing stalks of buffalo grass and curly mesquite grass and king ranch bluestem. The group had pressed the grasses forward. Now at a vestigial angle, the stalks reflected the light more directly, like opened compact mirrors, whitening it. McCarson described this effect later; for the moment, my experience remained completely secondhand--even after he had amplified the sign by walking right over it, my glances fell nowhere.

I could see that moisture had strengthened and limbered up the grasses: instead of breaking and lying inert after receiving their foot-blows, which makes for the brightest reflection, they had been rebounding in slow motion for many hours, and had nearly regained their posture, although adhering rain droplets now pulled them down like sinkers and a jittery wind ceaselessly repositioned them. Under the dingy, dropped-down clouds, the reflective power of the grass was negligible, and the rainy light shrank the color differences.

The mesquite thickened; the grasses faded out; the static of drizzle hardened the ground and corroded the sign. "This is some tough cuttin'," McCarson said, neutrally. The terrain was too stingy for prints, but the walkers had scored the earth with the soles of their shoes, and McCarson was following the scuff marks, which were often the size of fingernail clippings and about as much lighter than the surrounding earth as a No. 2 pencil is lighter than a No. 1.

In cow shit or ant-processed dirt or hoof-crushed earth, McCarson found fractional footprints. Largely dismantled by drizzle and generally separated by a quarter mile, they were about the size of suit buttons. For retracing purposes, he marked them by scraping a line in the adjacent dirt with his cane. He kept moving and held the silence of his concentration, but there was satisfaction in the absoluteness of the stroke and the granular conclusive sound itself: shhhick!

The ground was still hardening; the rain-erosion got worse. McCarson slowed, occasionally bending down a little and poking something diagnostically with his cane, but he never crouched and almost never came to a full stop.

"Well, we're just suckin' on the hind teat on this one," he said. A minute later he said, "Oh, by God, that's them!" and accelerated down an alley of spectral sign. Five minutes later, he slowed again. "The odds of following these five are pretty slim in these ground conditions," he said. "We're losin' so much time just tryin' to stay on the sign, we won't really catch 'em unless we take a risk."

So we held their line and walked fast down numberless, seemingly uninstructive trails. McCarson's trail choices appeared to be random, but he was sensing the path of least resistance, relating it to the group's hypothetical line of travel and behavioral tendencies, and occasionally seeing candidate sign. A few times, where the ground turned permissive, we swept perpendicularly back and forth, hoping for trapped sign. After fifteen minutes, McCarson said he was beginning to doubt the trueness of the line.

We walked windingly for maybe a mile. Suddenly McCarson stopped, reached down, and picked something up. All the quick walking had reminded me that we were operating in undifferentiated wilderness the size of two Rhode Islands. McCarson stood and opened his palm. In it was an aspirin-size mud clot distinguished by a sole-honed ridge with a strict curvature. He handed it to me and kept going. A ludicrously deteriorated trail of disturbance had nonetheless held its integrity, in McCarson's eyes, for about five miles. The clot seemed talismanic. But we never got a forward cut, the weather didn't improve, and the trail died.

"We never know who we're gonna catch," McCarson told me on the way back to the station. "The weather plays a tremendous role. You get into drought conditions and you can run groups until you lose daylight and never stall out. A hard rain and you lose all your sign. It's just shithouse luck if we catch 'em.

"Our real effectiveness is in actin' as a screenin' mechanism--we're a deterrent, which is not something you can really see out in the field, and some agents that might not love tracking get really hung up on that. They can't get over it, and they turn into sorry, disgruntled agents. What this job boils down to is desire--you started with a hundred sets of tracks on your drags, and you're trying to get ten. It's not a factory job--there's no boss, you're not stamping out x number of product. What determines that you should try to catch those ten when ninety are already gone? I know what you really wanna ask, which you haven't asked it yet, is how many get away. Well, they all get away. That's the answer. Eventually, they all get away."

McCarson grew up working on his grandfather's ranch, outside Comstock, which is about fifty miles northwest of Brackettville. The ranch didn't keep him entirely busy; as soon as he was old enough, he began hiring himself out to bigger local ranches, where he worked through the daylight hours and then traded money at cards and dice in bunkhouses until he could spend it in town on his day off. He never felt a desire to leave southwest Texas. He studied government at Angelo State University, but by the time he graduated livestock-raising in the region had lost most of its commercial viability.

McCarson had always liked the gunslinging look of Comstock's Border Patrol agents, and noticed that they often worked without supervision. "That's what really did it," McCarson told me. "Because I liked to hunt and fish, and it looked more like huntin' and fishin' than workin'--they'd give you a vehicle and you were on your own."

When he began tracking, he saw immediately that his hunting skills were almost useless for following people. Border Patrol trainees learn to track informally and opportunistically, tailing journeymen on live trails and asking questions as circumstances permit. McCarson augmented the process by walking exhausted, still-legible trails whenever he could, as far as he could, which is generally encouraged but not frequently undertaken because agents have to do it on their own time.

"Everything you need to know about tracking I can explain in about two sentences," he told me. "You're evaluating the ground for the difference between the disturbance made by humans and the disturbance made by any other force. After that, it's all practice. It's all just looking."

A planetary difference in vision separates great trackers from ordinary trackers. At Brackettville, McCarson and about a dozen other trackers occupy a paramount plane. "There's just a certain level you get to, where you can't say who's better," McCarson says. "And I can't even tell you what that level is as far as particular sign--it's just, if there's something there, we'll see it."

One slow day, after driving the length of a blank drag, McCarson gave me a concentrated lesson. We parked at the edge of a ranch road. In the adjacent dirt, day-old prints held faintly for a while and then disappeared into mesquite. McCarson checked the angle of the sign and pointed at the horizon, toward the group's destination. Then he pointed at a deer trail. "I can see all kinda sign through there," he said, and strolled off to make a cell-phone call.

But there was nothing to see: vacant inflexible earth, sparse mesquite-branch detritus, mesquite leaves in inconsistent profusion, various and occasional flat square-inch plants--beggar's-lice, horehound, hedge parsley--and rare tufts of short grasses. All color fell along a drastically shortened mustard-olive-ash continuum. The longer I went without seeing anything, the harder I looked at tiny things close by, and the more obdurately flawless they seemed.

McCarson ambled back. "You wanna see more but you ain't," he said. "That's the maximum the earth's gonna give you." He aimed his cane at some beggar's-lice. I got close and saw that several stalks had been nudged forward and now leaned at a slight angle, maybe thirty degrees from their natural posture. McCarson pointed to two seedpods the size of ball bearings which had exploded under downward pressure. He pointed to a half-inch mesquite twig that listed a little; its bark at the contact point was a fine-particulate smear. He pointed to a piece of ground the size of a playing card: half the stalks in a tuft of buffalo grass were stabbing ardently forward, and the adjoining earth had also been compressed--once spherical granules and clods of soil urged down toward two dimensions--and this compression was continuous and equivalent in degree through the two mediums of grass and earth.

These pygmy symbols were all within six feet, but they were isolated, camouflaged, and enclosed by a lot of pristine terrain--they didn't relate to one another directionally. McCarson waited for a moment while my eyes struggled to absorb them, and then strolled off. My eyes didn't absorb anything. I was afraid to move and contaminate the sign: I'd been immobilized by tininess. From somewhere in the brush, McCarson said into his cell phone, "Did those spearguns come in yet?"

After a while, he came back and pointed to a small area in a patch of mesquite leaves, which are very thin. I couldn't see anything; the leaves appeared to be flush with the ground. Then I got to within a few inches and noticed that they hovered microns above it, as if they'd been levitated by some exceptionally weak force--static electricity raising arm hairs or surface tension holding water above a glass rim. McCarson's area--he kept his cane trained on it--was composed of undamaged leaves that were truly flush with or partially embedded in the earth. The compacted region was a few inches long and about the width of a shoe sole.

"See those dingleberry bushes?" McCarson said. They were the size of Ping-Pong balls, and from a distance I couldn't tell that they'd been crushed: in two dimensions, they retained their fundamental color and some unbroken seeds and a remnant network infrastructure, the way road-killed toads can look like living toads minus a dimension. Up close, I saw fine fragmentation and a uniformity of flattening--a broad pad of compression--that only a shoe, not an angular hoof or a puncturing paw, could have made.

Sign cutting is overwhelmingly hushed and uneventful, but the screen of the ground shows action. Trackers find bits of skin and sock on the fishhook spines of horse-crippler cactus and watch the stride transformation and inexorable decline in mobility as the injury worsens. They watch as groups exhaust themselves and start resting more frequently, and as disoriented or fractious groups splinter. The day after a moonless night, the ground shows walkers equivocally chafing their way through mesquite thickets.

Agents watch as weak people stop so that everyone else can go on, and as they later stand up and try to keep going or give up and try to get caught. Very occasionally, in summer, the ground leads Brackettville agents to corpses. Death in the brush is notable for the attempts that dying people make to undress: a dehydrated, overheated body swells as death approaches, so the dying remove their shirts and shoes and socks, and unbuckle their belts. They seem to find comfort in order, folding items of clothing and arranging their belongings. They prefer to die under the boughs of trees, on their backs.

After a few nights sleeping in the brush, people emanate an odor of campfire smoke and sweat that can float for hours over an abandoned campsite. In wetter weather, the smell of canned sardines persists for a day. Cutting trails, agents step over shit and piss and blood and spit. They find handkerchiefs, pocket Bibles, bottles of Pert Plus, and photographs of daughters with notes on the back ("Te extrano. Regresa a casa pronto. Te Amo, Isela"). They find message-board graffiti on water tanks ("23/2/03 Por Aqui Paso Costa Chelo Felipe L. Miguel"), and the roasted remains of emus, doves, jackrabbits, and javelinas. The great majority of illegal immigrants are adult men, but sometimes agents find diapers and tampons and tiny shoes.

As trackers move through diffuse fields of abandoned objects, fixing the age of the sign, they assure themselves that they're walking through a group's recent past. They want to walk right into its present. They want the sign to turn into its authors. On live trails, the metamorphosis feels imminent, because it always could be. Groups stop unpredictably; if your group lays up at the right time, you'll find yourself disconcertingly deep in a marginal trail yet two minutes from an arroyo filled with snoring. The people you're chasing might appear on the other side of every rise you crest.

Because sign is so shifty and strongly insinuating, it's hard to avoid equating the trail with the people it evokes. Trackers say they're "running" or "chasing" a trail; they say they just "caught" a trail; they say, "That trail got away." The language of tracking treats the sign at the leading edge of a trail as the group itself. After cutting a trail's frontier, trackers say things like "They're in this pasture," or "They're right there trying to find a good spot to jump the fence," or, simply, "They're here."

When you start to see sign, everything unrelated to the trail vacates your mind. Sign cutting is a vigil with no clear object: the sign mediums continuously reconstitute themselves. You often find valuable dominant indicators, but you have to will yourself to remain receptively nonpartisan; otherwise you'll steadily grow blind to divergent marks, and terrain changes will instantly cloak the trail.

Eventually, the microworld entrances: every plant has distinct attitudes and behaviors beyond the obvious--the way it holds its berries, carries and orients and discards its leaves, shrivels and responds to wind, bends with the weight of raindrops. Soil classes reorganize themselves uniquely after a rain. Rocks erode and array themselves in singular patterns. And color and form at that scale are infinitely variable: a cluster of scrub-oak leaves is a thousand shades.

But every medium has, beneath its variability, a composite architecture and a native range of hues, and this is what you have to see, because divergence from it is sign. If you remain rigidly zoomed in, you pick up the endless variations and they hypnotize you and veil the composite. You can't memorize precise schemes of coloration and structure--although the best trackers hold in their heads very good approximations of the hybrid aesthetics of scores of terrain types--but you can learn to see how particular facets of unadulterated landscape acquire a range of colors and shapes over time, so that when you look at a pasture or stream bank or anthill you grasp its physical essence and all its natural deviations, and interloping shapes and colors quickly declare themselves to you. When rain liquefies the ground beneath branch litter, the twigs sink and the liquid soil adheres to the million unique contours of wood and bark and stiffens into a perfect seal. Later, running the trail after the ground has hardened almost beyond compressibility, you'll unconsciously seek out broken seals: millimetre-wide earthworks of shattered crust.

After about an hour and a hundred confirmed sightings, the trail's autonomous sign began to cohere. First it was a stirring paraphrase of recent movement, and then an expression of willfulness pressed into the ground: the overarching intent of a journey. This filled me with an almost violent exultation whose energy was instantly focussed on the next span of information. I couldn't help experiencing sympathetic sensations: I'd see a sneaker-cracked branch and feel it breaking underfoot; I'd see a recently embedded rock and feel my sole bending over it.

McCarson gradually taught me to look for the identifying rhythm of the trail. Group size and behavior correspond to a certain frequency of potential sign; the potential is realized according to the receptivity of the terrain. If two people walk fast and abreast of each other over hard ground while concealing their sign, they'll leave transparent traces; if fifteen people ingenuously plod single-file over impressionable earth, they'll basically plow a new road. Our linear group of five was mainly concerned with speed; they didn't brush out on drags or attempt to walk along the hard edges of animal trails; they didn't bother to avoid print-trapping sandy soil. They didn't stop, and they left nothing behind. Without quite realizing it, I began to think of them as professionals, moving wordlessly in a kind of improvisational accord.

Ultimately, I began to see disturbance before I'd identified any evidence. A piece of ground would appear oddly distressed, but I couldn't point to any explicit transformation. Even up close, I couldn't really tell, so I didn't say anything. But I kept seeing a kind of sorcerous disturbance. It didn't seem entirely related to vision--it was more like a perceptual unquiet. After a while, I pointed to a little region that seemed to exude unease and asked McCarson if it was sign. "Yup," he said. "Really?" I said. "Really?" "Yup," McCarson said.

McCarson experienced this phenomenon at several additional orders of subtlety. In recalcitrant terrain, after a long absence of sign, he'd say, "There's somethin' gone wrong there." I'd ask what, and he'd say, "I don't know--just disturbance." But he knew it was human disturbance, and his divinations almost always led to clearer sign. Once, we were cutting a cattle trail--grazing cows had ripped up the sign, which had been laid down with extreme faintness--and McCarson pointed to a spot and said, "I'm likin' the way this looks here. I'm likin' everything about this." He perceived some human quality in a series of superficial pressings no wider than toadstools, set amid many hoof-compressions of powerfully similar sizes and colors and depths. I could see no aberration at all, from five inches or five feet: it was a cow path. But McCarson liked some physical attribute, and the relative arrangement and general positioning of the impressions. Maybe he could have stood there and parsed the factors--probably, although you never stop on live trails--but he wouldn't have had any words to describe them, because there are no words.

A few days later, some agents were chasing a group of twelve who had crossed the river well after midnight. They had incised their tread marks on many powdered-up roads and drags--the chief identifier was "a motion wave with lugs around it in a horseshoe in the heel"--and tracking conditions in the brush were good, so at all points the line of travel was easy to establish and forward cuts came quickly.

As McCarson drove fast down ranch roads in quest of cuts, the radio transmitted updates: "I got 'em here at this deer blind--they got a real good shine on 'em." "They're crossing another road right here--hold on, there's some fresh toilet paper." "I got 'em laid up here in a real new jacal"--a branch shelter that illegal immigrants sometimes make--"and I got their campfire, still real warm." Around the fire were slick peach pits and cans with sardine juice in them--a scene so vibrant it was like seeing the group disappear around a corner. "This is lookin' like one of those rare storybook-endin' trails," McCarson said. "Just click-click-click-click."

Two cuts later, we were at the trail's apex, where six Border Patrol S.U.V.s and about ten agents had converged. The sign described a sudden dispersal into opaque brush: the group had heard its pursuers and taken off. The agents were staring contentedly into meshes of mesquite. A green-and-gold Border Patrol helicopter appeared and dropped to about twelve feet above the thicket, raucously rotor-washing everything; it nosed around the mesquite for about three minutes, and then the pilot's voice came stereophonically out of everyone's radio, saying the group was thirty yards away, prone beneath an absurdly undersized camouflage tarp. The junior agents surrounded the hiding place and yelled instructions; twelve depleted men crawled out. The agents told them to sit in a row, perfunctorily searched them, and began discussing transportation arrangements. The captives were all young men; without speaking, they moved from vigilance to dejection to resignation. They received permission to eat, and pulled out Cokes and canned corn and tuna and slices of Wonder Bread and plastic jugs of biologically tinted water.

When I was in southwest Texas, I watched Brackettville agents catch twenty-six people in five groups--all, except this one, by sensor or routine observation or accident. Every capture was quiet: no running, no resistance. In eighteen years, McCarson has drawn his gun two times. He has never fired it. The captured groups were representative: no one had drugs or warrants or enough previous captures to justify detention and criminal charges. The illegal immigrants were all adult men seeking work, tired and disinclined to flee: the chances of escaping after visual contact are slim, and the turnaround time is fast--the Del Rio sector runs a daily shuttle back to Ciudad Acuna. After the capture and pat-down, agents and immigrants, in a momentary common languor, stood around and talked sparingly about the weather or the river level or noteworthy episodes from the immigrants' voyage.

"We just got lost," an older man with a bronchial cough told me one cold dawn, after he and his four underdressed companions, loitering at a highway intersection, had been searched and ushered into a Border Patrol S.U.V. "The stars--the stars were our map at night, but look at the clouds. Look at the sky." It was impervious, and had been for the past two nights.

Once, I lay on my stomach in a mesquite thicket, waiting with two agents for a group of four that had tripped a sensor. The agents yelled and leaped forward only when the men were within ten feet; the group seized up and went submissively slack in a single motion. After the pat-down, one man began ruefully emptying from his pockets a collection of pretty rocks; for some reason, the agents and I looked away. Walking back to the truck, the men began heedlessly climbing a barbed-wire fence; one of the agents gave them a patient lesson in how to scale it. Another time, a guy whose group had been spotted from the air sociably handed an agent a big rattle and said he'd killed a six-foot snake. The agent examined the rattle deferentially, shook it, said, "That's a big snake," returned the rattle, walked the group of men to his truck, and locked them in the back.

Now McCarson and a few other senior agents were leaning against an S.U.V., watching the junior agents, who were watching the detainees finish their food. I was thinking that as trackers follow illegal immigrants, often right at the mesmerizing limit of what they can detect, they're mustering up the emotions and sensations of mercurial imaginary travellers, and then the imaginary is suddenly, alienatingly replaced by the real: men physically homogenized by days in the brush, all with the same propulsive need. It's a need no Border Patrol tracker will ever be able to identify with. Before the twelve men finished eating, McCarson walked back to his truck and radioed Brackettville to see if anything else was pending.