In his search for the routes used by the West's early travelers, archaeologist Jack Pfertsh has become a detective of detritus. Today he's on the hunt for old tin cans and fragments of purple and green glass. The mid-November sun is sinking as we walk the windswept land just north of Delta, Colo. Brown grasses like broom bristles throw long, sharp shadows across the rolling plateau. Pfertsh points out a slight matting and thinning of the vegetation -- a faint path running south, its edges lined with chunks of volcanic rock.

"Once you know what you are looking for," he says, "you start seeing these old roads everywhere, cutting across the land." The 49-year-old Pfertsh is tall and lean and wears a newsboy cap and black T-shirt. He gestures with quick hand-chops, the overlapping tribal-style tattoos on his arms adding a tinge of color to the landscape's sea of beige. He's spent most of the last two years seeking out faint patterns on the land for Montrose-based Alpine Archaeology. The ones he's showing me today happen to be remnants of one of the West's earliest, most important and least documented road systems: the Old Spanish Trail.

The trail runs for some 2,700 miles across six Southwestern states and was designated as a national historic trail by the National Park Service in 2002. Its period of "historical significance" lies between 1829 and 1848, when traders followed it from New Mexico to Los Angeles to swap woolen goods for California-bred horses. "The Old Spanish trail physically tied these distant settlements together in a way that had never been done before," says Glade Hadden, Bureau of Land Management archaeologist and lead manager of the trail through Colorado. "It created the whole concept of North America as a unit."

Today, it cuts across the mosaic of the modern West -- through sprawling suburbs and isolated towns, across oilfields and open range, reservoirs and free-flowing streams. Like a palimpsest, it preserves the overlapping writing of many eras. The result can be dizzying, a confusion of times and places -- a Zane Grey Western set to a thrash metal score.

The Old Spanish Trail is not the only historic Western trail threatened by development. Wyoming's South Pass, a break in the Rocky Mountains used by hundreds of thousands of wagons during the mid- and late-1800s -- perhaps the best-known trail segment in the entire country -- lies in a corridor long coveted by mining and energy companies. In 2003, on the 200-year anniversary of the Corps of Discovery, the Sierra Club led an initiative to protect the still-intact stretches of that historic route. Meanwhile, many of the landscapes of the "great unknown" explored by Lewis and Clark are now largely unrecognizable, inundated under vast reservoirs.

As development permanently alters more and more of the West, public-lands agencies are scrambling to identify and preserve these traces of the past. Using an array of technology -- from century-old government plats and expedition reports, to aerial photography and GPS -- Pfertsh and his colleagues have scoured hundreds of square miles in Colorado and Utah, racing against time. "It's become critical to figure out exactly where (the Old Spanish Trail) goes and to document it," says John Horn, the principal investigator at Alpine Archaeology. "The physical traces we see today are fading quickly. They probably won't be here in another 50 to 100 years."

To fill in the gaps, Alpine Archaeology and two other archaeology firms were commissioned in 2010 by AECOM, an engineering and cultural resources consulting firm, under a two-year contract from the BLM to study 400 miles of the Old Spanish Trail on public lands in six states.

The trail passes through or near 11 national parks, 15 national forests and one national wildlife refuge. Further complicating oversight is the fact that half of it lies on state, city, tribal or private lands. AECOM's findings will eventually be used to create a comprehensive management plan headed by the National Park Service that will coordinate the various agencies involved. For example, a "trail certification" program will allow the government to partner with private landowners to protect and provide access to important trail sections.

Aaron Mahr, superintendent of historic trails for the National Park Service's Intermountain Region, says the on-the-ground work will help the agencies better understand the cultural resources on their land -- and ensure that there's proper  review  when  those  resources are threatened. "We are not in a position to stop development," says Mahr. "But in areas where there is development without consultation, we stand the risk of losing very valuable trail resources. We need this information to make good decisions."