Portillo—the facilities chief at Fort Union National Monument in northeastern New Mexico—leads me around to the north side of the wall. Here, prevailing winds, a lack of moisture-fighting direct sun, and snow melt have resulted in erosion. Large sections of “shelter coat” (made of soil, sand, and water, sometimes with a chemical binder) have cracked and, in some places, sheared off, allowing original adobe bricks, molded and stacked more than 150 years before, to emerge. The grass and sand that bind them are nubby to the touch.

In the 1860s, this wall protected United States Army soldiers garrisoning Fort Union, as well as travelers and settlers in the region. Today, it is one of more than 60 standing remnants of the fort, revealing the long and complicated process of adobe preservation.

I’ve come to the Southwest to research a book I’m writing about the Civil War in Colorado and New Mexico. I am particularly interested in the forts where my book’s protagonists (Union and Confederate soldiers, Navajos, Apaches, surveyors, and gold miners) spent time during the 1860s. Wandering through these places today, you hear mostly the chirping of birds and the crunching of your shoes on gravel pathways, but it was a series of violent clashes that brought many of these structures into being.

In 1821, Mexico’s newly gained independence from Spain opened up commerce between Anglo and Mexican merchants, hunters, and trappers along the border. These men needed places to conduct trade and protect their goods, so they built commercial “forts” throughout the region. Twenty-five years later, the Mexican-American War and two subsequent events, the Treaty of Guadalupe Hidalgo and the Gadsden Purchase, brought New Mexico, Arizona, and parts of Colorado and Utah into the U.S. In order to guard these new holdings, the federal government built military forts along major waterways and roads.

Whether commercial or military in origin, the adobe forts typically consisted of one- and two-story buildings organized around a central plaza or parade ground, with living quarters, animal corrals, and storehouses for food and trade goods. They were located on well-traveled thoroughfares: the Santa Fe Trail, which brought travelers around the Sangre de Cristo mountains to the New Mexico capital; El Camino Real de Tierra Adentro (the Royal Road of the Interior Lands), which moved along the Rio Grande from Mexico to Santa Fe; and trading paths that wound through the Rocky Mountains.

Southwestern forts were meant, in part, to symbolize the power of Anglo-American traders and soldiers and their intent to “civilize” the region. But the adobe forts, in particular, were also material evidence of the cultural mixing that colonization produced. American Indians in the area had first used adobe—a blend of sand, clay, and silt—to build their pueblos, as wood and stone were scarce. When the Spanish missionaries and conquistadors arrived in the 1500s, they introduced molding forms to native communities. Artisans could now manufacture adobe bricks, which would cure more quickly in the sun. Structures made from these bricks—forts, but also churches, stores, and dwellings—were and are distinctive forms of regional architecture. Travelers from the eastern U.S. in the mid-19th century knew they had crossed from the Great Plains into the Southwest when adobe buildings began to appear along the trail.

Bent’s Old Fort outside La Junta, Colorado, towers above the banks of the Arkansas River. The “Castle on the Plains” was a convening site for Anglo, Cheyenne, Arapaho, and Mexican trappers and traders to barter animal skins and other supplies: flour, wagon wheels, blankets, and firearms.

As I walk through the gate and into the fort’s central plaza, I can smell the cook fire before I see its smoke, billowing up into the air above the adobe walls. A stocky, dark-haired man dressed in ragged-cut woolens greets me. His name is Joseph Frausto, and he is a living historian who portrays a Mexican laborer working at Bent’s Old Fort in the 1840s. Frausto leads me and several other visitors on a tour, demonstrating blacksmithing and adobe brickmaking.

Bent’s Old Fort was a vibrant place of trade, but the war with Mexico in 1846, the death of co-owner Charles Bent, a cholera epidemic, and the decline of the buffalo population all led to its eventual abandonment. By the 1920s, the fort’s ruins were destroyed by the region’s harsh weather; only the foundations remained. When the National Park Service declared Bent’s Old Fort a historic site in 1960, it turned to archaeological evidence and archival documents—sketches, diaries, and a blueprint—to help it reconstruct the buildings.

During my visit, most of the rebuilt adobe walls appear to be in good shape. But as I look more closely, I can see the marks of past preservation efforts and failures. Lumps of concrete stucco added in the 1980s bulge out of the adobe surface on one of the fort’s parapets, while huge sections of shelter coat, also known as adobe plaster, are sloughing off the outer wall in sheets. Rick Wallner, chief of interpretation, says that despite annual re-plastering and frequent patching, the staff is fighting a constant battle against a most formidable adversary: nature.

Strong winds take their toll on adobe, but its greatest enemy is water. Rain and groundwater seepage attack walls from above and below, finding tiny crevices and pushing through the layers of earthen bricks and shelter coats. The resulting fissures erode the adobe walls from the inside.

I find similar conditions at Fort Garland, nestled in a mountain valley 130 miles west of Bent’s Old Fort. Fort Garland was a frontier outpost, built in 1858 by Hispaño artisans and U.S. soldiers. They worked together to construct 22 adobe buildings on a path used by trappers, traders, and Utes. Once completed, the fort had an unobstructed view in all directions, ideal for defense against raids.

Today, Fort Garland retains its views of the mountains and the San Luis Valley. At the center of its five preserved (and one reconstructed) adobe buildings is a grassy parade ground, where Union Army volunteers from Colorado trained during the winter of 1861–62 before leaving for Santa Fe and Albuquerque.

After the Civil War, Fort Garland housed “Buffalo Soldiers”—African-American cavalry regiments—as well as Anglo and Hispaño troops, who policed the interactions between Utes, settlers, and miners. After the Utes were forced to move to a reservation, the need for troops declined. And once the railroad came to the region, the fort was no longer needed as a center for commerce. It was decommissioned in 1883, and many of its buildings deteriorated beyond repair or were demolished. Local preservationists campaigned to save the remaining structures, and the Colorado Historical Society (now History Colorado) acquired the site in 1945. Artisans patched and plastered walls and inserted new adobe bricks where necessary. But this initial restoration was just the beginning.

Anita McDaniel, director of the Fort Garland Museum, joins me on the parade ground, pointing out recent re-plastering work on the adobe buildings. The spring of 2015 brought torrential rainfall to southern Colorado, wreaking havoc on the fort. “Adobe preservation is a process, not a one-time thing,” McDaniel says.

Fort Union, Fort Garland, and Bent’s Old Fort each sit among miles and miles of undulating grasslands dotted with scrub brush and frequented by herds of antelope. But their isolation has served them well. Because they are so far from urban centers and sprawling suburbs, they still exist to tell the modern-day tale of people fighting with the elements to save the material remnants of the past.

On my way out of Fort Union, I try to imagine what it must have been like to see it for the first time, rounding a bend in the Santa Fe Trail in a wagon, looking forward to a night spent inside its walls after weeks on the road. As I head back toward the highway, I can see the adobe ruins in my rear-view mirror, still gathering the warmth of the sun’s rays.

Megan Kate Nelson is a writer and historian who has contributed to The New York Times Disunion blog, The Chronicle of Higher Education, and Civil War Times. She grew up in Colorado and is working on a book about the Civil War in the Southwest.