Driving onto the Taos Plateau from the south, the terrain opens up dramatically. The soil is dark — darker than most of New Mexico — because the basalt weathers into iron-rich clay. The Rio Grande Gorge cuts invisibly through this plateau; from the road you see only flat open land until you are suddenly at the rim. In the Taos valley proper, old adobe construction tells you how the soil has behaved for centuries. Cracked walls that have been plastered over many times. Acequia corridors with heavy clay that has been wet for generations.

The basalt-derived soils contain montmorillonite — one of the most expansive clay minerals. It swells significantly when wet and shrinks when dry. At Taos elevation (around 7,000 feet), frost penetration reaches 24 inches or more, which adds a seasonal heave cycle on top of the moisture-driven expansion. Foundations that are not deep enough for frost — or that were built in the adobe tradition without continuous footings — move with every season. The pattern is often mistaken for settlement when it is actually heave. Shrink-swell behavior is the signature of these soils.

Mora Valley is off the usual routes — you have to intend to go there. When you arrive, the terrain is beautiful and the foundations are often quietly troubled. The valley floor soils are dark and organic from millennia of wet meadow conditions. The surrounding slopes are rocky and competent, which creates a false sense that the whole area is solid ground. Old structures on the valley floor have often been settling slowly for decades. Spring snowmelt concentrates moisture in the lowest areas precisely where structures tend to be built.

Frost depth in the Mora Valley reaches 36 to 48 inches at elevation, which is the deepest in New Mexico. Footings that do not reach below that depth will heave. Valley floor organic soils compress under load — slowly, unevenly, and often in ways that look like differential settlement because the organic content varies across a site. The combination of frost heave and organic compression can be hard to distinguish without proper evaluation: one pushes up, the other pulls down, and the foundation can show both patterns at different corners. Settlement from organic compression is a slow, ongoing process.

Raton is a working town at the base of Raton Pass. The older neighborhoods near the historic coal mining areas have a characteristic pattern: walls that have cracked and been repaired multiple times, structures that lean slightly, ground that has a subtle undulation if you look at it from the right angle. Not every structure has mine influence — the town extends beyond the old workings — but in the affected zones the evidence accumulates. The surrounding country is Cretaceous shale and sandstone country, and the Pierre and Niobrara shales are expansive.

Underground coal mines from the early 20th century — some mapped, some not — create subsidence risk in portions of the Raton area. As unmaintained mines deteriorate, surface subsidence can be gradual or abrupt. Separately, the Pierre and Niobrara shale formations that outcrop in the region contain smectite clay minerals that expand significantly under moisture. Expansive soil behavior throughout the broader basin is independent of mine influence. Angel Fire and the mountain communities do not have the mine hazard but do have frost depth and some shale expansion.