Driving into Farmington from the south, the terrain changes character around Bloomfield. The hills take on a gray-green color — the Mancos Shale outcropping and weathering at the surface. The soil is dark gray to olive in cut areas, with a greasy texture when wet that tells you immediately what you are dealing with. In residential areas, you see the evidence before you see the foundation: driveways that have humped up, sidewalks that have cracked and heaved, garage floors that are not flat. The pattern repeats throughout the Farmington, Aztec, and Bloomfield area.

The Mancos Shale contains smectite clay minerals — specifically sodium montmorillonite — that expand dramatically when they absorb water. Volume changes of 10 to 15 percent are documented. In a dry year, the soil shrinks and foundations settle. In a wet year or when irrigation is introduced, the soil swells and foundations heave. The cycle repeats. Most visible foundation damage in the San Juan Basin is from heave, not settlement — but because heaving soil eventually drops back when it dries, the visible pattern often looks like settlement to the untrained eye. This distinction matters enormously for repair decisions. Swell potential testing is the only way to confirm the mechanism.

In Bloomfield and along the San Juan River corridor, the picture is more complex — river terrace and alluvial fan deposits sit above or adjacent to the shale, and these materials are collapsible. The first significant wetting event can produce rapid hydrocompaction in soil that has been stable for decades. Two hazards, same basin, opposite mechanisms.