Receiver-function analysis across the Cheyenne belt, an Archean–Proterozoic suture, images a 100-km-wide zone of 50–60-km-thick crust. This thick crust is thought to be a remnant from the original 1.8 Ga suturing event. The thick and presumably buoyant crust is remarkable in that it is not associated with a large topographic or gravitational anomaly, suggesting isostatic balance and a high-density mass at depth to compensate the crustal root. These features could account for the lithosphere's long-term stability. The presence of eclogite below the crustal root is a likely source for the high-density mass, as it would not produce a seismically unusual Moho, yet it would provide the required excess mass.