The central mass of the Raft River Mountains consists of Precambrian metamorphic rocks with Elba quartzite and interlayered schist on the southern slopes and Cambrian quartzite outcrops on the western part of the range. Below the Sublett Mountains the Phosphoria Formation, a basal phosphorite overlain by a thick sequence of chert and cherty sandstone, reaches its greatest thickness. Soils in the northern part of the forest are generally deep and highly fertile in lowlands but shallow and less so on steep slopes. In the Minidoka Ranger District, soils are generally productive, derived from volcanic and sedimentary material, shallow on steep slopes, and deep in the lowlands.