Root systems of crucifixion thorn (Koeberlinia spinosa Zucc.) were excavated on an alluvial-fan toeslope in a desert area of southern New Mexico. Highcarbonate toeslope deposits of late Pleistocene age overlie low-carbonate middle Pleistocene deposits of an ancestral Rio Grande. These two deposits and their soils provide markedly different geomorphic, pedogenic, chemical and physical environments for the development of tap roots and their branches. Roots descended through the fan toeslope sediments and penetrated the river deposits to a depth of at least 5·2m, much deeper than is usual for root penetration in this area. Remarkably, 2nd and 3rd order branches originated at depths of 2·3 to 3·8 m and grew vertically upward, branched profusely in the top 1 m of soil, and extended to within 10 cm of the soil surface. It is believed that occasional deeply penetrating soil water moves down channels once occupied by roots and other openings in the soil, and that this is a source of water for growth of the deeply penetrating roots, as well as for the roots that grow upward.