The central Wasatch Front, the major population center in Utah, is underlain by as much as 300 m of sediments of Great Salt Lake and its predecessor, Lake Bonneville. Much of this sediment, which is the foundation for major construction in the valley, consists of silicate clays that were deposited in saline water. Fresh ground water from the adjacent Wasatch Mountains has leached the saline clay, producing sensitive and extra sensitive clays in sediments of the central Wasatch Front. These clays contain montmorillonite and vermiculite and low-salinity pore water. Their extensive distribution in an area of known seismicity indicates that earthquake hazards are extreme.