A residual aeromagnetic map of Idaho, western Montana, western Wyoming, southwestern Oregon, Nevada, Utah, western Colorado, and northern Arizona illustrates magnetic patterns that are related to regional geology. The magnetic map provides useful information on the development of the crust in this region of the western Cordillera since Precambrian time. A major feature of the map is a broad zone extending from southern Nevada to northern Idaho, where magnetic anomalies from basement rock are not apparent. This feature is here named the “quiet basement zone.” To the west of this zone abundant magnetic anomalies are produced by Phanerozoic intrusive and extrusive igneous rocks. West-trending zones of magnetic anomalies in western Utah and Eastern Nevada contain abundant igneous rocks and most of the known mineral resources of this region. The major magnetic anomalies in the area east of the northern part of the Basin and Range province and east of the overthrust belts in southeastern Idaho and western Montana reflect lithologic contrasts in the Precambrian basement with prominent northeast and northwest trends. A magnetic high in north-central Nevada and another over the western Snake River Plain suggest north-northwest-trending Miocene rifts. The eastern Snake River Plain and the Yellowstone caldera are part of a much more extensive northeast-trending feature here called the Humboldt zone. In east-central Idaho, Tertiary intrusive and extrusive igneous rocks produce large magnetic highs. The Idaho batholith has little magnetic expression, but the Boulder batholith and related volcanic rock of western Montana produce a major magnetic high.