The Snake River Plain in southern Idaho is a major geologic structure of uncertain origin. Surface geology is generally well defined, but subsurface geology is poorly defined below about 500 feet. Rocks that underlie the plain form the framework for a regional ground-water system that supplies large quantities of water for irrigation and makes the plain nationally important in terms of agricultural production. The 15,600-square-mile Snake River Plain is a grabenlike structure that formed in middle Miocene time. The graben may have been formed by oblique extensional forces resulting from interactions between the North American and Pacific tectonic plates. The oldest known rocks underlying the plain, penetrated in a 14,007-foot-deep test hole northwest of Boise, are of middle Miocene age. Miocene volcanic rocks at the plain's margin that dip toward and underlie the plain were highly faulted and se- verely eroded before the plain was formed. Faults along the margins of the eastern part of the plain are not visible at land surface and have been defined chiefly by geo- physical methods. However, well-defined fault systems bound the western part of the plain. The eastern plain is underlain predominantly by Quaternary basalt of the Snake River Group, which is intercalated with sedimentary rocks along the margins. Basalt crops out or is less than 10 feet below land surface in the central part of the east- ern plain and is usually less than 100 feet below land surface elsewhere. Geophysical data and drillers' logs indicate that Quaternary basalt in the central part of the eastern plain is as much as 5,000 feet thick. A test hole about 10 miles northeast of the Snake River near Wendell provided the first information about deep subsurface stratigraphic relations in that part of the plain. The stratigraphic sequence penetrated in the test hole is similar to that in the north wall of the Snake River canyon be- tween Milner and King Hill. In that area, basalt of the Snake River Group thins toward the river and is underlain by sedi- mentary rocks and basalt of the Tertiary and Quaternary Idaho Group. The western plain is underlain mainly by unconsolidated and weakly consolidated Tertiary and Quaternary sedimentary rocks as much as 5,000 feet thick. Basalt also is present in the west- ern plain and is most extensive near Mountain Home. Quaternary basalt of the Snake River Group, which composes much of the Snake River Plain regional aquifer system, is highly transmissive. In the eastern plain, a thick sequence of thin- layered basalt flows yields large volumes of water to wells. Wells open to less than 100 feet of the aquifer yield as much as 7,000 gallons per minute; yields of 2,000 to 3,000 gallons per minute with only a few feet of drawdown are common. Transmissivity commonly exceeds 100,000 feet squared per day and, in places, 1 million feet squared per day. Large springs in the Snake River canyon between Milner and King Hill issue at the contact between highly transmissive pil- low lava and less transmissive underlying rocks. In 1980, ground-water discharge between Milner and King Hill, largely spring flow, averaged about 6,000 cubic feet per second. In the western plain, coarse-grained sedimentary deposits are thickest and transmissivity is highest along the northern mar- gins. The percentage of coarse-grained sedimentary deposits de- creases to the southwest, where lacustrine sedimentary deposits predominate. In most of the eastern plain, the upper part of the ground- water system is unconfined. At depth and in much of the west- ern plain, aquifers are confined. Across most of the plain, Quaternary basalt aquifers overlie aquifers in the Tertiary Idavada Volcanics and Banbury Basalt of the Idaho Group. The older volcanic rocks are typically much less transmissive than the Quaternary basalt. Faults and frac- tures are permeable zones for water storage and conduits for water movement. In places near the margins of the plain, the Idavada Volcanics contains important geothermal aquifers.