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Compiled in the immediate postwar period by USGS geologists Clyde P. Ross and J. Donald Forrester, this was the first large-scale statewide geologic map of Idaho to be printed in full color. It remains one of the most ambitious regional syntheses issued prior to the tectonic reinterpretations of the 1970s. The map represents an attempt to consolidate the scattered geological surveys of the early twentieth century into a single, comprehensive reference for state-wide stratigraphy, structure, and economic potential.
The map uses over thirty lithologic and stratigraphic colors to delineate bedrock units from Quaternary alluvium to Precambrian metasedimentary series and igneous intrusions. Known, inferred, and concealed faults are indicated by linework keyed in the legend. At top, six cross-sections trace transects across key regions of the state, including the Snake River Plain, the Idaho Batholith, and the miogeoclinal fold belts of the north. An inset map titled “Bibliographic Index Map of Idaho” assigns numbered sources, offering transparency about the patchwork character of existing geologic data in the 1940s. This is an interesting and unusual commitment to record-keeping.
The map remains significant for illustrating the regional understanding of Idaho’s complex geology before the widespread adoption of plate tectonic theory. It encompasses the late Cenozoic volcanism of the Snake River Plain, the massive granitic exposures of the Idaho Batholith, the folded and faulted Proterozoic Belt Supergroup, and Basin and Range faulting along the southern border.
![[Bancroft's Map of the Pacific States]](https://storage.googleapis.com/raremaps/img/small/54924.jpg)
![Map of Oregon, Washington and Part of Idaho [ First Appearance Of Idaho Territory ]](https://storage.googleapis.com/raremaps/img/small/110550.jpg)
