Geological Map of a Part of the State of California Explored in 1853 by Lieut. R. S. Williamson U. S. Top. Engr.

Cartographer: Lieutenant Robert S. Williamson, U. S. Topographical Engineers (expedition leader); William P. Blake, Geologist and map compiler

Year: 1853

Prepared for the Pacific Railroad Surveys, this hand-colored sheet plots rock formations along Williamson’s proposed rail routes from the Oregon border south to the Tehachapi passes. Eight pastel tints distinguish granites, metamorphic belts, volcanic ridges, serpentine zones, valley sediments, and coastal alluvium, with a keyed legend at lower left. The intended railroad line threads through the Sacramento Valley, skirts the Sierra Nevada foothills, and crosses the Coast Range at strategic gaps marked by sharp hachures. Rivers, lakes, and selected settlements anchor the geography, while dashed lines show reconnaissance paths into the “Great Basin.” By combining topography with bedrock geology, the map illustrates engineering obstacles—hard granites, unstable serpentine, and marshy valley fill—facing any trans-California rail link in the early 1850s.