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Detailed geological map of the region immediately north of Los Angeles, colored by geological features.

This map was produced as part of the Pacific Railroad Survey, which sought to map possible routes that  could be used by a transcontinental railroad. The Tejon Pass was an important contentdor as it offered a relatively easier path into the Central Valley than other known routes.

The Pacific Railroad Surveys

This work was produced by William Phipps Blake, a "Yankee Gentleman and Pioneer Geologist of the Far West" during the Pacific Railroad Surveys of 1853. These were a series of five surveys that took place along several routes during the years 1853 to 1855 and were conducted in order to gather valuable information regarding possible routes of a transcontinental railroad. Four surveys followed east-west routes, comprised of a Northern Pacific, Central Pacific, and two Southern Pacific surveys. The fifth survey followed a north-south route instead, going from San Diego to Seattle. This is the survey that Blake was attached to, and was led by Lt. Robert S. Williamson.

Blake, a Yale graduate and relative of Eli Whitney, was attached to the Williamson survey. He was a proficient mineralogist whose first job was to collect minerals for the New York City world fair prototype. He left this role less than a year after his graduation to join the 1853 Pacific Railroad Survey at the age of 26. At first, he was assigned to a party that surveyed half a dozen unsuitable passes throughout the Sierras, and he was sent on an unhopeful quest to survey a legendary pass in southern California. Blake would cement his importance in the survey when he discovered the excellent San Gorgonio Pass, which had, as of then, not appeared on any maps. After this thrilling discovery, Blake would explore the rest of the Colorado Desert before being sent northwards to explore the Coast Ranges. Blake would return to the East but soon realize that he was happier in California, where he would decamp and spend most of his life exploring, usually in a steep professional rivalry with his longtime colleague Josiah Whitney.