MLA

Frémont, John Charles, 1813-1890, cartographer. Map of an exploring expedition to the Rocky Mountains in the year 1842 and to Oregon & north California in the years 1843-44. [1845?]. https://collections.library.yale.edu/catalog/15812923.

APA, 6th edition

Frémont, John Charles, 1813-1890, cartographer. ([1845?])[Map of an exploring expedition to the Rocky Mountains in the year 1842 and to Oregon & north California in the years 1843-44]. https://collections.library.yale.edu/catalog/15812923.