Orestes Hawley St. John (1841 - 1921) was a geologist notable for his work with the U.S. Geological and Geographical Survey of the Territories (also known as the Hayden Expedition). He began his career with the Geological Survey of Illinois, with which he was intermittently associated from the 1860s until 1883. He also served as the principal geologist for the Second Geological Survey of Iowa from 1862 to 1868. Given his extensive experience, he was chosen to join the Hayden Expedition and led parties that surveyed the Wind River Mountains, portions of the Gros Ventre Range, the Wyoming Range, and the Snake River Valley.