The Mormons, who had settled the Great Salt Lake Valley in 1847, were aware that Fremont had named the land mass Disappointment Island, but because of its shape, the Saints renamed it Castle Island. To Stansbury, however, fell the official responsibility of placing it on a U.S. map. He chose to recognize the adventurous explorer who first set foot upon its shore. He set it down as Fremont Island, and so it remains today.

When Fremont's party departed on the morning of Sept. 10, the lieutenant was dismayed to discover he had left the lens cap to his "spy glass" on the summit, and ruefully observed it would probably remain there undisturbed by Indians, to furnish "matter of speculation" for some future traveler. (The lens cap was found in the 1860s by Jacob Miller, a Mormon using the island as a sheep range.)