[Phantasm]

Don't forget that by 1870 lever-action cartridge rifles were coming into play, having first been fielded in 1864 or '65 during the Civil War.

One of the first things the fort would do would be to send out scouts; not just to map the area they fell into, if it was noticeably different (two moons, different constellations, etc.), but to find fresh sources of food, water, and materials. These scouts often included men of Amerindian descent, so the practice of hiring locals - and here I mean an adventuring party/the PCs - has precedent.