Here are some monuments I recoverd this past weekend around the former Fort Niobrara Military Reservation near Valentine, Nebraska. They are 12"-square sandstone posts about 5 feet above the ground. I have a newspaper article that says a member of the Corps of Engineers was to "mark substantially and accurately" the boundaries of the fort, but to my knowledge no actual survey plat has been found.
In 1913, well after the fort was abandoned, the GLO resurveyed several townships in that area and set brass cap on pipes after finding many of the original GLO pits and mounds. The 1913 survey notes, however, do not mention the sandstone posts even though one GLO brass cap is placed next to a sandstone post. In other instances the 1913 brass caps vary greatly from the posts. It appears that the 1913 GLO surveyors pretty much ignored the early sandstone posts for whatever reason.
Nebraska also has substantial cast iron monuments around former fort boundaries. I have found 1940's brass caps on pipes anywhere was three to ten feet near these cast iron monuments.
