Corner of Colorado-Kansas-Oklahoma
That photo was taken looking to the west; the road runs along the north boundary of Oklahoma. The camera is in the southwest corner of Kansas, and I am standing in the southeast corner of Colorado. (However, it's interesting to note that this corner of Colorado is isolated from the rest of the state by the Cimarron River. There are no bridges over the Colorado section of the Cimarron, so the only way to access this corner of the state is to approach from Kansas or Oklahoma.) The tri-state marker is at my feet; it's shown close-up below:
In Kansas, on the hill just above the marker, it looks like someone took the support for a windmill and welded this sculpture to the top:
The three arms say "COLO", "KANS", and "OKLA"; they point toward each respective state. There's a bison in the center. It so happens that the southwest corner of Kansas is a parcel managed by Cimarron National Grassland; the sign on the fence indicates that it is called "State Line Pasture".
According to the Congressional definition of the boundary of Colorado, this point is theoretically where the 37th parallel north latitude intersects 25 degrees west longitude as measured from the Washington Meridian*. However, the south line of Kansas was surveyed in 1857, and this corner upon it was first located by John Major during his 1872 survey. So, due to the technological limitations of that age, this point is not exactly where it ought to be (although these surveys do define the official boundaries, regardless of their accuracy). The original sandstone marker is gone now, and apparently there was nothing else at the site until the current plate was installed in 1990.
* Many American surveys between 1850-1884 were based on this meridian, which was officially abolished by Congress in favor of the Greenwich Prime Meridian in 1912. "25 degrees west of Washington" is a few miles away from the 102nd degree of longitude west of Greenwich (more info on this page).
NOTE: The book "Colorado Mapology" (Erl H. Ellis, 1983, Jende-Hagan Book Corporation) is an excellent resource, from which I obtained much of the information presented on these pages.