This island is man made. It is the result of the earth works that encompassed the gristmill built here in 1837. It was the damming of the Tippecanoe River at this site that casued three main lakes to rise and form what is now known as Webster Lake. The northwest shore of this island is formed by the mill race from the mill. The south east shore is formed downstream of the dam on Webster Lake emptying into the Tippiecanoe River. These two shorelines meet just a dozen yards southwest from the cache to form the southern tip of the island. This land is owned by the county and not posted.
Just up Wysong Road (at the intersection by this cache) at N 41.318388° W 85.689316° is the actual site of the old mill. Here you can see the actual foundations of the what formed the race and chute for the mill turbine. To date I know of no other documentation on the internet showing the actual location of Boydston's Mill. Numerous references are made that it is in the area but none give exact latitude and longitude information.
You can also get a glimpse of the dam on the Tippiecanoe River that forms Webster lake by driving up Wysong Road (this would be the down stream side of the dam).
You are looking for a magnetic key holder.