What I had found, I'm almost certain, is something called a cache. Made by the French Voyageurs  in the late 1700's, it was a place where goods could be traded. The Native Tribes would leave valuable furs that they had trapped in this structure to be collected at a later time by the Frenchmen, who in turn would leave behind money, tools, weapons, whatever the agreement called for. These places were usually deeply hidden as to not be plundered by a neighboring, rival Tribe. The bluffs along the Mississippi River provided a perfect location for many of them.
   While most of these places have been found and mapped, I have yet to find this one in any published document. You can find a lot of material on this topic at your local public library. Just look in the county history section in which you are interested in. There will be a number of books with maps and locations of Native burial mounds as well. Sometimes county plat books contain this information also.

                                                            Southeast Minnesota