This place is real similar to a spot I've been studying for several years. It is also on a tributary going into the Mississippi and about a mile and a half back from that river. You might have a cache spot up on top of the bluff, maybe with an open hole. Have you checked up on top yet? You said the bluff was 150 feet high, there might be a trail that goes from the bottom of the bluff to the top. If there is such a trail, it could have a gradual slope, easy enough for a horse to move up the bluff. This trail, if it's there, could also be hidden, or maybe just not real obvious.
If your spot is like the place by me, most of your markers and carvings will be at the bottom of the bluff and there will be only a few markers up on top. The first marker down below, the beginning of the trail, will be very big and easy to spot. If there's a cache spot on top of the bluff, it could be right above this first marker and within a hundred yards.
Is your spot on public land? Also, do you know of any treasure legends that would call attention to the county you're in?