This image shows both a rock formation and a well-known tourist boat of the era. The rock formation is called Lone Rock, as it is found at some great distance from other formations in and around the lake.
There is no description on the back of this card.
The boat shown is named Josephine (it can be viewed more easily by clicking on the image to get the full sized picture). It was one of a small fleet of such vessels available for tourists to cruise the Lower Dells.
This card likely dates from the 1940s.