This is an image of a large rock formation off the coast of Michigan in Lake Huron. The formation is a single rock that has become separated into two large sections by erosion.
The description states:
183-S MICHIGAN THUMB SCENERY
Sister Rocks — off Broken Rocks, Port Austin, Michigan
–The Broken Rocks area of the lake refers to the off-shore formations of rock that have become separated from the shore and are now free-standing in the lake.
This postcard is dated 1957.