Just beyond the lanes of Interstate 55 but west of the South Riverfront are the remnants of a very old quarry, whose original purpose has been lost. It zigzags back and forth, and even appears in Pictorial St. Louis. I originally thought these cliffs were caused by prehistoric water erosion in this post from August of 2017.
A developer back in the late Nineteenth Century even platted some lots among the bluffs, laid out exactly with the right angles of the cliffs, naming the subdivision after the nearby Marine Hospital, a landmark in the area.
Now, the land sits vacant, except for some very tall billboards.