That represents a lot of lost sand and dirt: In some places, large objects are falling into the sunken areas, trees and weeds are growing (because the Park District lawn mowers are worried they'll fall in), and caves are forming under the stones. My brother-in-law lost his Jack Russell Terrier down this particular hole, and I'm not kidding:
Let's look at this example. This is what you see from the lake side --
Now, imagine east-wind waves smacking this shoreline, and washing right under that icing, wave after rhythmic wave.
This is what results on the other side of that section of revetment:
Try this test yourself: walk along the edge of the meadow, near the limestone blocks. Everywhere that you find the ground is especially sunken, scramble over the limestone blocks and really look at what's on the other side. I bet you'll find damaged girders, missing pilings, caves, and tumbled limestone on the lake side.