These "sponge rocks" have, for over 80 years, been important to my family. In the early 1930s, my mother lived with her parents in St.Louis, Missouri. (She always pronounced it St. Lois.) Her father, my grandfather, worked in a print shop and hated it; he hated being indoors all day long. Weekends, he would putter in his urban back yard and he built a fish pond there. He rimmed the edge with these large pieces of "sponge rock."
Mom remembers going out to Rock Hollow (they called it) somewhere near but outside of the city. There they could spend a picnic day and gather rocks for the fishpond.
In the 1940s, my grandparents moved from St. Louis to near Kalamazoo, Michigan, to live on very small Pickerel Lake. Oh, did Grandpa enjoy living on that lake! They took the sponge rocks with them and they formed my grandmother's flower bed.
Upon her death in 1987, my Mom hauled the rocks back to Spokane, Washington, and they've been around her favorite lilac bush for these past 28 years. Upon Mom's passing in 2014, the rocks were mine and now they reside in a space near the garage entry where I walk past them several times a day. (The brown insulator was added to show the scale of the rocks.)
To this day I don't know where "Rock Hollow" was, but doing a Google search I did find a Rock Hollow (bicycle) Trail as part of the Meremec River Greenway. I'm sure today it would be a tremendous legal offense to haul rocks from this spot but back in the 1930s it was wilderness and unprotected.
And my grandfather, then my mother, and now me, are committed to protecting these very special rocks.