Today I found an article about growing dutch iris that didn't explain it being used as a top layer, but it did say something interesting that seemed worth sharing:
If mice and voles are a problem, add a few handfuls of crushed seashells or oyster shells to the hole{the iris bulb is placed within} to discourage them. They don’t like the sharp, pointy crushed shells.
I kept looking, focusing on calcium as a search word instead of seashells, and eventually I found more info about calcium supplements for plants, although I had to weed through a lot of posts related to aquarium plants. I thought this article was worth sharing as well. At the end, he shares a list of edibles that are keen on calcium. No mention of seashells, though. It also reminded me of a lot if plant facts I had forgotten from high school biology.
I did find another bit on that same site, from the forum. It was posted by someone trying to decide if a few seashells in their rose bed had killed the flowers. (If you don't want to read the whole thread, the consensus was no.)
I found a few also related to pH and seashells, but sadly they were on the old Gardenweb which was taken over by a site that adores putting pop ups in your face at every opportunity, so I won't link to that.
Ever since I first noticed the shells, I've been wracking my brain trying to recall if my Nana ever put them in her beds, since we lived at the shore, and her branch of the family has loads of fisherfolk. I honestly can't recall ever seeing them.
Anyhoo, just a side exploration for what I thought was unusual for being in a land locked area.