Elevation required?

SO? That's how this happens? 

When I did this, back in the mists of history according to Blip; it well pre-dates 2009 since The Heap, at the end of the pond was well established by then.
This edging was both, at grass level AND merely laid on soil with NO foundation.
Should you, perhaps, detect what resembles micro-Sago it's a whole generation of Slugs - DOOMED!