Way back in 2010 I spent some time Geocaching in the wheat farming district of South Australia near Cleve. There, on a fence that separates the Airport Runway from the towns Cemetery (possibly a scary sight as your plane comes in to land) I found this interesting grouping of feral snails.
They gather like this during rainy times, the ones at the bottom of the pile cemented there by the ones over the top. After the wet weather the top snails leave to eat and breed while the ones underneath no doubt die a horrible death during the very hot Summer.
Gardner's have been known to scrape the shells off into buckets to mix into potting soil as a Liming and water holding agent.
*Groups of introduced snails aestivating at the top of fence posts at Cleve, South Australia. (As can be seen in the detail image, the groups are mostly composed of the white garden snail Theba pisana, with some smaller individuals of the pointed snail, Cochlicella acuta.) *this bit nicked from wikipedia.org