Trapped on the planet Brontetaal, having fallen out of the artistic cup, Arthur meets Lintilla, an attractive young archaeologist who was presumably intended as a replacement love interest (Trillian didn't appear in the second series - I don't know whether they didn't want to recast, or just fancied a change). Lintilla is investigating a strange phenomenon in the archaeological record in Brontetaal - an entire archaeological layer of compressed shoes.
The Guide then explains (via a very entertaining if slightly disturbing interaction between a CompuTeach and a young student involving pressing exciting buttons) that, when a civilization is going downhill and everyone is depressed, they are all looking at their feet, so they go out and buy new shoes to cheer themselves up. Shoe shops start to take over the economy and eventually no other shops exist, and society reaches the Shoe Event Horizon and collapses, the survivors evolving into birds.
Anyway, the upshot of all this is that I am totally incapable of looking at the side of a trench in an archaeological site, or indeed at levels of geological strata in rockfaces, as I did the other day, without expecting to find an entire archaeological layer of compressed shoes. It's a good thing I'm not a practical archaeologist.