Flark

Content sourced from Wikipedia, licensed under CC BY-SA 3.0.

A flark is a hollow in a bog. They usually appear as a row of parallel depressions with ridges, called strings, between them. At first, people blamed frost heave for making flarks, but they also occur where frost heave doesn’t happen. Today it’s thought that thick peat at the bog’s base can slide downhill under its own weight. Obstacles in the ground stop the slide, causing the peat to tear away and form flarks downslope. Another idea is that flarks come from parts of the bog that decay faster, creating depressions.