Megapolygons with thrusted margins occur within the lowermost dolostone of the Kimmeridge Clay Formation (Upper Jurassic), at the type locality in southern England. Unlike similar features previously described, these structures did not develop at, or near, the sediment surface but at a considerable depth of burial. They formed at a time when the adjacent shales were already compacted but prior to tectonic jointing and faulting. Features distinguishing them from similar near-surface structures include sigmoidal thrust planes, complete lack of penecontemporaneous erosion, absence of internal sediments and association with localized deformation of surrounding, compacted sediments. Diagenetic growth of dolomite is proposed as the mechanism that gave rise to expansion.