The area is known geologically as the South Stack Formation. Its strata includes sandstones and interbedded shales which have been contorted by large folds and crumples. The folds can be seen in the seacliffs (best viewed from the steps leading down to the lighthouse). Thick beds of pure Holyhead Quartzite, which forms the bulk of Holyhead Mountain, lie above these folded sedimentary rocks. Recent studies suggests the rocks appearance was caused by large scale earth movements which is not a part of the normal sedimentary process; however, this theory remains controversial. Similar evidence of upthrusts can be seen in the cliffs to the south of South Stack.