The island is exposed to severe winter gales and is being gradually eroded. During one storm in 1953 the well known headland, "The Horn of Papa," crashed into the sea; and in 1981 one of the lochs above Kirstan Hole collapsed into the cave below. Wind, wave and tide combine with its unique geology to create the island's distinctive and beautiful cliff scenery, with an abundance of geos, stacks, natural arches and caves.
Papa's sea caves were described as some of the finest in the British Isles by the naturalist J.R. Tudor in his book, "The Orkneys and Shetland," written in 1883. The Holl o Boardie, which passes right through the north-west tip of the island, is nearly half a mile long and wide enough to row through; though no-one has attempted this for many years. Fogla and Lyra Skerries are also pierced through by several large caves. These caves are accessible only by boat; and then with difficulty: but those which pass through Brei Holm, just south of Housa Voe, are frequently traversed by small boats when conditions permit.
The island is divided into three distinct areas. The fertile in-bye land lies to the east of the hill dyke (which runs south from West Voe) and just beyond that to the west the island is bisected by a belt of glacial moraine about one and a half kilometres in length. Much of the rest of the area consists of a shallow stony soil probably derived from glacial till. And there is evidence to suggest that Shetland had its own ice cap!
The 21 miles of ruggedly beautiful coastline contain a wide range of volcanic, metamorphic and sedimentary rock formations. Papa Stour rocks originate from the Devonian age when Shetland was south of the equator and conditions were generally hot and dry with occasional very wet periods. The structure of the island is typically made up of ashes and lavas from the volcanic activity associated with this period. Of further geological interest is a Devonian fish bed at Lambar Banks, which due to its fragile state is not to be disturbed by the removal of rock samples.