[3] Like most of the other islands of Orkney, the bedrock is Middle Old Red Sandstone of Rousay type of the Devonian period, but much eroded and tilted.

[4] The islet is separated from Copinsay by Horse Sound, and to the southwest is Corn Holm.

The Blaster Hole is a sea jet, of the type known in the Northern Isles as a gloup.

When a storm blows in from the east, the aptly named Blaster Hole can produce a spout of nearly 60 metres (200 ft) highly, solely by wave power.

[4] The tiny islet was used for grazing, and appears never to have been inhabited (although a number of the smaller islands were often used by culdee anchorites).