All the Scottish island groups were once part of the mainland and became separated through huge geological forces and rising sea levels. St Kilda and Rockall are the islands furthest to the west that were subject to the same process. The terrain of Bernera with its U-shaped valleys and rounded boulders was formed by the movement of glaciers during  various ice ages and sea erosion. Over hundreds of years, the rising sea level  has affected settlement and land use and dramatically altered the shape of the island. This is an ongoing process, Little Bernera for instance, is becoming two separate islands as the narrowest part is constantly eroding and will eventually allow inundation by the sea. The small island of Cealasaidh was once part of Little Bernera.

At Bosta, the sea was much further out than at present, and the beach would have been a more usual semi-circular shape, between the two headlands. The whole area of sand comprising the present day beach is the result of erosion and rising sea level. The stream alters its course frequently and is causing the shoreline to tumble. High winds and seas transfer hundreds of tons of sand from one place to another. The Iron age Village was first exposed in 1993 after a raging storm.

There is a fault running North/South through the length of the island and is exposed in places, this is an intrusion of quartz, pink in colour and beautiful when low evening or morning sun strikes it. The rock is mostly gneiss or granite with schist. The violent upheavals of molten rock can be seen in contorted strata in the small quarries throughout the island. The rocks you see are amongst the oldest on earth,brought to the surface billions of years ago.