Hall's Ledge, from the summit on the rock's western face. It is the only named geographical location on the rock. The nearest point on land from Rockall is , east at the uninhabited Scottish island of Soay in the St Kilda archipelago. The nearest inhabited area lies east at Hirta, the largest island in the St. Kilda group, which is populated intermittently at a single military base. The nearest permanently inhabited settlement is west of the headland of Aird an Rùnair, near the crofting township of Hogha Gearraidh on the island of North Uist at . North Uist is part of Na h-Eileanan Siar council area of Scotland. The exact position of Rockall and the size and shape of the Rockall Bank was first charted in 1831 by Captain A. T. E. Vidal, a Royal Navy surveyor. The first scientific expedition to Rockall was led by Miller Christy in 1896 when the Royal Irish Academy sponsored a study of the flora and fauna. They chartered the Granuaile. A detailed underwater mapping of the area around Rockall undertaken in 2011–2012 by Marine Scotland showed that Rockall itself is a minor pinnacle, whilst Helen's Reef extends in a sweeping arc of fissures and ridges to the north-west of the islet. Between the islet and Helen's Reef is a deeper trench much used by squid fishermen. Rockall is located in the pathway of the warming and moderating Gulf Stream. Although the rock does not sustain any weather station, the isolated nature of the setting dictates an extremely maritime climate without heat or cold extremes. Geology Rockall is made of a type of peralkaline granite that is relatively rich in sodium and potassium. Within this granite are darker bands richer in iron because they contain two iron-sodium silicate minerals called aegirine and riebeckite. The darker bands are a type of granite that geologists have named "rockallite", although use of this term is now discouraged. In 1975, a mineral new to science was discovered in a rock sample from Rockall. The mineral is called bazirite, named after the chemical elements barium and zirconium. Bazirite has the chemical formula BaZrSi3O9. Rockall forms part of the deeply eroded Rockall Igneous Centre that was formed as part of the North Atlantic Igneous Province. It was formed approximately 52 ± 8 million years ago based on rubidium–strontium dating, as part of the breakup of Laurasia. Greenland and Europe separated