Geosites Project

Markle basalt, named after the particular composition of a basalt found in a local quarry, forms the shore platform of the Longskelly Rocks and is well-exposed at low tide, with the island of Fidra in the background. The basalt contains small crystals of plagioclase feldspar within a fine-grained grey/green matrix. A close look showed the reddish crystals of altered olivines.

Within the basalts are sizeable rounded inclusions of the same basalt, surrounded by red reaction rims. These may be blocks of lava overrun by new lava flows, causing chemical changes in the minerals on the outside of the boulder. Scale card is 9 cm long.

Markle basalt with thin quartz veins forms the bedrock, but boulders of basalt with large phenocrysts of plagioclase feldspar, such as the rock beside the yellow field notebook, are common.

Vugs up to 5 cm across with calcite crystals can be seen in a boulder of dark basalt with phenocrysts of plagioclase feldspar. Scale card is 9 cm long.

The Markle basalts at this exposure have thin irregular calcite-filled joints with many large calcite-filled vugs.

Mugearite, technically a basaltic trachyandesite named after a locality on the Isle of Skye, is a pale grey-purple fine-grained lava with fewer phenocrysts than the Markle basalts that form the shore platform in the distance. There are iron-rich calcite veins in places.

This extraordinary exposure of mugearite lava is thought to show bleaching of the lavas, resulting in pale patches surrounded by networks of iron-rich calcite. It is thought to be the weathered top of a lava flow within the sequence. Field notebook is 20 cm long.

At this well-studied exposure near Marine Villa, the curving planar surfaces beyond the yellow-lichened boulders in the foreground are thin layers of volcanic tuff overlying the irregular upper surface of a mugearite lava flow.

The volcanic tuff lies above the highly irregular upper surface of the mugearite lava flow, above the scale card. Below the scale card, it appears that the mugearite has been highly altered and bleached in places, probably by movement of hot volatiles through the lava flow. Scale card is 13 cm long.

Above the mugearite lava flow is a good exposure of volcanic tuff, of trachyte composition. The beds are regular which suggests that the volcanic ash had cooled in the atmosphere before it fell onto the lava flow.

A close-up of the lowest section of the volcanic ash lying directly above the mugearite lava shows the variation in sizes of the material, which includes coarse sand-sized particles as well as small pebbles.