‘S Airde Beinn, Grid Ref: NM 475527 (pron. SARSJE BINE approx) near the Mishnish Lochs just west of Tobemory is an elongate rocky hill, 295m in height. It is the highest point on the North side of the Mishnish Lochs.
The hill is a volcanic plug, composed of a rock called dolerite, the worn down stump of a magmatic conduit which probably supplied much of the lava that makes up the surrounding area. It is of early Palaeogene age (60 million years old - what used to be known as the Tertiary). The dolerite is quite coarse almost gabbroic in character. It stands out against the trap-featured basaltic lavas that make up the surrounding area. It is NOT a volcano. per se. Although the walls of the plug are quite steep, there is no obvious Field Trips between it and the surrounding lavas - they seem to grade into one another.
The rock is very fresh as it lies well away from the "zone of pneumatolysis" where alteration of the rocks due to hydrothermal action has occurred. Thermal alteration of the surrounding rocks has produced some unusual minerals such as reyerite.
The summit hollow is filled with a loch (Lochan ‘S Airde Beinn) which is probably the result of glaciation. It is sometimes referred to as a crater. It isn't!
Lochan 'S Airde Beinn, viewed from the north top, looking south.
‘S Airde Beinn is a great viewpoint on a good day – the islands of Rum, Eigg, Skye and Ardnamurchan can easily be seen. These are also of Palaeogene age and are mainly composed of volcanic rocks. To the west lie the islands of Coll and Tiree which are much older, being of Lewisian age (over 2000 million years old). And to the south, Mull’s highest mountain, Ben More is clearly visible
Satellite view of SAB with the Mishnish Lochs to the south. (Loch Carn an Amais, Loch a' Mheadhoin, Loch Peallach)
There are other volcanic plugs in the area. To the north, close to the Glengorm road is a prominent hill called Creag a Chrochair (The Hangmans Rock). South of 'S Airde Beinn, close to the forestry track by Loch Frisa, another two plugs can be found which display good columnar structures. These four plugs all lie on the same rough N - S trending line.
