The Highland Line is a 'terrane boundary', where two sub-continent chunks have slid past one another. The southern terrane, the Scottish Lowlands, has also sunk, bringing softer sandstone and coal measures down alongside the much tougher, older rocks of the Highlands. The very northern edge of the Lowlands is this Old Red Sandstone type conglomerate or puddingstone – reinforced with big cobblestones in the same way as your Christmas pudding is strengthened by the sultanas and raisins. Marking the Highland Line, the chunky conglomerate forms Conic Hill on the West Highland Way, and the line of islands stretching across Loch Lomond.

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