Geological Tables
Macleod's Tables

by David Stephenson

Tradition has it that the hills acquired their shape in the sixth Century, when they crumbled and flattened to provide a resting place for St Columba, who had been denied hospitality elsewhere in the area. But their present overall conical shape had probably been determined some 15 000 years ago during the retreat of small local glacier systems that had developed during the main late Devensian glaciation.

The hills are carved out of flat lying, predominantly basaltic lavas that were erupted 58 million years ago, during the Palaeogene, and which crop out over virtually the whole of the Duirinish Peninsula and over most of western and northern Skye. The detailed topography, probably acquired mostly in post-glacial times, is characterised by a series of steps and near-horizontal benches, commonly known as 'trap features', which are largely a result of variations in texture and degree of alteration within each lava flow. The central part of each flow is commonly the most massive and the least altered and hence gives rise to the steep steps in the hillside and any crag features. The tops and bottoms are rubbly, autobrecciated and with abundant zones of amygdales, resulting from the infilling of gas bubbles, 'frozen' during the escape of volatiles from the magma. Such zones are particularly prone to hydrothermal alteration, are more-easily eroded, and hence form the poorly exposed benches between the steps. The twin summits of Macleod's Tables are merely the resistant central parts of lava flows, composed of olivine basalt in the case of Healabhal Bheag and of a more-fractionated mugearite at Healabhal Mhor. The fact that different lavas, representing different levels in the lava stratigraphy, occur close together at a similar altitude is a result of post-eruption faulting.