Image above: The tor at the east end of the summit of Morven

Definition: a small rock hill produced by differential weathering and erosion

Summit area of Smean

© Copyright Trevor Littlewood / Geograph

Conglomerate tors are a striking element of the scenery of the inselbergs of southern Caithness, with the largest summit tors rising over 10 m above their surroundings. The tors present strikingly irregular silhouettes but the influence of dip on tor shape is strong. On Morven, a line of crags surmounted by tors relates to a particularly tough bed of conglomerate. The tors are products of differential weathering and erosion, resistant rock knobs which have merged due to local contrasts in the jointing, bedding and bed thickness in the conglomerate.

There was no strong ice flow across the tors – these rock knobs are too fragile to survive that. The tors at lower elevations and at exposed sites have been modified by the passage of ice, losing summit blocks. Càrn Mór on Smean, for example, has been reduced to a streamlined rock ridge. This detail tells us that the tors predate at least the last glaciation.

Smean Summit Plateau – View from the summit towards Maiden Pap

© Copyright Peter Standing / Geograph

Yet whilst the surfaces may appear fragile, with cobbles of quartzite loosening from the sand matrix under present day frost weathering, the tor forms have a long history of exposure. Cosmogenic age determinations for tors on Smean indicate a minimum time for exposure of 150 thousand years – implying that tors have survived at least the least two major glacial cycles. That places the tors amongst the longest continuously exposed rock surfaces in Scotland – only the tors of the Cairngorms and Cranstackie can match that.

More on dating the tors:

Ballantyne, C.K., Hall, A.M., 2008. The altitude of the last ice sheet in Caithness and east Sutherland, Northern Scotland. Scottish Journal of Geology 44, 169-181.

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