roches moutonnées

striae

zones of glacial erosion

Lunnasting

 

 

 

 

 

 

 

 

Vementry. Crooie Geo

 

 

 

 

 

 

 

 

Ice-moulded terrain in NE Walls, with Vementry in the background. Ice movement R to L. Photo by Hamish Ross.

 

 

 

 

 

 

 

Google Earth image of West Burrafirth.

Glacial erosion has picked out structural lineations and emphasised lee-side cliffs. Rock basins are now infilled by lochs.

 

Knock and Lochan Landscapes

Definition: a glacially-scoured lowland area which displays alternating roches moutonnées (cnoc: a small rock hill in Gaelic) and eroded hollows often containing small lakes (lochans). Fracturing often determines the locations of linear valleys and rock basins.

Linton(1963) coined this term to describe the highly irregular topography on the Lewisian gneiss of the NW Highlands and Outer Hebrides. The scenery is usually associated with deep erosion by ice sheets and bears many similarities with that of glaciated shield landscapes in Canada and Sweden and in Central Park NYC. There is a strong lithological and structural component to these landscapes, as they are most often found developed on Archean rocks, where there is a juxtaposition of hard and weakly jointed gneisses and linear zones of shattering along faults.

Inspection of 1:50,000 OS map extracts indicates that the best examples of knock and lochan terrain are on North Roe (granite and Lewisian gneiss), Lunnasting and on the northern part of Walls.