Striae and polished surfaces near Barnes Castle

Striae

Definition: scratches and grooves cut in rock parallel to ice flow by granules and stones embedded in the moving sole of the glacier

Glacial striae or striations are scratches cut by sand, grit and stones embedded in the glacier sole. The striae typically run parallel to glacier flow but when ice deforms to flow around an obstacle the striae will record faithfully the divergence and convergence of flow around the bump. The bed load of the glacier will itself become striated.

Striae usually occur as sets of sub-parallel scratches. Striae are important in recording the direction of ice flow. Sites where crossing patterns of striae are recorded are particularly important, as here changing patterns of ice flow through time can be identified.

In Edinburgh and East Lothian, the soft sedimentary rocks only retain striae where a cover of till has protected the bedrock from postglacial weathering. On limestone surfaces, calcite precipitates sometimes occur with ridges that flow the former direction of ice flow. It is on the harder volcanic rocks, however, that striae are most abundant. Striae occur not only on flat surfaces but also on the sides of hills, for example, the north side of North Berwick Law and on the south side of the Lion's Haunch on the Queen's Drive around Arthur's Seat.

Glacial striae in Scotland  caithness shetland

Glacial striae in Central Park NYC  central park

chatter mark  crescentic gouge  groove subglacial calcite precipitation  Corstorphine Hill