Striae or striations, with other signs of glacial abrasion and fracture
.
Chatter marks on granite
Crossing striae on Eshaness, with the
contrasting ice flow directions highlighted. Image courtesy of
Hamish Ross Glacial striae
elsewhere
Other glacial microforms
Striae or striations
Definition: striae or striations are scratches and grooves cut by debris embedded in the sole of the glacier. Striated surfaces often show many parallel striae. Occasionally, a single surface shows crossing striae, indicating two different phases of ice movement.
The movement of glacier ice
studded with rocks can generate a range of markings. Large blocks embedded in
the sole of the glacier create grooves up to a metre wide and many centimetres
deep. Smaller blocks and grit create parallel scratches or striae. Where a
large, sharp edged block catches on the bedrock floor, pressure builds up
until the rock fails, giving chatter marks or crescentic gouges. Here the
basal layer of glacier ice is moving by stick and slip.
Striae are often faint and best viewed in low angle sun or after rain. To determine the direction of ice flow it is often necessary to find small stoss and lee forms, like mini- roches moutonnées or other microforms.
On Shetland, striae have been perhaps the most important type of evidence used in the reconstruction of former ice flow patterns. Crossing striae are rare on Shetland, suggesting that the last ice to move across each area was usually sufficiently erosive to remove earlier markings.
