A deep groove gouged from the up-ice slope of a rock hill. Pencil for scale in an image taken lying in the groove
Definition: sets of parallel furrows which have been ground out of rock surfaces by boulders lodged in the moving sole of a glacier or ice sheet. The rock surfaces have often been polished by finer debris.
Glacial grooves can be
found on most outcrops in the southern part of Central Park, transecting the structures of the schist
bedrock. The thinnest grooves are the width of pencils and are little bigger
than striations or glacial scratches. The larger grooves are more than 30 cm
wide and extend for several metres across rock surfaces. The grooves are best
developed on the up-ice slopes of roches moutonnées, where ice was forced to
move uphill and where the resultant pressures intensified on the pulverising
blocks moving across the rock floor. Merguerson and co-workers
have shown that at least two sets of grooves exist. The younger set trends
NNE-SSW and was cut in the early stages of ice retreat when ice was moving
towards a margin on Long Island. The older set includes many deep grooves
trending NW-SE, cut by thick glacier ice moving across the Manhattan ridge
without significant deflection. Fainter, largely obliterated grooves can be seen
on rock surfaces sheltered from the latest ice movements.