A deep groove gouged from the up-ice slope of a rock hill. Pencil for scale in an image taken lying in the groove

chatter mark

crescentic gouge

striation

roches moutonnées

erratics and perched blocks

knock and lochan terrain

Glacial grooves

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.