Above - striated surface exposed beneath a glacial erratic, a former agent of abrasion
Right - polished, striated and grooved surface with NW-SE lineations. Image by Brett Bennington
Thin grooves or striae trending NW to SE, now partly effaced by post-glacial weathering
Glacial striae or striations
Definition: scratches cut by debris embedded in the sole of a glacier
Glacial striations are widespread in Central Park. Best seen after rain and in low-angle light, the parallel sets of striae transect the structures of the schist bedrock.
The striations form as debris is dragged across the glacier bed. The debris must be relatively hard to scratch the rock. The rocks, stones and sand grains are the tools of abrasion and will themselves becomes striated. Eventually rocks will fracture or pulverise.
Striae provide important evidence of patterns of ice flow. Because striae are simply parallel sets of scratches other evidence is needed to decide which way the ice was moving. This is provided by asymmetric glacier bed forms and by glacial erratics.
Under the magnifying glass, striae are revealed as far from smooth-sided and comprising multiple arcuate fractures, testament to fracturing by the edges of rocks grinding across the rock surface in the base of the ice. The rock surface between striae may be polished. Small bumps will tend to be prized off by debris in the base of the ice and sand and rock flour can complete the polish.
Map of striations on the Palisades Ridge