Some microforms of glacial erosion
Glacial microforms on a smooth surface near the Avenue of the Americas entrance
Definition: small marks on rock surfaces cut by debris beneath glaciers that record the direction of ice flow
The many rock knobs of Central Park display excellent examples of detailed sculpting by ice. The rock outcrops have not only been smoothed and polished by the passage of great ice sheets but also scratched, gouged and fractured. These microforms are most easily seen after rain and in low angle sun.
Glacial microforms provide evidence of the processes of glacial erosion. Polishing results from abrasion by sand trapped in the glacier sole. Scratching records the grinding passage of small rocks across the glacier bed. Bigger blocks forced down against the bedrock under the great weight of the overlying ice cut deep grooves. Some blocks move beneath the ice by stick and slip, fracturing the bedrock. Other fracture-bounded blocks are plucked from rock cliffs.