roches moutonnées

erratics and perched blocks

glacial microforms

key locations in Central Park

The USGS topographic map picks out the highly irregular rock relief and the lake-filled rock basins.

 

Knock and lochan scenery in Scotland

shetland

Knock and lochan terrain

Definition: a glacially-scoured lowland area which displays alternating roches moutonnées (cnoc: a small rock hill in Gaelic) and eroded hollows often containing small lakes (lochans). Fracturing often determines the locations of linear valleys and rock basins.

Central Park provides a striking and accessible example of knock and lochan scenery, a term first used by David Linton to describe the landscape of small rock hills and lakes in the NW Highlands of Scotland. This kind of scenery is characteristic of glacially-eroded lowlands developed in hard rocks. A succession of ice sheets over the last million years have acted as powerful agents of erosion over Manhattan. All pre-existing sediments and soils have been removed to leave polished bedrock, leaving mica glistening as brightly as when it was first exposed from the melting ice around 15 thousand years ago.

The hills have been streamlined beneath glaciers many hundreds of metres thick. The structure of the rock - the disposition of rock layers and fractures - exerts a fundamental control on the location and form of the rock hills. The hills are found on the more resistant rock masses, including Umpire Rock with its widely-spaced and tightly-closed joint systems. Zones of faulted, fractured and otherwise shattered rock have been gouged out to give linear depressions, like The Dene.

Even before landscaping, many of these hollows probably contained small lakes and swamps. The original drainage system has been utterly effaced and instead a deranged drainage pattern exists dominated by NW-SE trending trenches which run parallel to the main faults and which were excavated by thick ice moving across Manhattan.