Glacial erratics in Scotland
Perched blocks in Scotland
A pegmatite erratic near the Heckshcer Playground
A perched boulder in The Rambles
A very large perched boulder in The Rambles which has split under its own weight since deposition
Glacial erratics and perched blocks
Definitions: An erratic is a boulder transported and deposited by a glacier having a lithology different than the bedrock upon which it is sitting. Erratics are useful indicators of patterns of former ice flow. A perched block is a large block of local or far-travelled rock which has been left by a melting glacier on top of a moraine or roche moutonn้e.
Erosion of bedrock by glaciers generates myriads of rock fragments from sand to boulder size and beyond. These clasts may travel only short distances before being pulverised or worn away at the glacier bed. Some do escape from the grinding mill at the glacier sole when ice is forced to deform around obstructions and rising flow lines carry debris into the body of the glacier. This debris may be carried long distances on to quite different rocks and then be dumped as erratics.
When glacier ice melts its load of debris is usually left as a layer of boulder-rich debris or till. The bare rock surfaces in Central Park carry little by way of till cover but a range of erratic blocks have been recognised resting on the rock surfaces. These stones include diabase from the Palisades Sill along the Hudson River, together with stones of granite and diorite. Central Park also shows outstanding examples of perched blocks, where imposing monoliths of schist have been dumped in odd positions on the tops and slopes of rock hills.