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glacial erratics

glacial stratigraphy

Perched block of migmatite on a moraine at Drumhollistan near the county boundary

 

perched blocks in central park nyc

 

 

Perched block or blocks

Definition: large blocks left by glaciers on top of moraines and rock hills

Blocks perchés were one of the key criteria used in the 19th century to identify glacially-transported boulders and glacial moraines beyond the limit of contemporary glaciers in the Alps. The term "perched block" was later used widely to describe erratics and large boulders of local rocks found in anomalous position in formerly glaciated areas, including Scotland. Not all perched blocks are glacial in origin - major rock falls can also produce chaotic accumulations of large blocks - but the association of perched blocks with moraines is common and characteristic of ice margins. In the late 20th century the use of the term waned but the perched blocks remain important evidence of former glacial transport.

In Caithness, the best examples of perched blocks lie astride the county boundary between Reay and Melvich. Here the retreat of ice into Strath Halladale and southwards towards Ben Ratha left arcuate sets of moraines with numerous perched blocks. To the west the blocks are of migmatite and granite, derived from Strath Halladale. To the east large diorite blocks occur, plucked from the crags south of Reay and dumped on low ridges. In other parts of the county perched blocks, together with field boulders, were cleared during agricultural improvements from the 18th century onwards. These boulders may now be found in field boundaries or cairns.