Formation Tor (rock formation)

kit-mikayi, celebrated tor near kisumu, kenya.

for example, dartmoor granite emplaced around 280 million years ago, cover rocks eroded away afterwards, exposing chemical , physical weathering processes. joints closely spaced, large crystals in granite readily disintegrate form sandy regolith known locally growan. readily stripped off solifluction or surface wash when not protected vegetation, notably during prolonged cold phases during quaternary ice ages - periglaciation.

where joints happen unusually spaced, core blocks can survive , escape above weathering surface, developing tors. these can monolithic, @ haytor , blackingstone rock, more subdivided stacks, arranged in avenues. each stack can comprise several tiers or pillows, may become separated: rocking pillows called logan stones. these stacks vulnerable frost action , collapse leaving trails of blocks down slopes called clitter or clatter. weathering has given rise circular rock basins formed accumulation of water , repeated freezing , thawing. example found @ kes tor on dartmoor.

dating of 28 tors on dartmoor showed surprisingly young, less 100,000 years old, none on 200,000 years old. emerged @ start of last major ice age (devensian). contrast, in scottish cairngorms other classic granite tor concentration in britain, oldest tors dated between 200 , 675,000 years old, glacially-modified ones having dates of 100-150,000 years. may reflect dryer, more arctic climate.