The northern tip of the island of Tresco lies just within the limit of the Devensian glaciation of the British Isles. Much of the plateau to the north of Cromwell’s Castle is littered with erratics and thin patches of till, which means that glacier ice must have affected the land surface at least 40m above present sea level. Most of the erratics are small pebbles and cobbles, up to about 20 cms in length. Some of these show clear striations. Illustrated: above: one coarse reddish pink sandstone pebble; below: one cobble of dark red sandstone with thin quartz veins, and one whitish coarse sandstone cobble. Sources unknown.
Erratics and patches of till occur even further to the south in the strait between Bryher and Tresco, suggesting that a lobe of ice occupied the low land between the hills (Tregarthen Hill and Castle Down to the east and Shipman Head Down to the west) on either side.
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