Geology
The Tor consists of layers of clay and blue lias strata (Jurassic sandstone) with a cap of hard midford sandstone, whose resistance to erosion compared to the lower layers is responsible for its height. The iron-rich waters of Chalice Well, a spring, have been flowing out as an artesian well for millions of years, impregnating the sandstone round it with iron oxides that have reinforced it. Iron-rich but oxygen-poor water in the aquifer carries dissolved Iron (II) "ferrous" iron, but as the water surfaces and its oxygen content rises, the oxidized Iron (III) "ferric" iron drops out as insoluble "rusty" oxides that bind to the surrounding stone, hardening it. As the surrounding soft sandstone has eroded away, Glastonbury Tor has slowly been revealed.
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