Hill of Johnston

Also within

Aberdeenshire County

Condition (NatureScot)

Geology & landscape

Geological Feature: Igneous Intrusion · Mining / Industrial: Quarry

Landscape processes: Volcanic / Igneous

Description

HILL OF JOHNSTON The Hill of Johnston site, located 9km north of Alford, is important in providing information about the later stages of mineral crystallisation from the magma that formed the Insch igneous intrusion. The most significant geochemical aspects of the Hill of Johnston rocks are their pronounced enrichment in the elements iron, barium and zirconium as indicated in the minerals olivine and pyroxene, alkali feldspar and cumulus zircon respectively. This variation in mineralogy is reflected in the different rock types, from ferrogabbros and ferrodiorites in a small roadside quarry to syenodiorite further up the hill. Similar magma differentiation features are known to result from extreme fractionation of tholeiitic basic magma in other layered intrusions world-wide such as the Bushveld Complex in South Africa and Skaergaard Intrusion in Greenland, but the Hill of Johnston is the only example in Britain.