Exposed Units: Brockhill Dyke, Raglan Mudstone
Conservation Status: Local Geological Site
The Brockhill Dyke is a type of intrusive igneous rock with a dolerite/teschenite chemistry, which has been extensively quarried for aggregate at this site. It was intruded into the surrounding sandstones and marls at a time when these country rocks were cold. This created a marked temperature gradient between the sedimentary ‘country rocks’ and the hot, molten igneous intrusion and resulted in the formation of a baked margin.
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Dyke – A body of igneous rock that has been intruded into the surrounding rocks and has a ‘sheeted’ geometry. This ‘sheet’ cuts across the sedimentary layering in the surrounding rocks.
Marl – A type of mudstone that consists of clay and carbonate, i.e. a lime-rich mudstone.
Country rock – The host rock into which an igneous rock has been intruded. It is also termed ‘surrounding rocks’ in this entry.
Baked margin – The part of the country rock that is immediately adjacent to an igneous intrusion. High temperatures experienced by the country rock during the intrusion of an igneous body can cause clay-rich rocks to become baked in the immediate vicinity of the intrusion. The effect of this baking decreases with distance from the intrusion.
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