Tan y Grisiau
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Geology & landscape
Landscape processes: Volcanic / Igneous
Description
Description The site, which is notified for its geological interest, provides an example of deformed contact metamorphic spots in Tremadoc siltstones and pelites. Wtihin the contact aureole of the Tan y Grisiau microgranites dark spots are flattened in the plane of the north dipping cleavage and are extended down dip. Strain can be estimated using a combination of cleavage and joint surfaces on which various sections of the strain elipse can be measured. The site provides an excellent illustration of the pre-tectonic age of the contact metamorphic aureole of the Tan y Grisiau microgranite intrusion. Contact spots are flattened in the plane of a single cleavage which affects rocks in the aureole and elsewhere in the area. The spots are elongated in a N/S direction on the north dipping cleavage, consistent with the control on deformation of the dipping, sub-surface extension of the intrusion. Studies in the Tan y Grisiau area have an important bearing on the interpretation of the tectonic relationships, strain intensity and proposed thrusting in the southern part of North Wales.