Dykes on the beach

I had pretty much forgotten about this amongst everything else but I still think this is worth posting. On the way back from doing Dream of White Horses on Gogarth (2 weeks ago) we decided to have a quick stop on the beach and found some interesting geology to look at. From just a few simple observations on this beach some really key geological principles can be demonstrated.

Beach on the way back from climbing on Gogarth
Beach on the way back from climbing on Gogarth

The main rock type here is a metamorphic rock (picture below) known as a gneiss with abundant mafic intrusions cutting it perpendicular to the coastline.

Foliation in the Gneiss which the mafic dykes intrude
Foliation in the Gneiss which the mafic dykes intrude

An intrusion is when molten rock is injected into another rock and the word ‘mafic‘ refers to the composition of the intrusion. Here I am using mafic to describe an igneous rock which is dark coloured and with crystals too small to identify, but its actual definition is a silicate mineral (contains the elements silicon and oxygen) which is rich in the elements magnesium and iron. A metamorphic rock is a rock which was previously another type of rock which has been physically/chemically altered through increased pressure and temperatures in the Earth.

One of the larger intrusions observable on the beach
One of the larger intrusions observable on the beach

The patterns we can see in the gneiss (second photo down) are known as foliations. These are a planar feature caused by pressure forcing the minerals in the rock to align in a preferred plane.

Small offshoot of the dykes
Small offshoot of the dykes

This simple observation of the dykes on this beach also demonstrates a crucial geological principle, so called ‘cross-cutting relationships‘. The principle of cross-cutting relationships states that the geologic feature which cuts another geological feature is the younger of the two features. Geologists use observations like this to build up the sequence of events (relative ages as opposed to absolute ages) that led to the particular setup which we observe. From this we can reliably say (with no need for dating techniques!) that the dykes here are younger than the metamorphic rocks they intrude. This concept of relative dating was noted as early on as 1795 when James Hutton published his ‘Theory of the Earth‘, and is not just limited to intrusions, it can be applied to a whole array of geological features such as faults or erosional surfaces.

Not sure what makes this pattern in the intrusions? Maybe contraction during cooling?
This pattern was observable on some of the dykes, but I’m not sure what causes it!  Maybe contraction during cooling?

I would imagine most of the beaches this side of Anglesey have things of interest. If you find yourself passing by might be worth a quick look 😉

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