In Geology an unconformity is a surface separating two rock types of different ages from one another. An unconformity represents a period of erosion or non-deposition in the sedimentary record, i.e. a gap or hiatus where we have no record of what happened.
One such unconformity is Hutton’s Unconformity. The confusing part is that the name Hutton’s Unconformity is the name given to several unconformities identified by the famous 18th century Scottish geologist James Hutton. On a recent fieldtrip to Scotland we visited Hutton’s Unconformity at Siccar Point, but other unconformities identified by James Hutton can be found on Arran and near Jedburgh.
When sediments (fragments of previous rocks) are deposited (settle before becoming a rock) they form layers of progressively younger material burying the older material. If a sedimentary rock is the correct way up the further up the rock you go the younger (closest to present day) it is.
At Siccar point we can see two distinct units rock, with the layers of rock (bedding or strata) in the upper and lower units being at different orientations from one another. This means that before the second (upper) unit was deposited the lower (older) unit was tilted and eroded, producing a gap in the geological record.
Unconformities are crucial to our current understanding of geology. They paved the way for us to consider deep time, that is essentially an appreciation of the fact it has taken a very long, yet quantifiable length of time for the rocks we observe around us to have reached their current situation. For the rocks here at Siccar point that means; deposition of the older sediments, tilting of the older sediments, erosion, then the deposition of the younger sediments on top. This order of processes cannot possibly happen quickly!
Unconformities have also been used as evidence that the mechanisms governing the production of these rocks (and the universe) have, and always will operate in an assumption known as uniformitarianism. Uniformitarianism is a counter (and more accepted view) than the opposing catastrophism, which implies that the Earth was created in a series of sudden short lived events.
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.
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.
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.
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.
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 😉
Whilst on holiday in Finland last year we visited an Island not far from the centre of Helsinki and found some interesting things to look at…
The linear features here are called striations and have been gouged into this rock. They are caused by the movement of a glacier over the bedrock, through the process of abrasion. Abrasion is a term used to describe when fragments of rock captured by a moving glacier are pressed onto the surface of the underlying rock. As the glacier drags these rock fragments along lines are scraped into the rock. Striations are a common feature of glaciated landscapes and are just one of many features glaciers can produce.
Striations such as these along with other glacial landforms can help us to reconstruct the dynamics of past glaciers, and help us understand more about modern glaciers. The presence of glacial landforms where we no longer have glaciers (such as here in Helsinki) demonstrates how the environment can change over time.
If you find glacial landscapes interesting you may find Rock Paper Glacier! blog to be of interest.
This example near Helskinki really does show us that even in built up areas we can still observe the influence of the forces that shaped the landscape around us!