This weekend I went on my first geological field trip in Torbay, Devon.
Our guide was Hazel Gibson, a geologist based at Plymouth university, who also has a blog here: https://mypatchworkplanet.wordpress.com/.
Hazel decided to take us to Torbay, because of its visually striking geologies, where she was able to talk about broad structural changes, geological faults and bedding and environmental changes (ocean to desert) in the different sedimentary rocks that make up the GeoPark.
The first stop was Berry Head Nature reserve, because of the vantage point, offering a view of the progression of geology, moving forward in time as our eyes track the coast, starting with Devonian rocks (350 million to 416 million years ago ), which we are standing on, through various geological phases, including red rocks (explained further below) from the Permian period (about 50 million years after the Devonian period), as well rocks from the Jurassic and Cretaceous periods.
Below, a view from Berry Head, of Torbay.
Below: Hazel explaining the local geology to us on her map.
Here a limestone quarry with a very high quality limestone. (Limestone is a sedimentary rock, which is less strong than igneous or metamorphic rocks, which are created through heat).
Inside the quarry, we were able to study some of the different rock formations.
Here lime stone, created in a relatively peaceful environment, through slow deposition of sediments (such as corals in tropical seas), with horizontal layers of sediment. Right next to it a completely different material, going in a vertical direction. This is a different rock type, a sand stone, which would have been deposited later, falling in from the surface, into what would have been a fissure in the limestone.
In the picture below is an example of a vein of calcite, formed by a water forcing its way through the rocks, finding the path of least resistance, and depositing minerals along the way. Hazel explained that this way of reading a landscape allows for an understanding of sequence in which rocks appeared, in terms of chronology.
Below, a red sandstone (coloured red by iron), soft and crumbling in comparison to the old lime stone surrounding it.
Below, observing quartz crystals formed in the rock, with a hand lens used by geologists.
Hazel also explained that real quartz crystals have irregular sides, with 3 sides slightly larger, while fake quartz crystals are completely symmetrical.
The second location was a beach, with cliffs made of red sandstone from the Permian period. The colour is very indicative of the environment they would have existed in, suggesting these rocks would have been laid down when this was a desert, located near the equator.
Hazel pointed out that some of the rocks have bigger lumps of stone within them, which are from flooding events in a desert, called wadis.
One can see various layers of different events recorded in the sediments…here breccia, a conglomerate of rocks with sharp edges, which tells us it comes from a much more extreme event, a flooding. So this is like a photograph of what happened in the wadi millions of years ago.
Below Hazel explaining why some layers of sediment go in opposite directions. This is because the direction of the water has changed, so that at different times we see it as it was coming from at us, hence horizontal lines, while at other times we are getting a view in profile, at an angle.
And here layers created by sand dunes, i.e. air moving across rather than water.
The following day we went to Plymouth University Geology department, where Hazel introduced us to a their technician, Ian King, who kindly showed us their resources, including an interesting collection of rocks, minerals and fossils.
Above, holding a “volcanic bomb”. Below a fossil collection.
And below some fluorite, characterised by cubic crystals.
and under UV light, transformed.
Below Hazel peeling some mica, from which glitter for make up is made.
Thin slices of rocks to be observed under a microscope.
Below Hazel explaining how students learn to distinguish rocks by understanding refraction of light, allowing for their identification . Here an app that allows students to explore rocks as if under a microscope, in thin slices, to learn these very specific distinguishing features.
Hazel explained other methods of identification, for instance through making marks with minerals and rocks, observing the traces and colours they leave on a piece of china clay.
With many thanks to Hazel Gibson and Ian King, for their time and wealth of geological knowledge.
Plymouth, Cornwall, April 2017.
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