Geological Forensics 5
Part of a series of EarthCaches that demonstrate how Geology and Forensics are often use together to explain features we find in rocks.
Geology is the study of the Earth; its rocks; its structure; everything within it - and also the processes that shape its surface. Forensics is the use of science and technology to investigate and establish facts. Geologists, and especially palaeontologists, use the evidence found in rocks to deduce the environment that a sedimentary rock or fossil was deposited in, or the 'story' of how the crystals in a metamorphic or igneous rock formed.
As stated, there are three main types of rock:
1 – Igneous Rocks
- These form when molten rock (known as ‘magma’), cools. As a general rule, the slower it cools, the larger the crystals found within it, and vice versa.
- Common examples include:
- Granite - formed when a specific type of magma cools slowly underground over millions of years, granites typically have a paler colour, containing large crystals, giving some quite pretty textures
- Gabbro - formed when magma of different chemistry to granites cools slowly underground over millions of years. Gabbros typically have a darker colour than granites, but do have larger (>1cm) crystals.
- Basalt – formed by magma being extruded as lava at the Earth’s surface from a volcano. These typically cool very quickly, giving us igneous rocks which contain small (millimetre-sized) crystals.
- Obsidian – formed in the same way as basalt, these cool even more rapidly, forming microscopic crystals, often giving a glassy appearance.
2 – Sedimentary Rocks
- These form when older rocks have been weathered and eroded, then transported, and then deposited, usually in layers. Over time, these layers are then squashed by the weight of overlying sediments, slowly turning the sediments into sedimentary rocks.
- Common examples include:
- Mudstone and sandstone - the names are determined by the size of the ‘grain’ in the rocks (mud being a smaller grain size than sand)
- Limestone – generally formed in ancient warmer, shallow seas (e.g. like the present day barrier reef off NW Australia), these can be formed around beautiful coral reefs, where fish, snails, worms, and various shells all lived, inter-acting with each other as a community, living in and on the mud and sand that made up the ancient sea-floor.
- Coal – formed in ancient (up to 300 million year old!) swamps, full of massive trees, ferns and other plants.
3 – Metamorphic Rocks
- These form when older rocks are buried deep within the Earth’s crust over millions of years, squashing and heating the rocks, so deforming them and causing the original structure within the rock to crystallise or re-crystallise, depending on the original rock. Common examples include:
- Marble – formed when ancient limestones are crystallised, destroying the delicate fossils that may have been present.
- Slate – formed when ancient mudstones are crystallised, squashing the mud grains together, then turning these grains into flatter minerals, all elongated in the same direction, so giving slate its characteristic property of being about to be easily split (e.g. for roof tiles).
- Schist – formed when slate is further deformed, causing the already-elongated minerals to grow larger and more prominent.
- Gneiss (pronounced ‘nice’) – formed when schist is further deformed, causing the already-elongated minerals to grow even larger still. These crystals can then often become bands of colour within the rock, these themselves also being deformed, taking on the appearance of wavy lines of differing colour to the ‘main’ mass of crystals that surround it.
Given how many times I have suffered the ‘hardship’ caused by having to be dragged around the shops that Gunwharf Quays is now (in)famous for, and the understanding that geological outcrops are rare in Portsmouth, I thought I would help my fellow Geocachers who have to ‘experience’ Gunwharf Quays by placing a nice easy EarthCache, and so also provide a real excuse to “nip off for a bit”.
At the coordinates, one will find a gateway in a high wall. BE CARFEUL HERE WITH YOUNG CHILDREN – the adjacent modern road outside the gate can get very busy, and it could be easy for a child to run towards/into the road. This gateway is known as ‘Vernon Gate’, and is the historic entrance to the former Royal Navy shore establishment HMS Vernon. It now provides pedestrian access to the Gunwharf Quays Plaza.
At the west side of the gate is a plaque, which reads:
On 17th May ???? a warrant was issued by Charles II for the purchase of the ground, buildings and wharf at Gunwharf for use by the Royal Navy.
The site remained in operational use from this date, with particular strategic importance during WWI and WWII, operating under the name of HMS Vernon from 1923.
In 1996 Berkeley Homes commenced the regeneration of the site for residential and commercial use, which was completed in 2010.
As usual with EarthCaches there is no physical cache. To Log this EarthCache, please study the road/building/walls that comprises Vernon Gate, then email the answer to these questions:
QUESTION 1: Look at the cobbles that make the original floor/roadway at the Gate – do you think these are igneous or metamorphic rocks?
QUESTION 2: From your answer to Question 1, of the 4 choices of such rocks as given above, which 2 do you think the cobbles were mostly carved from?
QUESTION 3: Look at the large bollard at the base of the northern gate post. What rock is this made from, and how can you tell?
QUESTION 4: What year did King Charles II issue his warrant?
If one then exits the Gunwharf Quays site at the Gate and crosses the road (there is a traffic-light-controlled pedestrian crossing directly adjacent to the Gate), look back and take in the sculpture and shapes of the whole impressive-looking ‘Vernon Gate’.
QUESTION 5: What pale-coloured softer rock was used to make the arches and decorative features above the original roadway into Gunwharf?
QUESTION 6: On top of the gate is a golden weather vane. On top of this is a model. What is the model of? Is it A) a wooden ship, B) a crown, or C) a dolphin?
Now either log this EarthCache and return to the hardships of the dreaded shops, or why not sit at the water’s edge and watch the world go by?
Good luck!