A Trip Down Memory Lane EarthCache
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Ground zero brings you to one entrance of the Sendall Tunnel
Construction began on the 21st Nov.1889 and was opened on 21st Oct.1895 and connects two parts of St. George .
The volcanic rock that forms the hill where Fort George stands was drilled straight through as an alternative to the steep slippery slope and the unpaved street that was used prior to the building of the tunnel .
The Tunnel is a narrow passage way 340 ft/104 metres long and 7ft/2 metres high that tunnels through the hill to connect Bruce st. to the rest of St.George .It was built in a time when horse and donkey carts were the main mode of transport coming down the mountainous inclines.The tunnel was the key to preventing these carts from heading into the sea when carrying heavy loads down hill .The tunnel was also the first walking path for those wishing to go from the Carenage to the Esplanade .
Today the tunnel is used by both pedestrians and vehicles.
Please be observant as you walk through the tunnel as it is a shared tunnel with vehicles .
just a bit of interest ,while researching for this cache discovered history says the height of the tunnel is 7ft high while that is not the case at the tunnel signage
To learn about the volcanic rock that forms the hill that was drilled through you have to learn a little about the geology of the island itself
The geological history of the island began approx. 38 million years ago in the Upper Eocene Period .At that time there was only a shallow sea where Grenada now exists. The sediment deposits were composed of sand,silt,mud and calcareous mud; the rock formed from this process is now known as the Tufton Hall Formation.In between this geologists found layers containing volcanic minerals and other deposits of volcanic origin .Volcanic activity became more frequent in the Oligocene Period (37-26 million years ago )The volcanic activity during and following the deposition of the Tufton Hall formation deformed and up lifted the rock ,resulting in the folding and faulting which can be seen just north of Levera Beach .
The oldest of the volcanic rock series are the Andesite domes of Northern Grenada which formed in the (26-5 million years ago)
The Pliocene Period (5-2 million years ago)witnessed the advent of Grenada'smost intense volcanism.In the south west of the island ,basaltic lava flows estimated at 3.5 million years old are inter layed with reworked volcanic sediments .
The final stage of volcanic activity involved the formation of explosion craters throughout the island.Most notibly The Carnege of St George and the Queens Park are both believed to be explosion craters ,giving the island its best harbour and the best source of scoria gravel .These are thought to have been formed 12,000 years ago .
What is Volcanic Rock
Volcanic rock is a rock formed from magma erupted from a volcano .
It differs from other igneous rock by being of volcanic origin
How Volcanic rock is formed
Through cooling and solidification of magma or lava
Igneous rock may form with or with out crystallization either below the surface as Intrusive (plutonic) or on the surface as extrusive (volcanic rock).
What does volcanic rock look like
Igneous rocks that form too quickly for crytals to form have what is called a glassy texture,Obsidiar is the only glassy igneous rock,and can be identified by its dark colour
At high temperatures only minerals that have structures stable under those conditions will be able to crystallize. Typically those minerals are olivine, pyroxene, and plagioclase feldspar. Therefore igneous rock that forms at high temperature would be composed of these minerals and hence be dark or mafic in color.
To log this cache you must send me the answers to the questions and then go ahead and log cache .I will contact you if need be .
Q1
What type of rock was drilled to allow the tunnel to be built.
Q2
Explain what the Tufton Hall formation is
Q3
A building above ground zero shows many different types of rocks that have been used in the construction
a-What is the average size of those used in the construction not including the cnr stones
b-what colours are they
c- what type of rock is it
We hope you have enjoyed your visit to Grenada
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