Dale Fort Blog Number 3

3 04 2012

The History of Dale Fort (part 1)

This post is the first in a series that will describe the pre-history and history of Dale Fort.

We begin with the rocks the place sits on:

The Dale peninsula is made of sedimentary rock.  It was formed when particles of material worn from other rocks, settled out from water as mud or sand and were compressed over an immense period of time to make new rock.

400 million years ago, the American and European continental plates collided.  Our part of what is now Britain was somewhere near the edge of the continent formed by the merging of the two landmasses.  Geologists call it the Caledonian Continent and the mountains raised are known as the Caledonian Mountains.

The Caledonian Continent was roughly twice the area of Australia. The parts of it that straddled the equator were extremely hot and wet.  Among the mountains there were lakes and saltpans; there were no plants and no soil.  What is now Wales was around 1500 miles south of the equator on the southeast coast.  Over the next 50 million years, the continent drifted northwards, (at a speed roughly equivalent to the rate at which your fingernails grow) until the part that was to become Britain was on the equator.  During this time thousands of metres of rock, stained red with iron (III) oxide wore away from the mountains.  They were washed as sediments down the rivers.  Not all the sediments were to reach the sea, some settled out at the edges of wide, sluggish rivers and in their associated lakes.

The Dale Peninsula (Dale Point below middle right)

To the south of the mountains, nearer the sea (the Rheic Ocean) the rivers slowed and massive deltas formed.  A small fraction of these sediments, much compressed, is what makes up the old red sandstone of the Dale Peninsula today.

It is possible to detect what appear to be bands or layers in the rock which show that deposition may have been seasonal. There are also surface cracks in the rock, which may have formed during periods of drying.  The extent of the Rheic Ocean seems to have varied, extending northwards flooding the land and then retreating southwards, re-exposing the land and river deltas.  These marine incursions leave their trace today in the form of occasional bands of green rock in the mostly red sandstone.  Old red sandstone is stained red because it contains an iron compound, which is fully oxidized (like red rust). The sandstone consists of a matrix of this compound (iron (III) oxide) holding together harder particles of quartz.  When the sea flooded the southern part of the Caledonian Continent, the red sediments were altered chemically by the seawater and the iron (III) oxide was reduced to iron (II) oxide which is green in colour. This produced the green sandstone seen in exposures at Castle Beach and running more or less east west underneath Dale Fort.




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11 10 2013

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