Geology
The rocks of Bowerchalke that can be seen today were deposited underwater between 120 and 70 million years ago (mya) until they were then uplifted from the sea and have been sculpted by periglacial weathering and erosion over the last 1 million years.
The unseen underlying rocks of Bowerchalke started to form in shallow seas surrounding volcanic islands up to 1,000 million years ago (Proterozoic), circa 60-70 degrees south of the equator, the latitude of present day Argentina. 'Proto-Bowerchalke' and the rest of England and Wales are part of Avalonia, a micro-continent that broke away from the southern landmass, and was 50 degrees south of the equator circa 500 mya. It joined with Baltica just south of the equator 400 mya, and was crushed between Baltica, and Laurasia (North America), and Gondwanaland (Africa, Australia, Antarctica and South America) 350 mya whilst near the equator (Variscan orogeny). Avalonia was landlocked and buffeted within Pangaea just north of the equator 270 mya, and became part of a desert in the northern tropics 230 mya. Eventually it formed the west coast of Eurasia as America split away to form the Atlantic Ocean 120 mya. Bowerchalke then laid under warm chalk forming seas until being uplifted 70 mya as Africa started to collide with the European plate in the (Alpine Orogeny), and in the cooler shallow waters river clay sediments capped the chalk. In the last million years the periglacial weathering has completely removed several hundred feet of chalk.
The main portion of the village is formed on the unique 'Bowerchalke greensand inlier' (an area of older rock completely surrounded by younger layers), highlighted in green on the adjacent map. It is not immediately obvious to the naked eye or on the standard Ordnance Survey maps with 10 metre contours but the apex is strikingly characterised by the 'island' drawn on the Andrews 1773 map. Its presence can be detected in the friable sandy soils in the centre of the village.
The Greensand inlier is a slightly dome shaped area of hard, coarse, olive-green coloured sandstone rock which has had its covering of softer chalk eroded away by 60 million years of weathering since the region was lifted out of the sea. The Cretaceous Upper Greensand is circa 120 million years old and was deposited in brackish, oxygen depleted, water when Bowerchalke was located at around 35 degrees north of the equator, roughly equivalent to southern Spain and Portugal. At that time it was still part of the supercontinent Pangea which was just starting to split and form the Atlantic ocean. The nearest continuous Upper Greensand exposure is along the A30 in the Nadder valley at Fovant. The closest 'unique greensand inlier' is near Andover in Hampshire.
Surrounding the greensand is a ring of younger Lower Cretaceous Chalk which is circa 100 million years old (darker blue on the adjacent map). The chalk was formed in warm, shallow, well oxygenated waters from the remains of micro-organisms (coccolithophores), over millions of years. At this time the Atlantic Ocean was circa 100 miles wide and Ireland, Scotland, Wales and Somerset were a single island with rivers draining their nutrients into the warm sea that covered Wiltshire, Hampshire, the east of England, Northern France, Denmark and northern Germany.
Surrounding the lower chalk is a ringlike region of still younger Middle Cretaceous chalk (circa 80 my)(light blue on the adjacent map). The whole area is bounded by the 'vast expanse' of Upper Cretacious chalk (shown white on the adjacent map) that continued to form when Bowerchalke was circa 45 degrees north, roughly equivalent to Bordeaux or the Dordogne in France. It was still under a shallow sea but Somerset, Ireland and Scotland had become separate islands. Bowerchalke was probably 15–30 miles offshore from the 'coast' between Shaftesbury and Dorchester.
Above the exposed Cretaceous chalk slopes of Marleycombe the hilltops are covered with a very young layer of Pleistocene 'clay with flint' that is circa 1-10 million years old and formed by alluvial sediments in cold shallow waters (pink on the adjacent map). The flints were formed in multiple layers as the clay sediment built up, and were then concentrated into a single dense layer as the last million years of sub glacial weathering washed the minute clay particles away.
The steepness of the north facing Marleycombe Down contrasts with the gentle rolling slopes towards Ebbesbourne Wake. This is mainly due to erosion during the sustained permafrost and tundra like conditions in the periglacial zones of multiple ice ages. Although the southern limit of the main glaciation is a line across North Wiltshire that corresponds to the M4 corridor, the sun rarely melted the north facing snow pockets on Marlecombe Down, thus they eroded the soft chalks and clays by eating back into them, leaving the very steep scarp faces. The sporadic melting of snow and ice was forced to drain north east along the course of the River Chalke and River Ebble in occasional summers, plus scouring the now dry channel that forms Church Street and Costers Lane. The southern boundary between the greensand and chalk, is concealed beneath a layer of heavy clay that has accumulated at the bottom Marleycombe Down due to the periglacial solifluction. This scouring has also located the natural spring that supplies the River Chalke, whereby the rainfall from the surrounding watershed, having been filtered and channeled through the porous chalk, rises at the natural spring at 'Mead End' where the water table sits on the underlying impervious greensand layer.
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