Sunday, 27 January 2019 19:41

Potted History Rocks!

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By Woudloper - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8020209 By Woudloper - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8020209

The Winnie-the-Pooh stories owe much of their magic to the iconic illustrations of E H Shepard. Many of these depict the inhabitants of the Hundred Acre Wood in recognisable heathland scenes, typical of parts of the High Weald ridge.

In fact, the rocks under your feet as you walk around the Ashdown Forest on your Potted History tour are the very reason the landscape around you looks the way it does, and they have their own story to tell.

The sandstones, shales and mudstones hearabouts comprise part of the Ashdown Formation, in turn a part of the Wealden Group of the Early Cretaceous. They range in age from about 140 to 130 million years old, and are broadly similar to related rocks which outcrop along the coastlines near Hastings and the Isle Of Wight. The rocks commonly exhibit structures such as cross bedding, and pebble bands, and often form cyclothems, or repeated sequences of coarser then finer grained sediments. These features, along with their mineral content, indicate that the deposits were laid down in river deltas and shallow lakes, and that the Weald was once located just south of a high landmass around the modern River Thames, and to the north of an open sea somewhere around Central France. The level of this sea fluctuated regularly, possibly as a result of minor climate changes, thereby causing the cyclic deposition.

Iguanodon Footprint
Iguanodon Footprint

While on the subject of climate, the area would have been warmer and generally wetter that it is today, and although fossils are scarce, remains of ferns and tree ferns have been found in East Sussex. Oh yes, and Iguanodon footprints and bones. Fancy meeting a dinosaur in the forest!

As the panoramic views indicate, the sandstone has found its way well above sea level since the Early Cretaceous. This is due to the Alpine Orogeny (before you get any funny ideas, an orogeny is merely the geologist's term for a period of mountain formation), which got underway about 65 million years ago, and is still going, to a lesser extent. Imagine a large layered cake - sponge - jam - sponge - jam, and imagine slowly but surely pushing together on opposite edges. The layers ruck up, or fold. This is exactly what has happened to the rock strata in this area, albeit as a fairly modest side-show to the larger mountain building event that created the Alps, powered by the African tectonic plate converging on it's Eurasian counterpart. The folding here has formed an extensive gently domed rock structure ranging from the Weald to Artois, known as the Wealden Anticline.

Over millions of years the inner parts of this dome have eroded, exposing ever older rocks, with younger formations like the chalk framing the picture. And aside from dictating that the Ashdown soil is acidic, the sandstones here shape many of the older buildings. They also contributed economically to the long defunct smelting industry, once a major factor in forest clearing to form the heathland, as there are local pockets of low grade iron ore to be found through the region.

So, despite not being quite so visible in outcrop as their contemporaries in Tunbridge Wells, the Ashdown sandstones have quietly formed a very special landscape, which is easily recognised and loved by fans of A A Milne's characters.

Read 1030 times Last modified on Monday, 27 April 2020 17:35
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