Chesil Beach

6 Apr 2011

At www.jurassiccoast.com/380/the-coast-uncovered-30/geo-highlights-226/ches... ... the Jurassic Coast was featured in BBC TVs Countrywise April 5th 2011 - and Chesil Beach was pictured and discussed. I had a quick look on the Net by search engine and came up with the above site. I can remember reading some years ago a tourist booklet that said the pebble and shingle beach was raised up in a storm. However, its origins are more interesting as it is formed of material from collapsed cliffs along the Dorset coast, in the bowl between Portland Bill and Exmouth. It is basically made up of pieces of stone that have fallen into the sea as the cliffs have receded further and further inland. I assumed this meant its origin was recent as the Channel has formed during the Holocene - and was dry land during the last Ice Age. However, it seems that during the Eemian inter-glacial, some 125,000 years ago, sea levels were much the same as today - and the Channel existed then, as well, and likewise the current cliff line was roughly in the same position as now. When the last Ice Age set in sea levels, the article claims, fell, and erosion of the cliffs stalled - they were untouched by the sea (for 100,000 years). Massive amounts of debris from the fallen cliffs formed huge fans and when the sea levels rose again as the Holocene kicked in the material was carried east by long shore drift, and formed the Chesil Beach formation. 

Now, there are several points to ponder here - was the cliff line really nearly the same as it is now 125,000 years ago? Anyone who has done any coastal path walking will be aware that cliffs in Dorset are desperately prone to collapse - with or without tectonic activity. Some of this has happened in quite recent time - the National Trust have a booklet on the Jurassic Coast that describes some of the landslips that have taken place over the last couple of hundred years. Some parts of the cliff are made of soft rock such as mudstone - and this virtually crumbles in the hand. Do we have a change in mind on the geology? The tourist booklets of a few years ago may have faithfully produced the old geology but since then they have been overtaken by the new geology - but what exactly is going on? I can see the effect a changing geoid might have in redistributing ocean waters, dry land being periodically submerged, and vice versa, at continental margins. This is not what the new geology implies however as they are talking in terms of the Ice Ages locking up water as an ice sheet at high latitudes and causing lower sea levels. If the scouring of the bowl west of Portland Bill did not occur in the Holocene and then it perforce must have happened in the last inter-glacial - is this what is at the bottom of this revision? If so it is based on some dodgy assumptions as Chesil Beach has all the hallmarks of a catastrophic origin.