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chalk erratics

23 February 2015

An article in February's Down to Earth (geology magazine), issue 90, ISSN 0969-3408 … concerns discoveries in chalk quarries over the years. Most finds are never reported. It is only the odd quarrymen who will bring attention to an oddity, and inform a geologist. In turn, most academic geologists rely on others to do the dirty work, and in spite of this there are a surprising number of erratics in museum collections (donated over the last 100 or so years).

Quarrymen and geologists are used to finding pyrites in chalk. They are fairly common. So, when a large brown oval stone was found that was too big to lift they set about it with a sledgehammer – and broke it into manageable pieces. It was at that point one of them noted it was not a pyrite – but was a crystalline structure. It was in fact a granite. What was a granite boulder doing inside a chalk formation?

The oval shape indicates the boulder had been rounded by water – and resembled the sort of stone found on beaches, smoothed due to countless tides. It also obviously came from somewhere else as granites are not the sort of stone you will find on the North Downs, not too far from Croydon. It was found the nearest match was in Scandinavia, close to Oslo. How did it get inside the chalk?

As it turns out it is not the only erratic found in chalk (as opposed to above the chalk). Cherts, green schists, porphyry, quartz, and sandstone pebbles, cobbles and boulders have also been found over the years – a fact usually left unmentioned. Geologists that visit museum collections do of course know about them – and they are somewhat of a mystery deposit.

Chalk is the dominant Upper Cretaceous rock, an almost pure limestone. It is thought that it was laid down in warm seas on the ocean bottom. It is 97 per cent composed of coccolith debris (broken shells of minute sea creatures, or algae). It is so pure it is thought it represents the flooding of some of the world's land masses in the Cretaceous – as a result of global warming. The reality of the chalk and its widespread distribution is thought to represent former sea beds. They are so widespread the climate is claimed to have been much warmer – why else was the North Pole region vegetated. Britain, it is said, enjoyed a sub tropical climate. It is and was thought that because of the purity it was unlikely much terrestrial drainage occured as there is simply a lack of run-off material from rivers and rainfall. Great thicknesses of what was originally soft chalk mud built up in many parts of western Europe, and in other parts of the world. In places the chalk is up to 1km thick – so why does it contain water rounded stones with a geographical origin elsewhere. Is that not a little embarrassing?

Indeed it is. Various ideas on the transportation method have been aired over the years, from transport by sea weed, transport by glacier (in a global warming environment), and transport inside the stomachs of marine reptiles such as the Pleisosaur (for digestion or ballast), etc. Why would a Pleisosaur swallow a boulder?

On no occasion was the idea broached that erratics in chalk were transported by water. The reason is that chalk is assumed to have been laid down over countless thousands of years, rather than quickly. It represents a long geological episode – most of the Cretaceous. Chalk is said to represent the bottom sediments of warm seas, built up over time, and later raised above sea level by tectonic processes – with a terrestrial mechanism. In the Cretaceous world there were many dinosaurs – and a host of marine reptiles. None of these are found in chalk deposits. They are always below the chalk. What is that telling us?

I would suggest it is telling us the chalk was laid down quickly, by a huge tidal wave that dredged the bottom of the ocean and deposited the sediment on the surrounding land masses. In that scenario the erratics would not be mysterious – and neither would the lack of fossils from the dinosaur era. The culprit? Well, a big asteroid is said to have struck Earth at the end of the Cretaceous – could that not have caused a tidal wave?

PS – the chalk forms three distinct layers and a succession of events may be appropriate. There is also the possibility that the asteroid caused the oceans to change place with the land, the Earth moving at the axis of rotation and creating a new geoid. What I found interesting is that water transport for the erratics was not on the agenda – why?

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