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Geology of Maiden Newton and the Surrounding Area

I am entirely indebted to Doreen Smith for the following article and accompanying photographs.

The uppermost geological layer in the Maiden Newton area is composed of sands, gravels and clays deposited by rivers during the Tertiary period. There are millions of years of these deposits much of which has subsequently been worn away by weathering. Rainfall is the principle agent of wear and tear on earth. Slowly but surely weather of all kinds changes the landscape.  These slumps in the hillside above Maiden Newton are almost certainly the result of freeze-thaw action during the last ice age some 10-15000 years ago.

The Upper Chalk immediately below the river deposits absorbs and holds rainfall. Although glaciers never reached this far south, because there were vast ice sheets to the north, several hundreds of metres thick, tundra-like conditions prevailed in Dorset. The water in the Chalk froze, the ice increases in size thus shattering the chalk particles. When warmer conditions returned the chalk was a mush and slid downhill if it was on a slope, carrying the river materials with it.

The patches of gorse are a good indication of the Tertiary materials which tends to be acidic rather then alkaline like Chalk. You can see the patches on most of the hillsides. Where there was principally gravel, man has used it for various commercial purposes. The best place to see exposures of the gravels is at Hardy’s Monument. You can find flint fossils in the gravels. They were originally in the Chalk, chemically changed from calcite into silica, a very hard fine material and were removed by the rivers. They were then redeposited all over Dorset.

There are few exposures of the Upper Chalk as even the small farm quarries are rarely used these days. A fine-grained material it does have fossils within it but they can be very difficult to see as they are white on white. The original shell of the fossil has dissolved leaving a chalk cast, as it does in much of the Middle Chalk, well exposed on White Sheet Hill. Sometimes the calcite shell remains and fine specimens of the sea urchin Micraster can be found. The Middle Chalk at White Sheet was mainly used for producing lime for agricultural purposes. Lime kilns can be found all over Dorset wherever there is a limestone layer, chalk being a particularly fine form of limestone.

The Lower Chalk is rather different as can be seen at Rock Pit Farm, the car park and picnic site in Chilfrome Lane. It is also one of Dorset’s Important Geological Site. (DIGS). It is fossiliferous and glauconitic. The glauconite is a dark green mineral which gives the chalk a much darker look than that of the Middle and Upper Chalk.

Below the Chalk lies the Greensand on which the Frome and the Hooke both flow. Again the mineral glauconite is much in evidence and its colour gives the material its name (from the Greek glaukos meaning blue-green or blue-grey). There are two layers to the Greensand. At Tolliford the hard uppermost layer needed much heavy machinery to put the gas/water pipes through. Exposures of this hard layer can be seen at the western end of Eggardon, again a DIGS. It is a well cemented pebbly layer. Beneath that is the same material but now it is uncemented and has been used as a sand component of cement. It is easily seen round the river banks. Certain layers are full of a small fossil oyster called amphidonte, formerly called Exogyra. The soft Greensand is also a large component of the fertile water meadows as the river removes it from the banks and re-deposits it across the valley when in flood. It is what turns the water the familiar dark brown as it rushes under the bridge at Whitehall.

Alacrify Ltd. 01300 320076. jon@alacrify.co.uk