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Almost all of the Upper Jurassic is made up of a thick series of mudstones, clays and shales. These are all very soft rocks and are easily eroded; as a result they form the low areas to the east of the limestone ridge. In the Fens they form a wide area beneath the younger sediments, but then thin and narrow northwards towards the Humber along the valley of the River Ancholme.
In the past these mudstones were frequently quarried for brick and tile making in areas where building stones were not easily available. The remains of many former pits can still be found, though they are not always easy to recognise since many are flooded and surrounded by woodland that has grown up since they were abandoned.
The mudstones were deposited on the continental shelf; marine animals, including ammonites and belemnites lived in these seas and their shells were well preserved because there was little life on the sea bed. Because the rocks are soft and finely bedded, the fossils often crumble as they are revealed.
A genus of ammonite found near Market Rasen in the 19th Century was given the scientific name Rasenia. In the past these were the best beds
for finding remains of the great Jurassic sea monsters like the Ichthyosaurs and Plesiosaurs, which preyed on the ammonites, but now it is difficult to find good exposures anywhere in Greater Lincolnshire. Some of these rocks were particularly rich in organic material and are thought to have been the source rocks for many of the hydrocarbon deposits (oil) under the northern North Sea.