lining it with London Clay (which is virtually impermeable) it prevents waste and hazardous substances from entering the groundwater. Tunnels in London Clay London Clay is an ideal medium for boring tunnels, which is one reason why the London Underground railway network expanded very quickly north of the River Thames. However, south of the Thames, the stratum at tube level is composed of water-bearing sand and gravel (not good for tunnelling) with London Clay below, which partly explains why there are very few tube tunnels south of the Thames. London Clay has a stand-up time long enough to enable support to be installed without urgency. It is also almost waterproof, resulting in virtually no seepage of groundwater into the tunnel. It is over-consolidated, which means that it was once subject to an overburden pressure higher than it is subjected to today, and expands upon excavation, thus gradually loading the support, i.e. it is not necessary to stress the support against the ground. Uses Due to its impermeability especially when exposed by ploughing, London Clay does not make good agricultural soil. In Middlesex, ploughing London Clay land so deep that it brings up clay has historically been called "ploughing up poison". It was also disliked for building houses on, though inevitably most of Greater London is built on it. Jane Ellen Panton in her Suburban Residences and How To Circumvent Them (1896), remarks: "I do not believe clay is or even can be fit for anyone to reside upon ... though roses flourished magnificently children didn't, and coughs and colds [lasted through autumn and winter, this at Shortlands]". Many London buildings have ultimately been constructed with London Clay. When compressed and burnt, London Clay can be fashioned into brick known as London stock, identifiable by its yellowish brown hue. Fossil flora and fauna Plant fossils, especially seeds and fruits, are found in abundance and have been collected from the London Clay for almost 300 years. Some 350 named species of plant have been found, making the London Clay flora one of the world's most diverse for fossil seeds and fruits. The flora includes plant types found today in tropical forests of Asia and demonstrates the much warmer climate of the Eocene epoch, with plants such as Nypa (Nipah palms) and other palms being frequently encountered. Notable coastal exposures from which fossils can be collected are on the Isle of Sheppey in