Kent coastline of Kent is continuously changing, due to tectonic uplift and coastal erosion. Until about 960, the Isle of Thanet was an island, separated by the Wantsum channel, formed around a deposit of chalk; over time, the channels silted up with alluvium. Similarly Romney Marsh and Dungeness have been formed by accumulation of alluvium. Kent's principal river, the River Medway, rises near East Grinstead in Sussex and flows eastwards to Maidstone. Here it turns north and breaks through the North Downs at Rochester, then joins the estuary of the River Thames as its final tributary near Sheerness. The Medway is
Maidstone in Kent to be turnpiked was from Rochester to Maidstone, in 1728. The A20 runs through the town and the M20 motorway runs to the north. Opened in 1960 as the "Maidstone Bypass" A20(M), this was the first motorway south of London. Maidstone is a hub for major roads: the M20 motorway, the A229, A249, A20 and A26. The M2 motorway is a short distance to the north and the A21 also runs close to the town. The historic centre of the town is largely pedestrianised or with restricted access for private vehicles, which are diverted around a one way