Kent is the south-easternmost county in England. It is bounded on the north by the River Thames and the North Sea, and on the south by the Straits of Dover and the English Channel. The continent of Europe is 21 miles across the straits. Geology The major geographical features of the county are determined by a series of ridges running from west to east across the county. These ridges are the remains of the Wealden dome, a denuded anticline across Kent, Surrey and Sussex, which was the result of uplifting caused by the Alpine movements between 10-20 million years ago. The dome was formed of an upper layer of Chalk above subsequent layers of Upper Greensand, Gault, Lower Greensand, Weald Clay and the Hastings Beds. The top of the dome eventually eroded away through weathering and ridges and valleys resulted across Kent and Sussex due to the exposed clay eroding at a faster rate than the exposed chalk, greensand and red sandstone and normal sandstone. The following ridges and the valleys have formed across Kent, listed from north to south: the low lying London Clay marshlands along the Thames/Medway estuaries and along the North Kent coast; the chalk North Downs, containing the highest point of the county, Betsom's Hill, at ; the Vale of Holmesdale formed from Gault Clay overlaid in the north with the upper layer greensand; the Greensand Ridge, formed from the lower layer of greensand, containing the source of the River Medway and its tributaries; the Low Weald, a Weald Clay valley the sandstone High Weald. The chalk comes in three layers: the upper layer, about 500 feet thick, is a pure white limestone bedded and jointed with localised masses of flint (ideal for cement); the middle layer, about 170 feet thick, is a compact white chalk occasionally hard enough for building; the lower layer, about 170 feet thick, is a greyish marly chalk. Dartford, Gravesend, The Medway Towns, Sittingbourne, Faversham, Canterbury, Deal and Dover are built on chalk. The eastern part of the Wealden dome was eroded away by the sea. The White cliffs of Dover occur where the North Downs meets the coast. From there to Westerham is now the Kent Downs Area of Outstanding Natural Beauty. The chalk displays all its characteristic features such as steep sided dry valleys, and sunken roads. Greensand is a calcareous sandstone containing an uneven distribution of the mineral