The second or central division of Berkshire is the district of the chalk land. The downs of Berkshire are separated from the Chiltern Hills, which are the chalk hills of Oxfordshire, by the valley of the river Thames, whilst on the west the chalk downs run on into Wiltshire without any natural break. The chalk ridge rises sharply up from the Vale of White Horse, and a large part of the crest is over 700 feet above the sea. White Horse Hill attains a height of 856 feet, and the village of Farnborough is 712 feet above sea level. There is a general slope of the chalk surface downwards towards the south, so that even the high part of Lambourn Downs is well below the 700-feet contour line, and long and beautiful valleys run up from the Newbury district into the chalk downs. The northern border of the chalk district is a well defined line; not so the southern border. The chalk gradually bends downwards underground and is covered by sand, gravel, and clay, so that in many places we find the upper part of the hills sandy or clayey whilst the valleys beneath them are chalk. Thus Bussocks Camp and Snelsmore Common near Newbury are situated upon a ridge of gravel, sand, and clay, but the road from Chieveley to Newbury in the valley below the camp runs for most of the way along a chalk valley, and the chalk extends all around, but underneath the sand, gravel, and clay. Hence there is no definite southern boundary to the chalk district, and there is a bit of chalk country near Inkpen. The projecting part of Berkshire, bounded on the south by a line drawn from Twyford to Maidenhead and on the other sides by the river Thames, is also mainly a chalk district. The southern division of the county has in consequence no definite northern border, but a line drawn from Hungerford in the west to Maidenhead in the east will have very little of chalk district to the south and very little forest country to the north, and is consequently a good practical boundary between the second and third divisions of Berkshire. The scenery of the southern division is quite different from that of the other two divisions. The country consists to a great extent of wide and flat table-land 300 to 400 feet above the sea, in which the rivers and streams have cut valleys. There are also extensive tracts of clay land, but the clay is often concealed under a few feet of gravel. 5. WATERSHED. RIVERS AND THEIR COURSES. LAKES. With the exception of a small tract in the south-western corner the county is wholly drained by the river Thames and its tributaries; that is to say, with a very few exceptions, every brook and stream in Berkshire is more or less directly a tributary of the Thames.