The river Thames or Isis becomes the boundary between Berkshire and Gloucestershire near Lechlade, and it flows in an easterly direction over a clay country, keeping a little to the north of the ridge of limestone hills upon which the villages of Buckland and Hinton Waldrist stand. Near Appleton the river bends to the north, curving round the outlying patch of limestone which forms Wytham Hill, and being joined by the river Evenlode. The united streams soon take a southerly course, and a little below Oxford are joined on the north by the Cherwell. The river then crosses the limestone formation near Sandford, and curves round by Radley to Abingdon. From Abingdon the river pursues a somewhat serpentine course with a general south-easterly trend towards Benson, being joined on the north near Dorchester by the river Thame. A little south of Benson the river, now the Thames proper, enters upon the chalk formation, across which it flows in a southerly direction to Streatley, and then takes a south-easterly course to Reading. At Streatley the river valley is deep, with steep sides separating the chalk downs of Berkshire from the chalk hills known as the Chilterns. The illustration above shows the Berkshire downs in the distance and the valley of the Thames in the foreground. At Reading the Thames is joined by the Kennet, and it is interesting to notice that the main stream adopts the direction of the tributary and flows with a north-easterly course to Wargrave, near which place the river Loddon meets it from the south, and again the direction of flow of the tributary is adopted, the Thames taking a northerly course past Henley. It is also of interest to observe that the river has turned away from the soft clays which form the ground south and east of Reading, and has cut a deep valley in the hard chalk from Wargrave onwards. Beyond Remenham the course of the river becomes easterly, and near Cookham it turns south and flows past Maidenhead to Bray. Near Bray the Thames leaves the chalk over which it has flowed for some 40 miles and enters upon a clay country, making its way in a fairly direct line to Windsor, the one place in the district where a knob of chalk sticks up through the clay. Windsor Castle stands upon this knob of chalk. The course of the river from Bray to Windsor is on the whole south-east, and after a big curve north at Eton the course becomes more southerly, with another big curve near Old Windsor. At Runnymede House the Berkshire boundary leaves the river, which flows on to London and the sea. The river Cole rises on the chalk not far from Ashbury, and flowing in a northerly direction joins the Upper Thames or Isis at the extreme western boundary of the county. The river Ock rises on the chalk near Uffington, and flows down the Vale of White Horse to join the Thames at Abingdon.