The South Downs are formed from a thick band of chalk which was deposited during the Cretaceous Period between 100 and 66 million years ago within a shallow sea which extended across much of northwest Europe. The rock is composed of the microscopic skeletons of plankton which lived in the sea, hence its colour. The chalk has many fossils, and bands of flint occur throughout the formation.
The term ‘downs’ is from Old English dūn, meaning ‘hill’. The word acquired the sense of ‘elevated rolling grassland’ around the fourteenth century. These hills are prefixed ‘south’ to distinguish them from another chalk escarpment, the North Downs, which runs roughly parallel to them about 30 miles (48 km) away on the northern edge of the Weald.
Take a look at the same view, but this time in winter, here.