The chalk contains very little mud or sand from neighbouring lands. The formation of chalk may have removed carbon dioxide from atmosphere. The long quiet period of the Chalk Age came to an end with a great folding movement which formed the Alps. Our various chalk ranges stand up because chalk is fairly hard, but permeable to water. The chalk downs are pierced by a number of narrow valleys, some of which still carry rivers, while others are now dry. The rivers were there before the surface had been lowered on each side of the chalk, leaving it as a range of hills. Once a river is formed, it can easily cut a gorge even through much harder rock than chalk. It is very likely that during Cretaceous period North America was drifting away from Europe, and the North Atlantic Ocean was forming; but certainly if this drifts occurred, it went on long after the chalk had been laid down.