Wide bedrock valleys are formed by streams cutting through rocks, but how this happens is not well understood. The speed at which valleys widen is influenced by the size of the stream and the type of rock it flows over. Valleys in softer rocks widen faster than those in harder rocks. New data shows that valleys in soft rocks widen downstream twice as fast as those in medium rocks. Different types of widening mechanisms control how valleys widen, and these mechanisms are linked to the type of rock.