Up until humans started to modify the landscape with machines using fossil fuels it could have been argued that the beavers had the greatest impact on the landscapes in North America. Two American geologists Ruedeman and Schoonmake in the 1930s were convinced that it was beavers working over many 100s of years that had changed a V-shaped valley into overlapping layers of sediments by damming streams to make small ponds which filled with sediment. This process is described in a book by Frances Backhouse, Once They Were Hats, In search of the Mighty Beaver.