<P> Drumlins are found in groups called drumlin fields or drumlin camps . One of these fields is found east of Rochester, New York; it is estimated to contain about 10,000 drumlins . </P> <P> Although the process that forms drumlins is not fully understood, their shape implies that they are products of the plastic deformation zone of ancient glaciers . It is believed that many drumlins were formed when glaciers advanced over and altered the deposits of earlier glaciers . </P> <P> Before glaciation, mountain valleys have a characteristic "V" shape, produced by eroding water . During glaciation, these valleys are often widened, deepened and smoothed to form a "U" - shaped glacial valley or glacial trough, as it is sometimes called . The erosion that creates glacial valleys truncates any spurs of rock or earth that may have earlier extended across the valley, creating broadly triangular - shaped cliffs called truncated spurs . Within glacial valleys, depressions created by plucking and abrasion can be filled by lakes, called paternoster lakes . If a glacial valley runs into a large body of water, it forms a fjord . </P> <P> Typically glaciers deepen their valleys more than their smaller tributaries . Therefore, when glaciers recede, the valleys of the tributary glaciers remain above the main glacier's depression and are called hanging valleys . </P>

What action did glacial ice take as it shaped north america