Before revision: During glacial periods, for example, the Pleistocene ice ages, it is in these locations that glaciers initially form and then, as the ice age proceeds extend downhill through valleys that have previously been shaped by water rather than ice.
After revision: ,During glacial periods, for example, the Pleistocene ice ages, it is in these locations that glaciers initially form and then, as the ice age proceeds extend downhill through valleys that have previously been shaped by water rather than ice.