Most varved deposits were formed during the melting of the great continental glaciers during the Pleistocene ice age, and form in glacial lakes near the ice margin. Loess were another significant factor in evidence of ice ages because they are usually associated to the Pleistocene Era. Loess deposits are often 10 to 15 meters thick, and they usually are not layered. The older the rock record of an ice age the less it is preserved, because rocks are altered by metamorphism over long periods of time. We will try to understand the cause of an ice age, but there are several theories about glaciation and ice ages.
Several conditions are necessary for an ice age to occur.
The only adequate source of water for such massive amounts of ice are the oceans, and wind and weather patterns have to be specific. For example, to preserve snow year around the summers have to be cool. Dust absorbs radiation and reflects some of the Sun's heat, thus cooling the Earth's surface. At first scientists thought it was volcanic dust, but it has not been any evidence of massive volcanic dust accumulation during ice ages. Another theory is based on evidence in dunes and loess blankets.