<P> The presence of ice over so much of the continents greatly modified patterns of atmospheric circulation . Winds near the glacial margins were strong and persistent because of the abundance of dense, cold air coming off the glacier fields . These winds picked up and transported large quantities of loose, fine - grained sediment brought down by the glaciers . This dust accumulated as loess (wind - blown silt), forming irregular blankets over much of the Missouri River valley, central Europe, and northern China . </P> <P> Sand dunes were much more widespread and active in many areas during the early Quaternary period . A good example is the Sand Hills region in Nebraska, USA, which covers an area of about 60,000 km (23,166 sq mi). This region was a large, active dune field during the Pleistocene epoch, but today is largely stabilized by grass cover . </P> <P> Thick glaciers were heavy enough to reach the sea bottom in several important areas, thus blocking the passage of ocean water and thereby affecting ocean currents . In addition to direct effects, this caused feedback effects as ocean currents contribute to global heat transfer . </P> <P> Glaciation has been a rare event in Earth's history, but there is evidence of widespread glaciation during the late Paleozoic Era (200 to 300 Ma) and during late Precambrian (i.e. in the Neoproterozoic Era, 600 to 800 Ma). Before the current ice age, which began 2 to 3 Ma, Earth's climate was typically mild and uniform for long periods of time . This climatic history is implied by the types of fossil plants and animals and by the characteristics of sediments preserved in the stratigraphic record . There are, however, widespread glacial deposits, recording several major periods of ancient glaciation in various parts of the geologic record . Such evidence suggests major periods of glaciation prior to the current Quaternary glaciation . </P>

What is thought to have caused the ice ages apex