Sussex's geology typically runs in bands running east-west. Running across the south of the county, from Hampshire to chalk cliffs of Beachy Head lies the chalk ridge of the South Downs. The South Downs form the southern rim of the Wealden anticline, a large and eroded dome of rocks, largely of Cretaceous age (140-65 million years old) which has had a significant influence on the development of the diverse topography of the county. Small exposures of rocks of Jurassic age (195-140 million years old) occur on the main axis of the Weald anticline and rocks of Tertiary age (65-2 million years old) are exposed along the coast in West Sussex. During the ice ages no glaciers reached Sussex, but the county was under the influence of very cold tundra-like conditions at the edge of the ice sheets. Processes of erosion and deposition during this period have contributed significantly to the formation of the present landscape.