Major climate changes in Britain causing sea level changes in the last 2.58 million years, with mini Ice Ages, the ice sheets did not extend to Surrey but sand and gravel deposits swept towards the fledgling River Thames were spread in all lower parts. Gravel terraces at various heights on the valley sides are the remnants of successive floodplains, the highest terrace being the oldest and the lowest the youngest. The most prominent terraces mark the former levels of the Thames in north Surrey. Along tributary slopes, a deposit, head, forms the main sediment of latest age. Head comprises angular pieces of rock and soil derived locally from the extensive frost-shattering of rocks and the subsequent movement of this material down valley slopes. Large areas of clay-with-flints, derived from the weathering of material overlying the present day chalk, occur across the North Downs. One particular suite of sediments that occur in the Guildford vale is known as the Headley Formation and comprises gravel and sand on top of the chalk. These sediments contain marine fossils and were probably derived from erosion of the Greensand and Tertiary rocks during the Paleogene.