In the Upper Cretaceous, some 100 million years ago, the level of the oceans rose above Upper Normandy (656 to 984 ft / 200 to 300 m higher). The drop in sea level that then began and the rise of the ocean floor generated cliffs with successive chalk and flint layers 100 to 300 ft (30 to more than 100 m) high. Chalk resulted from the accumulation of limestone particles or coccoliths over millions of years. The landslides at the foot of the cliffs are due to continental, marine, and biological erosions. The chalk quickly dissolves while the flint turns into pebbles.