Chalk is a soft, fine-grained, white to grayish variety of limestone that is composed of the calcareous skeletal remains of microscopic marine organisms including coccoliths and foraminifera. Some of the purest varieties can have up to 99 percent calcium carbonate (see Figure 6.82). The White Cliffs of Dover, England are one of the most iconic landscape features in the United Kingdom. The White Cliffs consist of Cretaceous-age chalk deposited about 89 to 85 million years ago in more tropical conditions than exist in the region today. The layers of chalk reach nearly 500 meter thick. The sediment the chalk formed from was coccolithopore ooze.