Why are the White Cliffs of Dover white?

The White Cliffs of Dover are white due to their composition of chalk, a soft, white, porous limestone formed from the skeletal remains of microscopic marine organisms. Over millions of years, these organisms accumulated on the seafloor and were compressed into chalk rock. The white color of the cliffs is a result of the high calcium carbonate content in the chalk reflecting sunlight, giving them their iconic white appearance.