As limy ocean mud hardens into limestone, water is squeezed out, and the rock shrinks slightly, creating a network of cracks. Rainwater is slightly acidic, and plants produce acids as they die and become part of the soil. As acidic water seeps through cracks in the limestone, calcium dissolves from the surrounding rock and the cracks grow wider and deeper, year by year. Dolomitized limestone is much more difficult for acids to dissolve, so in some places there are wide areas of smooth bedrock with few cracks and plants. Some of these also preserve deep scratch marks where moving glacier ice dragged boulders across their surface during different ice advances.