Noses are 20 feet long, and mouths stretch 18 feet. Had the presidents been carved full-body, they would stand 465 feet tall. Borglum chose to create his work from Mount Rushmore, knowing that it was a solid mass of granite. Granite is a tough, hard rock because it consists of interlocking crystals of the minerals quartz, feldspar, and mica. Weathering breaks rock down into smaller pieces.
Granite resists weathering more than layered and bedded sedimentary rocks do.
Rock outcrops are constantly subjected to the elements, which gradually cause chemical changes in the rock's minerals. Oxygen and carbon dioxide in runoff waters produce chemical weathering . Physical weathering occurs when water, lodged in fractures in the rock, freezes and expands in all directions, forcing the sides of the fractures apart. Tree roots have the same effect. Rocks exposed to long periods of alternating heat and freezing disintegrate into sand and clay.