KEYSTONE, South Dakota (CNN) -- Mount Rushmore is a towering tribute to four towering U.S. presidents, so it comes as no surprise that an annual check-up would be anything but routine.

George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln get a good going over every year, as park officials look for cracks that may have developed. Then a repairman scales the granite busts, filling in fissures with silicone, so that water can't get in and wedge the rock apart.

With regular maintenance, the busts -- carved between 1927 and 1941 -- are expected to have a long life; the monument erodes only at a rate of about one inch (2.54 centimeters) every 10,000 years.