Mount Rushmore National Memorial / A monument commemorating the conception, preservation, and growth of the great American republic

by Mount Rushmore National Memorial Society of the Black Hills

At the Battle of the Pyramids, Napoleon is reported to have exhorted his men by saying, “Soldiers, from these pyramids forty centuries look down upon you.” From the standpoint of human history four thousand years represent great antiquity indeed. But as one gazes upon the rugged slopes of Mount Rushmore, he is face to face with antiquity beside which the age of the Egyptian pyramids seems but a moment.

How old is the granite of Rushmore? We have a yardstick by which we can measure that quite accurately. Not far from the mountain, in a subsidiary mass of granite, there was found a few years ago a small piece of coal-black, lustrous mineral known as pitchblende or uraninite, of which the chief constituent is the heaviest known element, uranium. We know that uranium continually undergoes atomic disintegration, changing at a slow, but uniform and measurable rate into lighter elements. The end product of this change is the metal lead. If we submit the specimen of pitchblende to chemical analysis, determine how much lead it contains, how much uranium is still left, it is a comparatively simple calculation to determine from the known rate of change, the number of years that have elapsed since the pitchblende came into existence. That experiment has been performed and the result is one billion four hundred and sixty-five million (1,465,000,000) years. Bear in mind that this enormous figure represents the time that has elapsed since the molten rock came to rest at some depth under the surface of the earth, and cooled sufficiently to crystallize into granite. It represents the age of the solid granite.

But, although the granite of which the mountain is composed dates back to a period almost inconceivably remote, Mount Rushmore itself is much younger. We know that all of the granite mountains of the southern Black Hills were carved out of the rocks by the process of erosion. Field evidence indicates that fairly early in the Tertiary period, approximately thirty million years ago, erosion had carved out the topography of the Black Hills into much the same stage as we see it today. Perhaps Mount Rushmore was not fully born in that period; its form may not yet have been completely sculptured under the chisel of time, but we know that its age must be measured in millions of years and not in centuries.

Mount Rushmore is a child of weathering and erosion. They brought the mountain into being and gave it form. But those relentless parents will not be content to leave their child as they fashioned it. They will continue their work of disintegration on the surface of the rock and along the cracks, until eventually they will completely destroy the mountain they formed, and long before the mountain will have been destroyed, the magnificent carvings of man will disappear. “How long,” we anxiously ask, “will the carvings endure?” Two processes will tend eventually to destroy the memorial, chemical weathering and physical disintegration.

A typical view from the Needles highway with the Cathedral Spires in the background.

Fantastic formations in the Badlands. The variegated coloring is at its best in the early morning or the late evening.

Chemical weathering will take place very slowly, so slowly that if it were the only destructive process we had to consider, we could with some confidence say that the memorial would endure for hundreds of thousands of years. And the progress of chemical weathering will probably be impeded by the sculpturing of the memorial, for on the figures the rock will be smoother, water will drain off more rapidly instead of penetrating, lichens and other vegetation will not have as secure a foothold as on the natural face of the rock, and thus will not contribute to so great an extent their destructive acids to such waters as do penetrate.

Physical disintegration is somewhat more to be feared. This operates in two ways, by exfoliation due to changes in temperature, and by frost action. Differential stresses set up by unequal expansion and contraction, owing to the poor heat conductivity of granite, tend to spall off or exfoliate the surface layers of rock.