**Geologic Forces**

![](media/image1.jpeg){width="5.5in" height="2.7333333333333334in"}

The geologic forces of heat, pressure, deposition, uplift and erosion
have combined with millions of years of time to create the spectacular
setting for the carving of Mount Rushmore. The rocks which form and
surround the memorial are part of the "crystalline core" of the Black
Hills, the oldest rocks in South Dakota and among the oldest in the
western United States.

There are two distinct stories to tell-the origin of the rocks and the
creation of the current topography. The rocks are ancient, the
topography is relatively recent.

**Origin of the Rocks**

There are two main types of rock at Mount Rushmore, very old granite and
much older metamorphic rocks.

**Precambrian Sediments**

The metamorphic rocks began formation as sediments at the bottom of a
shallow sea during the Precambrian period-sometime between 1.6 and 2.5
billion years ago.(Dates this old are very difficult to determine
precisely, mainly because they pre-date all life on the earth).What is
now western South Dakota was then near the edge of the sea. Because
there was no plant life to hold the forces of erosion in check, erosive
forces acted much faster during the Precambrian. Vast quantities of sand
and clay were washed down into the sea.

![](media/image2.png){width="0.5736111111111111in" height="0.75in"}The
pressure of its own weight eventually turned this sand and clay into
sandstone and shale. Eventually these sandstone and shale formations
reached a thickness of several miles.

**Formation of the Granite**

About 1.6 billion years ago a great mass of molten rock began to rise
from deep within the earth's crust. As this mass cooled (still well
underground) it formed the granite which is now the core of the Black
Hills. The molten rock cooled unevenly, forming both blocks of
relatively fine grained rock (quick cooling) and, more commonly, blocks
of coarse grained pegmatite (slow cooling). A large block of fine
grained granite now forms the upper portion

of Mount Rushmore, providing an excellent carving medium for Gutzon
Borglum.

**Metamorphism**

As this mass of molten rock neared the surface it came in contact with
the older sandstone and shale. The intense heat and pressure melted some
of the sedimentary rocks and turned the rest into metamorphic rock. The
sandstone became quartzite, an extremely hard, glassy rock. The more
abundant shale became mica schist. What had once been shale now acquired
a structure called schistosity, or the physical ability to break easily
into thin slices. The mineral content was also changed by the pressure
as the original quartz and clay were converted into new minerals. The
most common are muscovite and biotite, but others include garnet,
sillimanite, staurolite, cordierite and feldspar. The rocks were also
bent and folded. The exact contact points between the granite and the
metamorphic rocks can bee seen in many places within the memorial as
geologic unconformities.

![](media/image3.jpeg){width="3.5in" height="2.3715277777777777in"}

*Geologic Unconformity at Mount Rushmore-the white rock on top is
granite-the dark rock is mica schist*

These granites and metamorphic rocks (mainly mica schists) make up all
of the surface rock visible within the memorial.

**Formation of the Topography**

**Erosion**

The geologic history of the region between 1.6 billion and 500 million
years ago is unknown. Either no rocks formed during that time or none
have survived. Several miles of rocks must have been eroded from above
the granite during that time, creating marine sediments which have been
found to the east of the region.

**Deposition**

The flanks of the Black Hills are covered with rocks which formed
between 500 and 100 million years ago. These rocks have been totally
eroded away in the area of the memorial but they can be seen on any of
the roads heading away from the central Black Hills. Most of these rocks
were limestone. They were deposited in shallow seas as millions of
marine organisms died and fell to the bottom, were buried and converted
to sedimentary rock. These limestone formations include the rocks of
both Wind Cave National Park and Jewel Cave National Monument.

**Uplift**

About 70 million years ago the area of the Black Hills began to
uplift*.* This uplift is still going on and is related to the general
uplift of the central Rocky Mountains. As the Black Hills began to rise,
the sedimentary rocks above the crystalline core began to crack, and
then erode away. This process of erosion is complete in the area of the
memorial-only the granites, pegmatites and metamorphic rocks of the
crystalline core are still visible within the boundaries of the
memorial. Much of the material eroded from above the crystalline core
was washed 100-150 miles to the east and forms the sediments now being
eroded again at Badlands National Park.

![](media/image4.jpeg){width="3.6756944444444444in"
height="2.2930555555555556in"}

*The view east of Mount Rushmore looking east toward Badlands National
Park.*

**Modern Topography**

Mount Rushmore is here today because the granite erodes very slowly
compared to the surrounding rocks. In the area of the memorial the hard
granite creates mountains or hills and the equally tough quartzite forms
knife ridges. The less

resistant mica schist tends to be eroded into canyons and gullies.

**Erosive** **Forces**

The primary erosive forces in action today are wind, rain, snow and
frost wedging*.* The forces of wind and weather slowly eat away at the
rocks of the memorial. However, the granite is extremely resistant. The
rate of erosion on the granite faces has been estimated at only 1/10
inch per one thousand years.

A more significant erosional force in the Black Hills is frost wedging.
Granite has natural cracks in it. If water gets into the cracks it
expands as temperatures dip below freezing-exerting great pressure on
the rock and gradually expanding the cracks. This is prevented by the
caulking of cracks on the memorial with a silicon solution that prevents
water from entering the cracks.

![](media/image5.jpeg){width="3.727777777777778in"
height="2.497916666666667in"}

*Winter weather at Mount Rushmore can be severe-increasing all the
forces of erosion*

**What You See Today**

The two stories of the rock at Mount Rushmore-the origin of the rock and
the creation of the current topography are as different as the rock
types found here at the mountain. The granite is a very hard rock and
the mica schist is much softer. The granite was once molten rock and the
mica schist was formed as a result of the heat and pressure exerted by
the molten rock. The granite erodes very slowly at only one inch every
10,000 year, while the mica schist is eroded relatively quickly into
canyons and gullies. The Black Hills continues to be uplifted today.
Water is the main erosive force threatening to make any real change to
the mountain. Water freezing over the years can wedge the blocks of rock
apart. The result of all these geologic forces created a place where a
colossal carving could stand. The four granite faces look down upon
millions of visitors each year from their lofty heights and will
continue to do so for thousands of years to come.