The Great Unconformity Frenchman Mountain Southern Nevada
The Great Unconformity
Frenchman Mountain Southern Nevada
Or, how I
Why we need
• What is an
• What is the great
• What creates them?
• Where did the name
• An unconformity is a buried erosion surface.
It is a surface of rock that was exposed on
the Earth's surface and was then covered by
• Unconformities are important because they
represent missing intervals of the geologic
record, like pages missing from a history
book. The Great Unconformity of Frenchman
Mountain represents about 1.2 billion years,
which is more than one fourth of the age of
• The concept of an unconformity arises from two of
the oldest principles of geology, first stated in 1669
by Nicholas Steno:
• Layers of sedimentary rock (strata) are originally laid
down flat, parallel to the Earth's surface. That's the
law of original horizontality.
• Younger strata always overlie older strata, except
where the rocks have been overturned. That's the
law of superposition.
Forming the Great Unconformity
FROM THE GRAND CANYON TO THE LAS VEGAS
BOWL…HOW A MOUNTAIN MOVES.
6-10 Million years ago western North America begins
moving west due to interaction between the North
American and Pacific lithospheric plates, which today
slide past one another along the San Andreas fault in
Movement of blocks becomes tilted as secondary faults
encounter a nearly horizontal master fault causing the
blocks to tilt eastward while moving westward.
Continuous sliding on the master fault causes
Frenchman Mountain to move about 50 miles from
Precambrian granite (about 1.7 billion years old) and beginning with the
white rocks exposed along the slope of the low hill, are the Cambrian
Tapeats Sandstone (about 500 million years old).
• Above the Great Unconformity we
see the sedimentary Tapeats
Sandstone with recycled large, light
gray, angular, quartz pebbles.
These rocks have a dark coating
called desert varnish or rock varnish
which is a thin manganese- and iron-
rich coating that develops on exposed
rock surfaces and is most common in
Bacteria, using chemicals from dust
particles,[produces this varnish over
thousands of years
FRENCHMAN MOUNTAIN WAS PARTLY CREATED BY
MOVEMENT ALONG THE FRONT RANGE FAULT
Granite intrusion in the schist.
Precambrian granite and schist / Tapeats sandstone
Vishnu granites and the overlying
•Clarence Dutton in his 1882 book Tertiary History of the Grand Cañon District.
•People assume that the Great Unconformity was so named due to the large
interval of time it represents in the Earth’s history.
•The truth is Dutton had no idea of the age of the rocks on either side of the
•Basically, he knew the importance of this find and that something was
missing but had no idea of the extent of time that had passed between the
Cambrian age Tapeats sandstone and the granite and schist that lie beneath it.
WHERE DID THE NAME “GREAT
UNCONFORMITY” COME FROM?
Great Unconformity Geologic
Interpretive Site andTrail