Transcript Tearing Down Mountains I: Weathering, Mass Movement

Deep Time I:
Stratigraphy & Sedimentary Record
GEOSC 10: Geology of the National Parks
Presented by Dr. Richard Alley
The Pennsylvania State University
ARCHES
NATIONAL
PARK:
Stories in
Stone
Photos by R. Alley
GEOSC 10 - Geology of the National Parks
Dr. Anandakrishnan, Arches National Park. The La Sal mountains,
background, are named for the salt beneath them. Motion of the salt
made joints that isolate fins that make the arches, such as the one next to
Dr. Anandakrishnan’s shoulder.
GEOSC 10 - Geology of the National Parks
End-on (top) and
angled (bottom)
views of fins,
Devils Garden,
Arches National
Park. The tough
sandstone has
been broken by
parallel, vertical
joints, and
weathering along
those joints
isolates “fins” of
sandstone that
then are eroded
to make arches.
GEOSC 10 - Geology of the National Parks
Desert varnish at Double Arch. The arrow points along a crack through
which rainwater flows to form desert varnish. Enlargement of the crack
will eventually lead to rockfall, changing the arch.
GEOSC 10 - Geology of the National Parks
CAUSE student Raya Guruswami in Glen
Canyon (right), and a cliff in Canyon de Chelly
National Monument (top). Rockfalls from the
sandstone cliffs have left arch-shaped
amphitheaters. Similar falls from fins help
make arches.
GEOSC 10 - Geology of the National Parks
The sandstone of Delicate Arch started
as a sand dune (note bedding, top-right
picture).
GEOSC 10 - Geology of the National Parks
Landscape Arch is the world’s longest natural
stone arch. Notice numerous large blocks (a
few are shown by red arrows) that have fallen
from the arch, many since the park was
founded, plus joints that make additional falls
likely (black arrows). The close-up on the right
shows the source of a recent fall. White and
yellow lines connect points that are common to
both pictures.
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
GEOSC 10 - Geology of the National Parks
BRYCE CANYON:
“A Hell of a Place to Lose a Cow”
–Ebeneezer Bryce
GEOSC 10 - Geology of the National Parks
Although Bryce Canyon is higher,
cooler and wetter than many of the
nearby parks, life is still tough on the
unstable slopes of the canyon. Here,
2-inch-tall dwarf columbine (right and
upper right) and a dewy Oregon-grape
leaf struggle to grow at Bryce.
GEOSC 10 - Geology of the National Parks
Guidebooks almost always refer to
Bryce as a “fairyland.” The wonders
of differential erosion—softer rocks,
and rocks along cracks, go faster—
really are amazing.
GEOSC 10 - Geology of the National Parks
The Park Service now discourages “naming” rocks, because visitors are so
disappointed when erosion then changes those rocks. This one was named
Queen Victoria back when naming was condoned. The layered sedimentary
rocks are beautiful by any name.
GEOSC 10 - Geology of the National Parks
Trees in Bryce Canyon, with CAUSE students Sameer Safaya (far right) and Sam Ascah.
GEOSC 10 - Geology of the National Parks
A decade or so ago, the Park Service put the logs (yellow arrow) across
the stream bed to slow burial of a trail by sediment. As we’ve seen for
dams, rocks filled the space upstream of the logs, and erosion
happened downstream. Fast erosion supplied lots of rocks to fill the
“lake” above the dam quickly.
GEOSC 10 - Geology of the National Parks
Bryce’s rocks are mostly limestones, but include river gravels such as the
conglomerate shown here. Many types of clasts are present; the orange
one in the middle is a conglomerate itself, and the clasts in it include
several types of sedimentary rocks. This picture shows a long and complex
story—try telling it.
GEOSC 10 - Geology of the National Parks
Sedimentary rocks, Waterton Lakes-Glacier International Peace
Park. Colored lines follow a folded layer. Right-side-up at the
green line becomes mostly upside-down at yellow, right-side up at
orange, and slightly upside-down at red, as shown by the black
GEOSC 10 - Geology of the National Parks
arrows.
Small region (about 6 inches across) of fossil sand dune in a cliff at Canyon de
Chelly National Monument. Wind from the right blew over the dune and
cascaded down, forming the slanting layers. Then the top was eroded, and
nearly horizontal layers were added above. This is right-side-up. Rust deposition
after formation of the dune colored some of the layers, making the layering
easier to see.
GEOSC 10 - Geology of the National Parks
Here is part of the
fossil sand dune from
the previous slide.
The picture in the
upper-left corner
shows the original,
right-side-up sand
dune. You might see
the others if nature
had flipped the rocks
various ways. The
blue arrows, which
point in the “up”
direction, will help
you follow the flips,
but as an
experienced GEOSC
10 geologist, nature
can’t fool you any
more.
GEOSC 10 - Geology of the National Parks
A cliff below the South
Rim of the Grand
Canyon, along the
Bright Angel Trail. The
redder Hermit Shale
(below the yellow line)
is a flood-plain deposit,
and sand from the
Coconino Sandstone
dunes (above the
yellow line) fell into a
huge mud crack
extending perhaps 20
feet downward (below
the red arrow). A sand
dune blowing onto the
flood plain of the Nile
could produce a similar
deposit today. This is
right-side up; the sand
fell down into the mud
crack.
GEOSC 10 - Geology of the National Parks
These mud cracks are from the Flagstaff Limestone of central Utah, very
similar in age and setting to the Bryce Limestone. This slab is right-side
up; the cracks go down.
GEOSC 10 - Geology of the National Parks
Mud deposited in a small lake near the sand dunes dried and cracked in the
sun. More mud washed in and filled the cracks. After the mud hardened, the
rocks were split apart, and the upper piece turned over; Dr. Alley’s index finger
points to the crack-filling mud.
GEOSC 10 - Geology of the National Parks
Lava flow, Sunset
Crater National
Monument, Arizona.
Bubbles tend to rise in
lava flows, as they do in
soda, but to be trapped
beneath the quickcooled upper layer.
Often, bubbles will be
bigger and more
numerous toward the
top of a flow, as in this
one. This flow is rightside-up.
GEOSC 10 - Geology of the National Parks
Along I-70 in central Utah. Rocks below are sediments from a coastal
environment, and are standing up on end (look just above the geology student,
shown by the black arrow). Above is a fossil soil or paleosol (blue arrow), and
above that are whitish lake sediments. Try telling the story shown here.
GEOSC 10 - Geology of the National Parks