Transcript Unit 9 – Which Way Up?

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 arrows.
Unit 9 – Which Way Up?
Dinosaur Ridge, near Denver. Above, a stone
surface that was mud when the dinosaurs stomped
on it. Our view is quite similar to what the
dinosaurs would have seen, looking down on their
feet going down in the mud. Right, the red outline
shows the footprint of a dinosaur (green coloration
of foot being pulled out of mud is purely
conjectural!), down into mud (notice that the layers
on the far left are broken or bend down at the red
line). Later, more mud washed in to fill the
footprint, and then everything was hardened to
stone. We’ll see more of these prints in unit 11.
(Dr. Alley for scale; that was a large dinosaur foot!)
Unit 9 – Which Way Up?
Part of a fossil sand dune. The picture
in the upper-left corner shows the
original, right-side-up dune. Wind
blew from the right to the left, building
a dune layer-by-layer as sand blew
over the crest and avalanched down
(the magenta arrow shows the sand
motion). Then, more erosion occurred
(along the green lines), cutting the tops
off the layers, and then more sand was
deposited above. If you had flopped
down on your back and looked at the
sky when this was forming, you would
have gotten sand in your eyes, but you
would have been looking in the
direction the blue arrow points--up.
You might see the other pictures if
nature had flipped the rocks various
ways. The blue arrows, which point in
the “up” direction in each case, will
help you follow the flips, but as an
experienced Geosc10 geologist, nature
can’t fool you any more. The key is
that sand was deposited and then cut
by the wind, wind cannot cut sand that
isn’t there, so the cut ends are “up”.
(There’s a bit more complexity in the far lower right of the original picture, telling of another event, but don’t worry.)
Unit 9 – Which Way Up?
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 floodplain 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.
Unit 9 – Which Way Up?
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, as
would happen if you
quick-froze the top of your
soda. Often, bubbles will
be bigger and more
numerous toward the top
of a cooled lava flow, as in
this one. This flow is
right-side-up.
Unit 9 – Which Way Up?
Mud cracks and Dr. Alley’s index finger, Arches National Park. Mud dried, cracked, a new mud layer was
deposited on top, both hardened, layers were split, and the top one turned over for you to see here. This
piece of rock is now upside-down. Compare the shadows from Dr. Alley’s finger and the reddish rock
ridges to see that these really are ridges and not valleys.
Unit 9 – Which Way Up?
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, as you can
see by comparing the shadows in the cracks with the shadow from Dr. Alley’s pocket knife.
Unit 9 – Which Way Up?
Along I-70 in central Utah. The reddish rocks below are sediments from a coastal environment, and the originally
horizontal layers are now vertical (look just above the geology student, shown by the black arrow, which also points
along the layers). Above the reddish rocks is a reddish fossil soil or paleosol (shown by the blue arrow; notice the
horizon of white caliche nodules just below the blue arrow, indicating a rather dry climate when the soil formed). Above
the soil are whitish lake sediments. Try telling the story shown here.
Unit 9 – Which Way Up?