Page 60 - visitorGuide
P. 60
■ Oil Shale
The fine laminations in this oil shale, from the Eocene
Green River Formation, probably represent annual sets
of seasonal layering of fine- and finer-grained sediment
deposited at the bottom of a large inland lake. Oil shales
do not produce oil directly. Instead, they contain kero-
gen, a solid material that produces oil when the shales
are distilled under extreme heat.
■ Modern Branched Coral and Modern Modern Tabular,
Tabular, Lobed Coral Lobed Coral
Coral is commonly found in warm seas
and has been abundant in the fossil record
since the Ordovician Period about 500
million years ago. Corals produce external
skeletons of calcium carbonate.They grow
as solitary individuals or in colonies.
■ Calcite-Cemented
Sand Crystals
This group of crystals, from South Dakota,
contains more sand grains than calcite.The
crystals take their shape from crystalline cal-
cite that cements the sand grains together.
Gift from the Geology Museum, Colorado School of Mines.
■ Sandstone
The shape of this sandstone specimen, from
the Upper Cretaceous Frontier Formation, Shirley Basin,
Wyoming, formed when it broke along fractures filled with
white calcite cement.The sandstone is about 90 million
years old. It was deposited in a shallow marine setting in
south-central Wyoming.
After you have viewed the specimens in the case, you will see two exhibits about three
quarters of the way down the hallway in front of you, on the right- and left-hand walls.
The first, on your right, Ripple Marks, was formed in Late Cambrian Potsdam Sandstone
found in Keeseville, New York.
■ Ripple Marks
Ripples form in sand that is moved by wind, waves, or water currents. Ripples
that are steeper on one side than the other are typically formed by directional
currents like those found in streams. Ripples that slope evenly from each side
of their crests are usually formed by wave action where water moves back and
forth across the sand. The broad rusty bands that cross these ripples are iron
oxide stains that precipitated from moisture between the layers of Late Cam-
brian Potsdam Sandstone.The rock layer with the mirror image of these ripples
is on display at the National Museum of Natural History in Washington, D.C.
Collected 4 December 1886 by Charles Doolittle Walcott. Gift from the National Museum of Natural History, Smithsonian
Institution, through George Switzer and Harold H. Banks, Jr.
The exhibit on the left-hand wall, Intertidal Sand Bodies, is directly opposite.
50 VISITOR GUIDE
