Geology Reference
In-Depth Information
◗
Figure 23.15
The Geologic Evolution of Cape Cod,
Massachusetts, and Nearby Areas During the Ice Age
Direction
of ice
movement
Terminal
moraine
b
Position of the glacier when it deposited a terminal moraine
that would become Martha's Vineyard and Nantucket Island.
Several basins in southern California are also important
areas of petroleum production. The Green River Forma-
tion of Wyoming, Utah, and Colorado has huge reserves
of oil shale and evaporites.
Diatomite is a sedimentary rock composed of the
microscopic shells of diatoms, which are single-celled marine
and freshwater plants that secrete skeletons of silica (SiO
2
).
This rock, also called diatomaceous earth, is used chiefl y in
gas purifi cation and to fi lter liquids such as molasses, fruit
juices, water, and sewage. The United States is the leader in
diatomite production, mostly from Cenozoic deposits in
California, Oregon, and Washington.
Huge deposits of low-grade lignite and subbituminous
coals in the northern Great Plains are becoming increas-
ingly important resources. These coal deposits are Late
Cretaceous to Early Paleogene in age and are most exten-
sive in the Williston and Powder River basins of Montana,
Wyoming, and North and South Dakota. In addition to
having a low sulfur content, some of these coal are beds
30 to 60 m thick!
Gold production from the Pacific Coast, particularly
California, comes mostly from Cenozoic gravels. The gold is
found in placer deposits, which formed as concentrations of
minerals separated from weathered debris by fl uvial processes.
A variety of other mineral deposits are also impor-
tant. For example, the United States must import almost
all manganese used in the manufacture of steel. The largest
manganese deposits are in Cenozoic rocks in Russia. One
molybdenum deposit in Colorado accounts for much of the
world production of this element. Cenozoic sand and gravel,
as well as evaporites, building stone, and clay deposits, are
quarried from areas around the world.
Sand and gravel deposits resulting from glacial activ-
ity are a valuable resource in many formerly glaciated areas.
Most Pleistocene sands and gravels originated as fl oodplain
deposits, outwash sediment, or esker deposits. The bulk of
a
Cape Cod and the nearby islands are made up of mostly end
moraines, although the deposits have been modifi ed by waves since
they were deposited 23,000 to 14,000 years ago.
Recessional
moraine
c
Position of the glacier when it deposited a recessional
moraine that now forms much of Cape Cod.
The United States is the third largest producer of petroleum,
accounting for about 19% of the world's total. Much of this
production comes from Cenozoic reservoirs of the Gulf
Coastal Plain and the adjacent continental shelf. On the Gulf
Coastal Plain, most of the petroleum is in structural traps
that formed adjacent to salt domes and other structures.
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