Figure 1 |
Outcrops of sedimentary rocks can be dramatically
beautiful. What makes them this way? In some cases,
as illustrated by the brilliant orange-red sandstone
of Monument Valley, Arizona , it's the color. The
late afternoon sun can make such rocks seem to glow
with warmth against the blue sky. In other cases,
the beauty lies in texture. For example, in cross-bedded
sandstone, each bed contains a fabric that results
from the accumulation of sand on the slip face of
dunes (large and small). |
Figure 2 |
In this example from the coast of Scotland , thin
sandstone beds have been tilted during later mountain
building. |
Figure 3 |
The giant cross beds of sandstone in Zion National
Park (Utah) weather out to form a mega-herringbone
texture on the face of outcrops. The girl has posed
just above a master bed plane, which truncates the
tilted portion of the underlying cross-bedded set. |
Figure 4 |
The size of these cross beds indicates that they
formed from sand deposited in large desert dunes.
Similar desert dunes make up the large ledge of sandstone
that protects the ancient Native American village
at Mesa Verde, Colorado. |
Figure 5 |
Most limestone units consist of calcite that was
extracted from the ocean by living organisms. For
example, the coral of this coral reef in Australia
represents limestone in the making. Some limestone
is made up of finer-grained fragments (even as small
as mud) from shells that have been broken up or from
skeletal flakes of certain species of plankton. |
Figure 6 |
Once deposited, calcite recrystallizes to form
a gray mass, as illustrated by this exposure of Devonian
limestone in central New York State. |
Figure 7 |
The grain size of sedimentary rocks, and the sorting
of sediment, tells us about the depositional environment
in which the sediment accumulated. For example, the
siltstone and shale at the base of the cliffs in
Monument Valley probably accumulated in a river system.
The massive cliff of sandstone is the remnant of
ancient dunes. Figure 7 shows a boulder of conglomerate
from a Precambrian sedimentary unit in Norway. The
rounding of the clasts suggests that they tumbled
around in a high-energy stream. The stream water
washed away most mud and sand, leaving this rock
fairly well sorted. |
Figure 8 |
In contrast, the glacial till exposed in this cliff
on the west coast of Ireland is poorly sorted — boulders,
sand, and mud are all jumbled together. That's because
the till was deposited by a glacier, and glaciers
are made of solid ice and thus can carry sediment
grains of any size. |
Figure 9 |
Fossils can be preserved in sedimentary rocks.
Figure 9 shows two examples. The fossil in the black
shale, which comes from a coal-bearing sequence of
strata in Pennsylvania, represents a Carboniferous-age
fern. The sandstone block contains the fossil of
a Cretaceous fish, which swam in a long-gone lake
in central Brazil. |
