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Bedding.
Layers of sedimentary rock that show
different grain sizes or compositions. Bedding indicates successive
depositional events.
Strata; Stratification.
The structure of
sedimentary rocks where
parallel beds of considerable lateral extent can be seen.
Photo: Horizontal layers in Titus Canyon, Death
Valley National Park.
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Strata
sometime are bent in odd ways by tectonic forces after they are laid
down.
Photo: Bent layers of Precambrian rock in Bass Canyon, Grand Canyon
National Park.
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Cross-bedding.
A characteristic of sedimentary rocks where different layers are laid
down at angles to one another, that is, not all are laid horizontally.
Sandstone formed
from sand dunes,
which is the case at Red Rocks, often shows cross-bedding. As shifting
winds move sand around, they create and destroy layers of sand. As
dunes move about, some layers are left from the sides of dunes while
others are left from the dune tops or dune swales. The
result,
solidified into sandstone, reveals thin bed at angles to one another.
Photo: Calico Hills, Red Rock Canyon National Conservation Area.
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Unconformity.
A
line between layered
strata of different
ages representing an interval of time in which deposition stopped,
erosion removed some material, and then deposition resumed. A period of
time missing from the geologic record.
Photo: Boucher Canyon, Grand Canyon National Park. The Great
Unconformity is a gap of more than 1 billion years between the lower
layer of 1.7 billion-year-old Vishnu Schist (Precambrian
rock) and the upper layer of 0.57 billion-year-old Tapeats
Sandstone (Paleozoic rock). Over 1 billion years of
geologic record are missing at this point; that is, the Vishnu Schist
eroded for over 1 billion years before the sea rose and deposited beach
sand that later became the Tapeats Sandstone.
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