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Bedding
Layers in sedimentary rocks, caused by changes in composition or grain size.
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On the left is shown flat-lying bedding in sandstone and shale. The
parallel layers (beds) were formed by sediments accumulating over a
long geological period. This photo taken in the Kaumajet Mountains,
Labrador.
The photograph on the right shows gently dipping
bedding in sandstone, at Freshwater Bay, Bonavista Bay.
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Graded Bedding
A type of bedding in which each layer is characterized by a
progressive decrease in grain size from the bottom to the top of the
bed. Graded bedding is caused by the differential settling by size
and weight of suspended particles in water. Graded bedding can be
used to tell which are the oldest and youngest rocks in a sedimentary
sequence because the larger, older grains are at the base of the bed.
This is known as younging direction or bedding tops.
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Graded bedding is shown in this example where pebbles form the lower
part of the bed and are replaced by sand and pebbles towards the top,
(King George IV Lake area, central Newfoundland).
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Cross-bedding
Cross-bedding is stratification inclined to the original horizontal
surface upon which the sediment accumulated. Cross-beds are formed
in beach, river and sand-dune environments. It is produced by
deposition on the downcurrent slope of a dune or sand wave. It is
produced by the deposition of fragments of sediment layers which are
then eroded and truncated by later water currents or winds that
deposit new sediments at an angle on top of the erosion surface
(scours). Cross-beds can be used to determine bedding tops because
of the truncation of the beds.
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The photo on the right shows large-scale cross-beds in sandstone.
(Note the person for scale.) The truncation of beds indicates that
bedding tops are toward top of photograph. This example is from
central Labrador.
The photo on the left shows cross-bedding
in sandstone. The truncation of beds indicates that bedding tops are
toward bottom of photograph, i.e., the beds are upside down. This
structure was photographed west of Swift Current, Burin Peninsula.
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Mud Cracks
Polygonal cracks are produced in fine-grained, muddy sediments due to
the shrinkage that accompanies drying. They are preserved when new
sediment is deposited in the cracks.
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The photograph shows mud cracks in red siltstone from central Newfoundland.
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Ripple Marks
Ripple marks are sand waves produced on a top of a bed by wave or
wind action. When new layers build up on top of each other, the
ripples are preserved.
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These ripple marks occur in red micaceous sandstone on Lockers Flat
Island, Bonavista Bay.
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Load Casts
Structures, such as bulbous, downward-pointing protuberances of sand
or other coarse-grained sediments that extend downward into finer
grained, softer underlying silt, clay or mud are called lode casts.
They form when soft, waterlogged mud is too weak to support the
overlying sand, which sinks into the mud.
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Load casts are shown to be well-developed, in the bottom-centre of
this photograph, in this interbedded red sandstone and grey/black
mudstone. Look just left of the pencil. Scour or erosion features
are also present. This example of load casts is from near the west
end of Gander Lake.
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Stromatolite Mounds
Stromatolites refer to generally domal- or subspherical-shaped,
laminated, calcareous sedimentary (organosedimentary) structures
that formed in a shallow-water environment. They form when a mat or
assemblage of sediment-binding blue-green algae traps fine, silty
detritus and precipitates calcium carbonate. This may lead to
development of large rigid mounds.
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Fossilized algal mounds forming limestone deposits near Flower's
Cove, western Newfoundland.
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