CONCRETIONS
DESCRIPTION: A Concretion is a compact mass of precipitated minerals which is commonly spherical to discoid in overall shape and is found embedded within a host rock which is common;y of a different composition.
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Figure 1 - Large Concretion growing within fine sediments. These large nodules found in northwest NY State near the Great Lakes shore are commonly Septarian in nature.
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Figure 2 - Large Concretion growing within fine sediments as for Figure 1. In the background is a Tillite
CROSS-BEDDING
DESCRIPTION: Cross-bedding is layering within a stratum that is at an angle to the main bedding plane. It indicates deposition within a flowing (air or water) medium.
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Figure 1 - Crossbedding of Sand Dunes. Zion NP
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Figure 2 - River Crossbeds. Eastern Tennessee.
CHANNEL-FILL DEPOSITS
DESCRIPTION: A stream or river channel that has been filled due to the lack of energy of water to remove material introduced to the channel.
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Figure 1 - Channel-fill deposit photographed from a moving vehicle on NY Route-17 near Hancock, NY.
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Figure 2 - River-Channel sandstones cut into lagoonal muds. Tennessee.
Figure 3 - River-Channel sandstones cut into lagoonal muds. Note the cross-bedding present within river sediments. Tennessee.
MUDCRACKS
Description: Mudcracks are sedimentary structures that occur when moist mud-bearing sediment dries upon exposure to air and shrinks with the resulting cracks forming a polygonal pattern.
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Figure 1 - Mudcracks in the Kittatiny Limestone. NY/PA. Original cracks caused by dessication are subsequently filled with coarser sediment that forms ridges due to differential erosion. (Photo 1976)
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Figure 2 - Modern day mudcracks. In the fossil record, the cracks are commonly filled in with coarser sediment that is more resistant to erosion than the contiguous muddy sediment. Raindrop impressions are commonly preserved in the sediment.
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IFigure 3 - Logan Pass, Glacier N.P.
RIPPLE MARKS
DESCRIPTION: Ripple Marks are ridges of sediment that form in response to a fluid (water or wind) moving across a layer of sediment. They form generally perpendicular to fluid movement. Fluid movement can be in the form of waves, currents or wind. Ripple Marks that are symmetrical commonly indicate periodic water movement in opposite directions such as occurs in tidal areas (oscillation ripples). In areas of unidirectional currents, ripples are asymmetrical with the leeward slopes being steeper than the opposite slopes.
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Figure 1 - Oscillation Ripple Marks in Cook Inlet, AK created by the ebb and flow of one of the largest tidal ranges in the world.
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Figure 2 - Oscillation Ripple Marks. Esopus Fm., Devonian, NY.
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Figure 3 - Oscillation Ripple Marks. Ordovician. Tennessee.
Figure 4 - Modern ripples in a stream. Ripples are asymmetrical. Flow is unidirectional towards the viewer.
SOLE MARKINGS
DESCRIPTION: Sole Markings are sedimentary structures found on the undersides of some strata. They commonly form when grooves, striations, or irregularities on the top of the bedding surface below the sole marking are filled with coarser sediment that eventually hardens to form a cast on the base of the overlying bed.
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Figure 1 - Flute Casts on the base of a stratum. Sample is from northwest NY State, probably Devonian.
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Figure 2 - Groove Casts in the bottom of a bed. Silurian Red Mt. Fm. Ringgold, GA.
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Figure 3 - Sole markings in a slab of the Pennsylvanian Smithwick Sandstone. Marble Falls, TX
POROSITY
Description: Porosity refers to the empty space between sediments, Permeability refers to how well these pore spaces are interconnected. In the petroleum industry, Porosity and Permeability are key factors in the ability of a Reservoir Rock to hold oil or natural gas.
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Figure 1 - Thin-section illustrating Porosity in Rhombic Dolomite. In order to better understand the potential Porosity and Permeability in a rock, a sample is brought back to the lab where it is soaked in a blue epoxy liquid within a vacuum. The epoxy then moves into the specimen to fill voids. Thin-sections are subsequently made to view the voids. Triassic Thaynes Fm. WY (Photo 1983)
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Figure 2 - Thin-Section of Fine Sandstone illustrating intergranular porosity in the bottom half of the photo (blue epoxy void-filling cement - see Figure 1 for more information). Triassic Thaynes Fm, WY (Photo 1983)
COLLAPSE BRECCIA
DESCRIPTION: Collapse Breccias are formed by the collapse of rock overlying an opening. Spaces between angular particles are subsequently filled with smaller sediment.
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Figure 1 - Collapse Breccia. Little Flat Canyon, UT. Note the Rugose Coral in the upper center of the photo. (Photo 1981)