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Geology of the Pacific Northwest

Geology-208

Week 2:  READING THE ROCKS OF WASHINGTON

Rock Forms

Sedimentary Rocks

Detrital and chemical and organic sediment rocks, formed from the erosion, transportation, and deposition of pre-existing rocks
DETRITAL Sedimentary Rocks:  formed from erosion and transportation of a pre-existing rock
ROCK
COMPOSITION
SIZE/SHAPE
SORTING
ORIGIN
Quartz Sandstone

quartz (white to tan); fossils (invertebrates, vertebrates, plants)
medium-grained; rounded (sandstone) or angular (gritstone)
well-sorted
marine (continent shelf or slope); coastal (delta; beach/barrier island; tidal flat); continental (stream channel, if rippled; desert; lake)
Shale

clay (too small to see); fossils (invertebrates, vertebrates, plants)
clay-sized (feels smooth); angular (too small to see)
well-sorted
marine (continental shelf or slope, deep marine); coastal (delta, tidal flat); continental (stream floodplain)
Siltstone
clay and quartz (usually too small to see); fossils (invertebrates, vertebrates, plants)
silt-sized (feels gritty); angular (too small to see)
well-sorted
marine (continental shelf or slope, deep marine); coastal (delta, tidal flat); continental (stream floodplain)
Graywacke
quartz and rock fragments mixed with clay; often gray colored
sand-sized (visible grains up to 2 mm); rounded to angular
moderate sorting
marine (continental shelf: cross-beds, ripple marks common)
Arkose
feldspar (usually orthoclase), quartz, and other minerals
sand-sized (visible grains up to 2 mm); angular; cross-beds
moderate to poor sorting (few grains >2 mm)
continental (alluvial fan); considered an immature rock because of high feldspar contnt; derived from granite disintegration
Breccia
fragments of any rock type
gravel-sized (> 2 mm); angular; cross-beds
moderate to poor sorting
marine; continental (alluvial fan); coastal (especially where mechanical weathering is active)
Conglomerate
fragments of any rock type
gravel-sized (> 2 mm); angular to rounded; may have scratches
moderate to poor sorting
continental (stream channel; glacier; lake, near shore)
CHEMICAL & ORGANIC Sedimentary Rocks:  organic sediments form from biological activity; chemical sediments form by primary precipitation in shallow marine environments
ROCK
COMPOSITION
SIZE
TEXTURE
ORIGIN
Chalk
calcite (acid reaction)
fine-grained; powdery; soft
black, gray, or white); earthy smell
marine (continental shelf, during the Cretaceous period)
Coquina
calcite (acid reaction)

shell fragments loosely cemented with very little matrix
coastal (beach/barrier island; tropical beaches)
Crystalline Limestone
calcite (acid reaction); fossils (invertebrates, coral, algae)
medium to coarse (visible) grains
crystalline
marine (continental shelf)
Fossiliferous Limestone
calcite (acid reaction); fossils (invertebrates, coral, algae)
fine to coarse grains
crystalline matrix
marine (coral reef)
Micrite
calcite (acid reaction)
fine grains (too small to see)
crystalline

Oolitic Limestone
calcite (acid reaction); fossils (invertebrates)
sand-sized
spheres (ooliths) packed together
marine (continental shelf; warm, shallow, strongly agitated marine conditions)
Chert
quartz (scratches glass)
fine grains (too small to see)
grayish in color; cannot be scratched with a knife; conchoidal fracture
marine (deep marine: seabeds)
Rock Gypsum
gympsum
fine to coarse grains
crystalline (vitreous luster); fibrous; distorted bedding; soft (can be scratched with a fingernail); uneven fracture
marine (arid, hot, restricted circulation); found with sandstones, marls, limestones
Rock Salt
halite; impurities of clay minerals and iron oxides (reddish brown)
fine to coarse grains
crystalline
marine (arid, hot, restricted circulation); saline waters (salt lakes)
Coal (bituminous and anthracite)
plant remains (carbon)

dense, black masses; depending on grade, may be dull to vitreous; lower grades may contain minor fossils
swamp
Peat
plant remains (carbon)

fibrous, brownish plant material; swamp
swamp


GLOSSARY



Send comments to Rob Viens (e-mail: rviens@bcc.ctc.edu) or call him at his Bellevue Community College office at (425) 564-3158. Office hours are by appointment.


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