Geology Journeys
Glossary of Terms

A geology-related blog by Sharon
Caliche A calcium carbonate sedimentary rock. Out here in West Texas, it's used to make a lot of our rural roads.
Clast A sediment grain of any size.
Conglomerate A sedimentary rock comprised of large rounded clasts in a fine-grain matrix.
Erosion The process by which material is removed from the Earth's surface, usually by gravity, water, wind or ice.
Evaporite A mineral formed by precipitation or evaporating aqueous solution (salt water)
Formation A lithologically distinct stratigraphic unit, large enough to be mappable.
Gypsum An evaporite mineral comprised of calcium, sulfur and oxygen molecules. This very soft mineral is more commonly known by its use as plaster or drywall and as alabaster ("thy alabaster cities gleam....)
Hoodoo A tall spire of rock capped by harder resistant rock that protects the layers below from erosion.
Lithology The study and description of the physical character of rocks.
Matrix The fine-grained mass of a rock in which larger clasts or crystals are present.
Mudstone A fine-grained sedimentary rock comprised of clay-sized grains.
Orogeny A tectonic process that folds, faults and uplifts the earth's crust, often accompanied by volcanic activity. A mountain building process.
Phytosaur An extinct crocodile-like Triassic reptile.
Precipitate To form a solid from a solution by chemical reaction
Sandstone A sedimentary rock comprised of sand-sized grains (2 to 0.5 mm) of rocks and/or minerals.
Sedimentary Rock that forms by processes that cause rock particles and minerals to accumulate, usually deposited by water, ice or wind.
Shale A sedimentary rock comprised of clay (<= 0.002 mm) and silt sized (0.5 to 0.002 mm) rocks and/or minerals.
Siltstone A sedimentary rock comprised of silt-sized (0.05 to 0.002 mm) grains of rock and/or minerals.
Strata A layer of sedimentary rock.
Stratigraphy The study of the characteristic of layered rocks: their classification, correlation, relative position, depositional environment to determine their geological history.
Tectonic A process that moves and deforms the earth's crust.
Weathering The physical or chemical breaking down of rocks, soils or minerals.


Grain Name Smallest size mm Largest size mm
Clay 0.00006 0.002
Silt 0.002 0.05
Sand 0.05 2.0
Gravel 2.0