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TALUS: A deposit of large angular fragments of physically weathered bedrock, usually at the base of a cliff or steep slope.

TAR SAND: A sandy deposit of organic matter impregnated with a tarry substance made up mostly of hydrocarbons, from which petroleum can be extracted.

TENSIONAL FORCES: Forces that stretch a body and pull it apart. Tensional forces dominate at divergent plate boundaries.

TERRACE (STREAM VALLEY): A flat, steplike surface above the floodplain in a stream valley, marking a former floodplain that existed at the higher level before regional uplift or an increase in discharge caused the stream to erode into the former floodplain.

TERRIGENOUS SEDIMENT: Sediment eroded from the land surface.

TEXTURE (ROCK): The rock characteristics of grain or crystal size, size variability, rounding or angularity, and preferred orientation.

THERMOREMANANT MAGNETIZATION: A permanent magnetization acquired by minerals in igneous rocks during crystallization.

TIDAL FLAT: A broad, flat region of muddy or sandy sediment, covered and uncovered in each tide cycle.

TIDAL SURGE: Waves that overrun a beach and batter sea cliffs when an intense storm passes near the shore during a spring tide.

TIDE: The rise and fall of the water level of the ocean that occurs twice a day and is caused by the gravitational attraction of the Moon and, to a lesser degree, the Sun, with greater force on the parts of the Earth facing and opposite the Moon (and Sun).

TILL: An unstratified and poorly sorted sediment containing all sizes of fragments from clay to boulders, deposited by glacial action.

TILLITE: The lithified equivalent of till.

TOPOGRAPHY: The shape of the Earth's surface, above and below sea level; the set of landforms in a region; the distribution of elevations.

TOPSET BED: A horizontal sedimentary bed formed at the top of a delta and overlying the foreset beds.

TRACE ELEMENT: An element that appears in a mineral in a concentration of less than 1 percent (often less than 0.001 percent).

TRANSFORM FAULT PLATE BOUNDARY: A boundary at which Earth's plates slide horizontally past each other, approximately at right angles to their divergent plate boundaries.

TRANSPIRATION: The release of water vapor by plants into the atmosphere.

TRANSVERSE DUNE: A dune that has its axis perpendicular (transverse) to the prevailing winds or to a current. The upwind or upcurrent side has a gentle slope, and the downwind or downcurrent side lies at the angle of repose.

TRELLIS DRAINAGE: A system of streams in which tributaries tend to lie in parallel valleys formed in steeply dipping beds in folded belts.

TRENCH: A long, narrow, deep trough in the seafloor; marks the line along which a plate bends down into a subduction zone.

TRIBUTARY: A stream that discharges water into a larger stream.

TSUNAMI: A large destructive wave caused by seafloor movements in an earthquake.

TURBIDITE: The sedimentary deposit of a turbidity current, typically showing graded bedding.

TURBIDITY CURRENT: A mass of mixed water and sediment that flows downhill along the bottom of an ocean or lake because it is denser than the surrounding water. It may reach high speeds and erode rapidly.

TURBULENT FLOW: A high-velocity flow in which streamlines are neither parallel nor straight but curled into small tight eddies. (Compare Laminar flow.)