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LAHAR: A mudflow of unconsolidated volcanic ash, dust, breccia, and boulders that occurs when pyroclastic or lava deposits mix with rain or the water of a lake, river, or melting glacier.

LAMINAR FLOW: A flow in which streamlines are straight or gently curved and parallel. (Compare Turbulent flow.)

LANDFORM: A characteristic landscape feature on the Earth's surface that attained its shape through the processes of erosion and sedimentation; for example, a hill or a valley.

LATERITE: A distinctive, deep-red soil formed in very humid regions, characterized by high alumina and iron oxide content, and produced by rapid chemical weathering of feldspar minerals.

LAVA: Magma that has reached the surface.

LEVEE: A ridge along a stream bank, formed by deposits left when floodwater slowed on leaving the channel; also, an artificial barrier to floods built in the same form.

LIMB (FOLD): The two relatively planar sides of a fold, one on either side of the axial plane.

LIMESTONE: A sedimentary rock composed mainly of calcium carbonate (CaCO₃), usually as the mineral calcite.

LINEAR DUNE: A long, narrow eolian sand dune that is aligned parallel to the direction of the prevailing wind.

LITHIFICATION: The chemical and physical diagenetic processes that bind and harden a sediment into a sedimentary rock. (See also Compaction.)

LITHOSPHERE: The outer, rigid shell of the Earth, situated above the asthenosphere and containing the crust, the uppermost part of the mantle, the continents, and the plates.

LOESS: An unstratified, wind-deposited, dusty sediment rich in clay minerals.

LONGITUDINAL PROFILE: A cross section of a stream from its head to its mouth, showing elevation versus distance to the mouth.

LONGSHORE CURRENT: A current that flows parallel to the shoreline; the summed longshore components of water motion of waves that break obliquely with respect to the shore.

LONGSHORE DRIFT: The movement of sediment along a beach by swash and backwash of waves that approach the shore obliquely.

LUSTER: The general quality of the shine of a mineral surface, described by such subjective terms as dull, glassy, or metallic. (See Table 2.3.)