Chapter 6 — Landforms and Their Evolution

Evolution implies the transformation of a part of the earth's surface from one landform into another, or changes in individual landforms after they form. A landmass passes through stages of development comparable to youth, mature, and old age.

The chapter covers running water, groundwater, glaciers, waves and currents, and wind as the primary geomorphic agents that cause erosion and deposition to evolve landforms.

A peneplain is an almost-flat lowland of faint relief formed when stream erosion over a long period lowers divides between drainage basins until they are nearly completely flattened, leaving only a few low resistant remnants called monadnocks.

A gorge is a deep valley with very steep to straight sides and is almost equal in width at its top and bottom. A canyon has steep step-like side slopes and is wider at its top than at its bottom; a canyon is essentially a variant of a gorge.

Karst topography develops in limestone or dolomite regions where groundwater causes solution and precipitation. It is named after the Karst region in the Balkans adjacent to the Adriatic Sea and includes sinkholes, dolines, uvalas, lapies, caves, stalactites, and stalagmites.

Calcium carbonate dissolved in carbonated rainwater is deposited when the water evaporates or loses carbon dioxide as it trickles over cave surfaces. Stalactites hang from cave ceilings and stalagmites rise from cave floors; they eventually fuse to form columns and pillars.

A cirque is a deep, long, and wide trough or basin with very steep concave to vertically dropping walls at its head and sides, found at the heads of glacial valleys. After the glacier disappears, water collects within a cirque forming a cirque or tarn lake.

Drumlins are smooth oval-shaped ridge-like features composed mainly of glacial till with some gravel and sand. Their long axes are parallel to the direction of ice movement, making them an indicator of glacier movement direction.

Barrier bars and spits form when sediment is deposited by longshore currents parallel to or across a bay mouth. A spit is a barrier bar keyed to a headland. When barrier bars and spits block a bay mouth, the enclosed water body becomes a lagoon, which gradually fills with sediment to form a coastal plain.

Barchans form with constant moderate wind and uniform surface. Parabolic dunes form on partially vegetated sandy surfaces. Seif dunes have one wing due to a shift in wind conditions. Longitudinal dunes form with poor sand supply and constant wind. Transverse dunes are aligned perpendicular to constant wind direction. Shape is controlled by wind direction, constancy, sand supply, and vegetation.

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Both form when streams lose velocity and deposit their load, but alluvial fans form at the foot of mountain slopes where streams break onto plains, while deltas form where rivers dump their load into the sea. Delta deposits are well sorted with clear stratification, unlike the coarser, less sorted alluvial fan deposits.