Page 106 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 89

drift is impeded, coastal landforms develop. Longshore a fluid medium (air, water, or ice) they become clastic
currents and beach drifting may act in the same or sediments.
opposite directions. Size is the normal criterion for grouping clastic sedi-
ments. Loose sediments and their cemented or com-
pacted equivalents have different names (Table 3.1). The
Biological activity coarsest loose fragments (2 mm or more in diameter)
Some marine organisms build, and some help to build, are rudaceous deposits. They comprise gravels of vari-
particular coastal landforms. Corals and other carbonate- ous kinds – boulders, pebbles, cobbles, granules – and
secreting organisms make coral reefs, which can be sometimes form distinct deposits such as glacial till.
spectacularly large. The Great Barrier Reef extends along When indurated, these coarse deposits form rudaceous
much of the north-east coast of Australia. Corals grow in sedimentary rocks. Examples are conglomerate, which
the tropics, extratropical regions being too cold for them. consists largely of rounded fragments held together by a
2
Coral reefs cover about 2 million km of tropical oceans cement, breccia, which consists largely of angular frag-
and are the largest biologically built formation on Earth. ments cemented together, and gritstone. Loose fragments
Calcareous algae produce carbonate encrustations along in the size range 2–0.0625 mm (the lower size limit
many tropical shores. varies a little between different systems) are sands or
Salt-tolerant plants colonize salt marshes. Mangroves arenaceous deposits. Indurated sands are known as
are a big component of coastal tropical vegetation. With arenaceous sedimentary rocks. They include sandstone,
other salt-tolerant plants, they help to trap sediment in arkose, greywacke, and flags. Loose fragments smaller
their root systems. Plants stabilize coastal dunes. than 0.0625 mm are silts and clays and form argilla-
ceous deposits. Silt is loose particles with a diameter in
the range 0.0625–0.002 mm. Clay is loose and colloidal
material smaller than 0.002 mm in diameter. Indurated
SEDIMENT DEPOSITION equivalents are termed argillaceous rocks (which embrace
silts and clays). Examples are claystone, siltstone, mud-
Sediments are material temporarily resting at or near stone, shale, and marl. Clay-sized particles are often
the Earth’s surface. Sedimentary material comes from made of clay minerals, but they may also be made of
weathering, from volcanic activity, from the impact of other mineral fragments.
cosmic bodies, and from biological processes. Nearly
all sediments accumulate in neat layers that obligingly
record their own history of deposition. In the fullness Chemical sediments
of Earth history, deposition has produced the geologi- The materials in chemical sediments derive mainly from
cal or stratigraphic column (see Appendix). The sum- weathering, which releases mineral matter in solution
ming of the maximum known sedimentary thickness for and in solid form. Under suitable conditions, the solu-
each Phanerozoic period produces about 140,000 m of ble material is precipitated chemically. The precipitation
sediment (Holmes 1965, 157). usually takes place in situ within soils, sediments, or
water bodies (oceans, seas, lakes, and, less commonly,
rivers). Iron oxides and hydroxides precipitate on the
Clastic sediments
sea-floor as chamosite, a green iron silicate. On land,
Clastic or detrital sediments form through rock weath- iron released by weathering goes into solution and,
ering.Weatheringattacksrockschemicallyandphysically under suitable conditions, precipitates to form vari-
and so softens, weakens, and breaks them. The pro- ous minerals, including siderite, limonite (bog iron),
cess releases fragments or particles of rock, which range and vivianite. Calcium carbonate carried in ground-
from clay to large boulders. These particles may accu- water precipitates in caves and grottoes as sheets of
mulate in situ to form a regolith. Once transported by flowstone or as stalagmites, stalactites, and columns