Geoscience Reference
In-Depth Information
will find two different rivers flowing over identical rock types in equal
proportions along their length. Some rocks are harder than others - the
rocks that make up the central and ancient 'cores' of continents tend
to be crystalline igneous rocks such as granite and granodiorite, and
metamorphic rocks such as schist and gneiss, and these are far more
resistant to erosion than the usually younger overlying sedimentary
rocks. Inequality of strength is also displayed in similar sedimentary
rocks; not all sandstones have the same physical characteristics
although all are composed of cemented grains of quartz (silicon diox-
ide), which itself is a hard mineral. On the mineralogical scale of
hardness, which was devised by the German mineralogist Frederick
Mohs (1773-1839) and carries his name, quartz is ranked 7 (diamond,
the hardest substance, is 10, while calcite or calcium carbonate which
makes up most limestones is 3). The strength of a sandstone or indeed
of any rock is only as strong as its weakest component; in the case of
sandstone, many are cemented by calcium carbonate or calcite, and so
in reality are more susceptible to erosion than those cemented with
silica.
Close examination of a sediment sequence or a sedimentary
rock will reveal information about sediment type, provenance, size
and sorting and this can be used to interpret something about the
environment in which the material was deposited and the mechanism
that produced the sediment, but often it is difficult to gauge the speed
or timing of deposition. In the past the rates of denudation were not
always uniform and varied from location to location and environment
to environment.
THE SEDIMENTARY HOUR-GLASS
Howdid geologists use the sedimentary record as an hour-glass? By the
1850s the geological community had taken on board the ideas of James
Hutton and accepted that the Earth's history was cyclical, but in the
context of sediment accumulation the uniformitarianism ideas spread
by Charles Lyell were more important. If the various active geological
processes of the past were similar to those active at the present time,
