GeologyRocks

Absolute Age

An age of a rock or formation given in years determined by radiometric means. See also Relative age.

Amethyst

Variant of quartz, purple in colour.

Angular unconformity

An unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers that are at an angle to the overlying horizontal layers.

Hutton's unconformity at Siccar Point is a classic example where Devonian sediments are deposited on Silurian turbidites that are tilted at 70°.

Aquifer

An aquifer is a layer of porous rock that contains water. See Artisan Well

Artisan Well

An artesian aquifer or well (also known as a confined aquifer) is an aquifer that is trapped between two impermeable layers. An unconfined aquifer is recharged rapidly from surface water. A confined aquifer takes much longer to recharge, but this also means it is not responsive to daily, annual or even millennial changes. An example of an artesian aquifer is South Dakota, USA (see diagram below). Note how the Dakota sandstone (the aquifer) is bounded top and bottom by an impermeable rock.

Asthenosphere

The asthenosphere is the region of the inner structure of the Earth between 100-200 km below the surface that is the weak or "soft" zone in the upper mantle. It lies just below the lithosphere, which is involved in plate movements and isostatic adjustments. The layer is best thought of as "plastic" in that it is solid, but deforms over time.

Cambrian

The earliest defined geological period spanning 542-483Ma. Time before the Cambrian is referred to as the Pre-Cambrian, making the Cambrian the first period of the Palaeozoic Era. All fossils from this period are marine and the beginning of this period saw the Cambrian Explosion; an apparent burst of evolution.

The Cambrian is split into four Epochs, Lower Series, Series 2, Series 3 and Furongian.

Carboniferous

The fifth period of the Palaeozoic. Spans 359-299Ma, and is famous for the extensive coal beds found throughout the UK.

Cline

A gradual change in an allele frequency or in the mean of a character over a geographic boundary.

Clint

A clint or flachkarren is a tabular block of limestone in limestone pavement. See also karst topography and grike.