Sandstone is a sedimentary rock formed from cemented sand-sized clasts. The cement that binds the clasts can vary from clay minerals to calcite, silica or iron oxides. Sandstone can be further divided according to:
Clast size - fine (0.06-0.2mm), medium (0.2-0.6mm), coarse (0.6-2mm);
Sorting - a sandstone comprising a mixture of clast sizes is poorly sorted, while one comprising mostly clasts of the same size is well sorted; a sandstone containing very little silt and / or clay is termed arenaceous; a sandstone containing a significant amount of silt and / or clay is termed argillaceous or a "wacke" (see greywacke for more information);
Mineral content - a sandstone consisting of more than 25% feldspar clasts is termed arkose; a sandstone consisting of more than 90% quartz clasts is called quartzose;
Other specimens - Click the thumbnails to enlarge
Grain size - 0.06 - 2mm; clasts visible to the naked eye, often identifiable.
Hardness - variable, soft to hard, dependent on clast and cement composition.
Colour - variable through grey, yellow, red to white reflecting the variation in mineral content and cement.
Clasts - dominantly quartz and feldspar (orthoclase, plagioclase) with lithic clasts and varying minor amounts of other minerals.
Other features - gritty to touch (like sandpaper).
Uses - if soft then generally of no use; if hard then can be used as aggregate, fill etc. in the construction and roading industries; dimension stone for buildings, paving, etc.
New Zealand occurrences - extensive occurrence throughout the country (especially North Island), well exposed on many coastlines, e.g. Auckland, Taranaki, King Country, Wairarapa, Southland; generally found interbedded with mudstone.