Conglomerate is a sedimentary rock formed from rounded gravel and boulder sized clasts cemented together in a matrix. The rounding of the clasts indicates that they have been transported some distance from their original source (e.g. by a river or glacier), or that they have resided in a high energy environment for some time (e.g. on a beach subject to wave action). The cement that binds the clasts is generally one of either calcite, silica or iron oxide. The matrix can consist solely of the cementing material, but may also contain sand and / or silt sized clasts cemented together among the coarser clasts. Conglomerates can be further divided according to:
Class - conglomerate can be divided into two broad classes:
Clast supported - where the clasts touch each other and the matrix fills the voids; and
Matrix supported - where the clasts are not in contact and the matrix surrounds each clast;
Clast size - fine (2 - 6mm), medium (6 - 20mm), coarse (20 - 60mm), very coarse (> 60mm);
Sorting - a conglomerate comprising a mixture of clast sizes is poorly sorted, while one comprising mostly clasts of the same size is well sorted;
Lithology - a conglomerate where the clasts represent more than one rock type is termed polymictic (or petromictic), while one where the clasts are of a single rock type are monomictic (or oligomictic).
Other specimens - Click the thumbnails to enlarge
Grain size - > 2mm; clasts easily visible to the naked eye, should be identifiable.
Hardness - variable, soft to hard, dependent on clast composition and strength of cement.
Colour - variable, dependent on clast and matrix composition.
Clasts - variable, but generally harder rock types and / or minerals dominate.
Other features - clasts generally smooth to touch, matrix variable.
Uses - as dimension stone for decoration of walls and floors; if hard can be used as aggregate, fill etc. in the construction and roading industries.
New Zealand occurrences - sporadic occurrence throughout the country, often exposed on coastlines (Hawkes Bay (Cape Kidnappers), Wairarapa), Canterbury Plains; generally found interbedded with sandstone and mudstone.