Conglomerate is a sedimentary rock formed by the lithification of rounded or sub-rounded gravel (grains larger than 2 mm in diameter). Conglomerate is strongly related to sandstone. It is actually a type of sandstone although it may not be technically correct to say so. Conglomerate is composed of clasts larger than 2 mm which is the upper limit of sand grains size.
Conglomerates are differentiated from sedimentary breccias which are composed of angular clasts. Sometimes it may be difficult to say whether the grains are angular or rounded enough. It is therefore unavoidable that one geologist’s conglomerate will be another’s breccia. I generally prefer to name these rocks conglomerates if there is at least some rounding apparent. Breccia in my opinion is composed of really angular, wedge-like clasts with a strong variation in size. Breccias often show clear signs of shattering which is a result of a sudden and energetic event. Conglomerates tend to be composed somewhat more evenly sized clasts. Conglomerates may be clast-supported (clasts are in contact) or matrix-supported (clasts are separated from one another by fine-grained matrix that binds the clasts together).
There are many different ways to make a conglomerate. Conglomerate deposit may be an ancient riverbed or a coastline. Conglomerates form at the base of mountain ranges or even as a glacial deposit. Tillite is a lithified till (poorly sorted glacial debris) which may be called a matrix-supported conglomerate if there are enough granules, pebbles, cobbles, and boulders present. At least 30% of the rock should be composed of clasts larger than 2 mm in diameter in order to call that rock a true conglomerate.
A conglomerate sample from Switzerland. Such colorful, decorative, and well-rounded conglomerates are sometimes named puddingstone. The width of the sample is 9 cm.
A conglomerate sample from the Bunter formation which is composed of conglomerates and sandstones from the Triassic. The width of the sample is 12 cm.
A metamorphosed conglomerate (metaconglomerate) from Norway. The width of the sample is 23 cm.
An outcrop of conglomerate in Cyprus consisting of rounded gabbro clasts from the Troodos ophiolite.
The same outcrop near Kouklia in Cyprus.
Weakly consolidated river deposits in Cyprus, north of Paphos.
Fine-grained conglomerate pebble with quartz clasts and dolomite cement from the northwestern coast of Estonia. Red grain in the middle is almandine garnet.
A boulder of conglomerate on a lithified lahar (volcanic mudflow) at the coast near Portraine in Ireland.
A close-up of the same boulder.
A conglomerate boulder in the Spanish Pyrenees consisting of gray limestone and red sandstone clasts.
Conglomerate is a widespread rock type in northern Spain. These deposits formed as a result of the orogeny (rapidly rising mountains were eroded rapidly as well).
A boulder of conglomerate in southern Ireland (Ballydowane Cove).
Vertical layers of alternating sandstone and conglomerate at Bunmahon Head, Ireland. These layers were originally horizontal.
More sandstone and conglomerate at Bunmahon Head. It is easy to see that original up direction was to the left.
Conglomerate deposits often give rise to peculiar landforms like this tower in northern Spain.
More landforms composed of conglomerate in Spain. Conglomerate is easily erodable. Once there is a crevice formed, it tends to widen which results in many tower-like rock formations.
A closer view of these conglomeratic cliffs.