Montserrat's jagged peaks are made of cemented river pebbles
The mountain near Barcelona whose name means jagged mountain is built from conglomerate: rounded pebbles and cobbles glued together by natural cement. Erosion carved vertical channels into it, creating its saw-toothed skyline, and the rock is strong enough that the abbey on its slopes was built from it.
Conglomerate is a sedimentary rock in which gravel-sized fragments, called clasts, sit in a finer matrix of sand, silt or clay, bound by calcium carbonate, iron oxide, silica or hardened clay. To qualify, at least 30 percent of the rock must be rounded to subangular clasts over 2 mm across, anything from granules to boulders. If the fragments are sharp-edged instead, the rock is a breccia; a mix of both is sometimes called breccio-conglomerate. Despite their visibility, conglomerates probably make up less than 1 percent of all sedimentary rock by weight.
How the clasts sit tells a story. When pebbles touch one another, the rock is an orthoconglomerate, usually laid down by flowing water and often cross-bedded. When they float apart in abundant matrix, it is a paraconglomerate, typically left by glaciers or debris flows. Geologists also classify by what the clasts are made of, by whether they came from outside the basin or were ripped up from within it, and by dominant size, as in pebble or cobble conglomerate. Flat-pebble conglomerates, made from slabs of lime mud, record storms or tsunamis scouring shallow seafloors.
Settings vary widely. Young, vigorous rivers leave rounded but poorly sorted gravels, since pebbles shrink as they travel. Where alluvial fans merge at mountain fronts, the thickest conglomerate deposits accumulate, and fanglomerates from these fans probably hold the largest gravel accumulations in the geological record. Glaciers produce tillites, jumbled and matrix-rich. Basal conglomerates mark the position of an advancing shoreline during a rise in sea level.
Striking examples exist beyond Spain. The Crestone Conglomerate at the foot of Colorado's Sangre de Cristo Range formed in prehistoric alluvial fans and shows hues of red and green. On Scotland's east coast, conglomerate cliffs run north from Arbroath, and Dunnottar Castle perches on a rugged conglomerate headland jutting into the North Sea near Stonehaven.
Source: Conglomerate (geology)
