Oceans are the final resting place for a good proportion of terrigenous (i.e. from the land) sediment that is transported by the elements, as well as being the location for the production of biogenic sediments like chalk and chert. The further away from the continental margin you go, the sediment generally becomes finer, and a large proportion of the deep marine environment is floored by fine clay and mud.

The oldest known rocks on Arran are thought to be deep sea mudstones and submarine debrites that have been later deformed to produce intensely folded slates and schists. These rocks, known as the Southern Highland Group of the Dalradian Supergroup, are thought to have been deposited about 550 Ma years ago when Arran lay at the edge the Laurentian supercontinent. These rocks outcrop mainly in the northwest and north of the island. Excellent exposures of folded greenschists can be seen at on the shore at Imachar. These rocks indicate that there was a very ancient ocean in this area, and that after these rocks were deposited there was a major phase or phases of metamorphism, presumably linked to major tectonic events. The original sedimentary grains can still be seen in some of the metamorphosed sediment.

Extensive shallow seas are often the site of deposition of carbonate rocks such as chalk and reef limestone, as long as the input of siliciclastic sediment is negligable. On occasion during the Carboniferous Arran was subjected to incursions of the sea. The resulting shallow marine environment allowed for the deposition of richly fossiliferous sandy limestone. Such a limestone can be seen outcropping on the shore near the harbor at Corrie and in the raised beach cliffs just behind. The presence of fossils is often a good indicator of the environment of deposition - in this case the presence of brachiopods indicates that these are marine sediments because these organisms are entirely marine, and do not live in rivers or lakes (where similar-looking sediments can be deposited). On Arran the Carboniferous marine sediments are thin, and only extensively exposed in a narrow coastal belt - please do not hammer these exposures - it is possible to see all the important features without damaging these scientifically valuable rocks.

During the Cretaceous the rise in sea level was considerable; enough to inundate nearly all of the British Isles! It was then that the Chalk was deposited. Made up of the microscopic skeletal plates of calcareous nanoplankton, this fine lime mud accumulated to great thicknesses but has since been removed by glaciation. The only chalk that remains on the island is in the form of a large foundered block which was trapped in the collapsed volcanic caldera of the Central Ring Complex. Again identifying fossils is an important clue as to environment of deposition - in this case the outcrops on Arran are not well exposed or large, and in any event fossils are relatively rare in many Cretaceous chalks. Microscope analysis of thin sections will however allow the nanoplankton to be recognised - in the field the carbonate nature to the rock is apparent by applying dilute acid and noting the fizzing. Note that chalk on Arran is rare and should not be hammered or collected without good scientific reason.