Formation of sedimentary rocks

sedimentary rock

Sedimentary rocks are formed when particles of animals, sea creatures, plant life and broken-down rocks are lithified.

Limestone

Limestone is an organically-formed sedimentary rock. It began forming 300 million years ago, when the bones and shells of fish and other sea creatures built up on the bed of a warm, shallow sea close to the equator.

When rivers deposited sand and mud particles on top of it, the weight of these sediments compacted the dead marine matter together, pushing the seawater out of the pore spaces between the different bodies and pieces of dead matter.

Calcium carbonate in the bones and shells of the marine life then cemented the compacted remains together.

Limestone was formed in different periods, with each period represented horizontal layers of limestone known as strata.

Strata are separated from one another by horizontal bedding planes. Limestone also contains vertical cracks, or joints, which were formed when earth movements folded and split the rock.

Sandstone

Sandstone is an inorganic sedimentary rock.

Old red sandstone was formed approximately 380 million years ago, when Ireland was south of the equator and experienced a desert climate.

Only the mountains of the north-west (Derryveagh Mountains), formed during the Caledonian folding period, were present above water.

Rocks were broken down by weathering and erosion and sediments of fine sand were transported from the region by flash floods.

Later, these sediments were deposited in strata (layers) on the bed of a shallow sea.

Over time, the sand was compressed by the weight of its own material and the weight of the seawater on top of it.

This pushed any water out of the pore spaces before calcium carbonate, iron and silica cemented the sand grains together, forming a solid sandstone rock.