Chalk is a sedimentary rock, formed from the remains of invertebrate & algal exoskeletons. Coastlines of chalk rock include those that are subject to inundation by the tide (littoral) & permanently submerged (sublittoral).

Key Characteristics

  • Site: Porous sedimentary rock; specialised invertebrate & algal communities
  • UK: 57% of European resource occurs along coast of England
  • NI: Ulster White Limestone

Habitat Description

  • Chalk, a sedimentary rock, is generally soft, friable & easily eroded. It was formed from the remains of invertebrate & algal exoskeletons, including foraminiferans (protozoans), particles of bivalves & coccolithophorid (calcite secreting) algae.
  • Unlike most of the chalk in north-west Europe, which was formed during Lower and Middle Cretaceous times, chalk in Northern Ireland was formed during the Upper Cretaceaous period, when most of north-west Europe was underwater. This chalk is known as the Ulster White Limestone.
  • Ulster White Limestone is typically of high carbonate purity (>95% calcium carbonate), with distinct bands of the siliceous rock, flint. NI chalk differs from the relatively soft chalks in England, as it is very resistant to erosion as a result of secondary calcite cementation within the pore spaces.

Current Status in UK & Northern Ireland

  • Marine chalk is a scarce habitat in UK & EU waters. The coast of England holds the largest proportion of European coastal chalk (57%) (NIEA, 2005).
  • Ulster White Limestone is present on Rathlin Island, along the north Antrim coast, with occasional outcrops on the east Antrim coast (NILS).
  • Littoral chalk is present on the northern & southern coasts of Rathlin Island, where it forms irregular wide platforms. There is a good example of this at Beddag, an extensive intertidal chalk area in the shelter of Church Bay, which is particularly species rich (Wilkinson et al., 1988; NIEA, 2005).
  • Little is known of the extent or nature of sublittoral chalk. Rathlin Island has extensive underwater exposures of chalk. Sublittoral caves are present in chalk down to at least 75m (NIEA, 2005). These caves support populations of rare species. For example, the NISS (Erwin et al., 1986) recorded up to three rare sponge species from chalk habitats, one of which was only known from the west coast of Sweden.
  • The White Rocks have diverse chalk formations, in the form of cliffs, arches, platforms & caves, representing littoral, sublittoral & supra-littoral habitats.
  • Chalk habitats in NI are threatened by physical disturbance such as benthic trawling but the sublittoral chalk habitats in Rathlin Island gain protection from the steep vertical faces & high level of wave exposure, which reduce fishing operations. Coastal defences have been constructed at Portballintrae & Ballycastle & new breakwaters have been built in Church Bay, Rathlin Island. These may negatively impact the biodiversity of the chalk habitats. Deterioration of water quality by pollution & nutrient enrichment may change community structure.

Designated Sites

  • Carrick-a-rede
  • Magilligan
  • Rathlin Island
  • White Park Bay
  • White Rocks

Further Information