The Ulster Chalk
The Cretaceous chalk (135-65 million years old) is a thickly bedded, almost pure carbonate rock and is the youngest rock unit in Ireland to exhibit karst characteristics. It is exposed as a narrow strip-like outcrop, which totals approximately 80 km2 around the periphery of the overlying Tertiary basalt (volcanic lava). The chalk dips gently beneath the overlying basalt. Before the eruption of the basalts, uplift and erosion of the chalk allowed the development of a karst landscape, and palaeokarst features are still evident at the basalt-chalk contact. The chalk, generally less than 50 m thick and with a wide joint spacing, differs from the chalk of England and other areas of north-west Europe, in that groundwater flow relies on the presence of fractures. Few open boreholes penetrate the chalk beneath the basalt, so aquifer investigations have been largely restricted to the areas neighbouring the outcrops.

Loughareema sink, the largest single sink recharging the chalk aquifer,
supplied by three permanent streams. A lake forms above the sink which
fills after heavy rainfall. No streams flow out of the lake. The water level
rises and falls depending on rainfall and the chalks ability to take in the
water (Donal Daly)

Numerous springs emerge from the base of the chalk at its junction with the underlying, relatively impermeable, mudstones. Chemical analyses of their waters show that the springs are mainly fed by surface water, which runs off catchment areas on the basalt plateau and sinks at the basalt-chalk boundary. The year-round point flow into the numerous stream sinks has resulted in active dissolution of the chalk matrix in the outcrop areas. These occurrences are more common in the east, as overlying glacial deposits in the west have reduced outcrop exposure and streams tend to be perched above the aquifer. Small dolines and numerous fissures showing solution features are also dispersed over the outcrop areas.

The narrow outcrop of the Ulster Chalk along the Antrim coast.
The overlaying rock is basalt (Stephen Barnes)

Flow from the chalk springs is closely related to rainfall: sustained flow is very limited during prolonged dry periods. This confirms that the contribution of 'old' sub-basaltic water to the chalk springs is only a minor component of the overall water budget in the chalk. Beneath the basalt, large scale dissolution is not thought to have occurred in the chalk as recharge water is restricted and is often close to saturation with calcite before reaching the chalk.

Water tracing experiments using artificial dyes have demonstrated links between individual sinks and springs, with groundwater flow velocities between 0.3 and 2.8 km/day.