The North Slob is underlain by two formations of Carboniferous limestone (named after Wexford and Ballysteen) that were laid down over 330 million years ago. There are no rock outcrops to be seen on the Slob as the area is thickly covered with marine muds and sands. The Wexford Formation can be seen in quarries on private land near Wexford Town, while, further south-west, the Ballysteen formation forms the southern tip of Hook Head and its fossils of corals, sea lilies and sea mats are beautifully exposed where sea waves etch them.
Both limestones are dolomitised which means that magnesium atoms replaced some of the calcium atoms, turning the calcium carbonate into Magnesium carbonate. This happened millions of years ago and resulted in shrinkage and fissuring of the rock which made it about 1% porous to water. As a result it is now a regionally important aquifer from which groundwater is pumped up at Fardystown) and piped to much of south County Wexford and to Wexford town.
From 1881 to 1924 the limestone quarries were of great importance to Wexford, as they were the basis of an industry that took limestone from quarries at Drinagh and baked it with muds dredged from the Harbour to make an award-winning Portland Cement. A tall chimneystack at Drinagh, set among trees on private property, can be seen from the N25 near the Rosslare road roundabout.
The soils on the North Slob are formed of silt and sand that were covered by the sea at high tides until the sea wall was built from 1847 to 1849 and pumps installed to keep them dry. To the east the artificial land is protected by the sand dunes of Raven Point. This narrow spit of land is composed of sand dunes that were built from the seventeenth century onwards as the wind blew sea sand into patches of Marram Grass that grew up through the accumulating sand and anchored it into hillocks or dunes.
North of the North Slob are the Screen Hills, which are a classic kame-and-kettle landscape. Kames are hills of sand and gravel that were left behind and washed by running water in complex processes associated with the melting of the Irish Sea Glacier over 13 thousand years ago. Kettle holes are large cavities left as ice blocks melted in the ground at the same time. There are over a hundred “kettle-hole” ponds and lakes in the Screen Hills formed in this way.
A heavy clay subsoil, part of the Macamores formation, forms low cliffs between Wexford Bridge and the Wexford Wildfowl Reserve. The cliffs running east of Rosslare Harbour as far as Greenore Point are formed from the same stiff clay. This clay is also the basis of the very heavy “strong” soils of the Macamores area, northeast Co. Wexford. This material was pushed up from the bed of the Irish Sea into its present locations by the same Irish Sea ice sheet.
The Sea Wall is faced with Quartzite and Greywacke rock of the Cambrian period that was quarried up-river around 1847. The blocks were originally grouted with lime-based mortar, cement mortar is used nowadays. The core of the wall was built of Macamores clay (see above), drawn mainly from a quarry that can still be seen as a vegetated depression near the east end of the structure.