Other rock types include hornblende gneiss, pegmatite and basalt dykes. The hornblende gneiss is dark in colour and occurs as pods and bands within the lighter coloured gneiss. Pink and white pegmatite typically occurs in veins and contains coarse grained quartz and feldspar. Basalt dykes are dark grey, fine grained, later igneous intrusions which can be eroded to form gullies and cliffs.
Rock was also excavated to form the breakwater and create a marshalling area for the ferry terminal on Eriskay at Ceann a Gharaidh. Excavations in this area uncovered shear zones which may be related to the Outer Hebrides Thrust Zone, a low angled fault which trends north to south along the length of the Outer Hebrides. Pseudotachylite has been identified which is formed by the crushing action when rocks are sheared and is also associated with the Outer Hebrides Thrust Zone,
Two sources of rock were used to provide the rock required to construct the causeway. The main source was the quarry situated on South Uist in a depression in the hills behind Ludag. The other rock source was the cutting for the approach road to the causeway on the Eriskay side.
The bedrock consists of Lewisian Gneiss which is a metamorphic rock that experienced high temperatures and pressures to take on its present form. This gneiss is some 2600 million years old and belongs to the oldest rocks in the UK. It has a light and dark grey banded appearance and contains light quartz and feldspar minerals with smaller amounts of dark hornblende and biotite. The banding is often curved and folded showing the severe deformation that the rock has undergone. Larger blocks of gneiss (up to 5 tonnes) were used as armour stone to provide the protective outer layer of the causeway.
