The Upper Llandoverian rocks of West Connacht that are present between the Highland Boundary fault and the Dalradian inlier of Connemara were deposited on the northern margin of the Caledonian geosyncline. They lie unconformably on Ordovician and Dalradian strata. The sub-Silurian unconformity indicates that in Silurian time the Ordovician rocks were terminated abruptly against the Dalradian strata of Connemara by a major fault. The "Connemara landmass" was an important sediment source in Ordovician time, but appears to have been absent in the Silurian.

During late Llandoverian time, the sea gradually transgressed northward across Connemara to the Highland Boundary fault. Two contrasting depositional environments developed. The "basin facies" consists of well-sorted cobble conglomerate, graywacke, and siltstone. The petrology of the conglomerate indicates a northerly source area of Ordovician volcanic rocks and Dalradian quartzite, partly of low metamorphic grade. The quartzite is mostly homogeneously reddened. The "shelf facies" consists of fluvial and shallow-water marine sandstone and siltstone that also indicate a northerly source. Penecontemporaneous faulting was important in producing steep basin margins and in affecting rates of subsidence in the shelf facies.