1. Fairymount - introduction to North Roscommon's geology & landscapes.
Fairymount Quarry is a good place to start examining north Roscommon's geology. Most of the County is underlain by limestones of Lower Carboniferous age. The limestones crop out sparingly in a number of low hilly areas: here at Fairymount, to the east near Rathcroghan and Tulsk, further east in hills to the north of Strokestown, and, last but not least, between Boyle and Carrick-on-Shannon, the ancestral lands of the MacDermots, the kingdom of Moylurg.
The geology of the area between Boyle and Roscommon is reasonably well-known. However, away from this area outcrops are sparse, and dips are low and variable. It's not possible to match outcrops against the "Carrick-on-Shannon succession". For this reason much of Roscommon is denoted as "Undifferentiated" limestone on the 1:100,000 Sheet 12
Examining the rocks in the quarry we can see:
1. near-vertical joints which run roughly north-south - the back face of the quarry (of which the picture is a detail view) is one such.
2. Bedding (layering of the rock) is nearly horizontal, between 30cm and 1m in thickness, defined by undulating planes along which there may be thin films of shale.
3. The limestone is fine grained and dark grey in colour; fossil remains are sparse.
4. Chert (hard, brittle, very finely crystalline silica) occurs as nodules (see just above and to the left of my glasses in the pic), also as patches of "festoon" silicification.
5. Solution of the limestone by groundwater: the picture shows a sub-vertical narrow cavity which has been filled later by clay. Joints and bedding planes in the upper 2 or 3m have been widened by solution (the "epikarst" zone).
The paucity of outcrop results from a blanketing of the area by glacial deposits of till, the landscape being formed of an apparently irregular mosaic of drumlins and ridges, the low ground between being occupied by lake and bog. However, there is some pattern in the till distribution. The map below, derived from the old GSI six-inch sheets with some field checking in 1999, shows a remarkable pattern in its southeast corner (just west of Kilglass Lough).
A series of northeast-southwest till areas are separated by lake and bog. Drumlins are oriented transverse to this alignment. A similar, though less spectacular arrangement, has been recognised in the Lisacul area during the making of a figure for these webpages, the drumlins there being at a low angle to the ridges. Robbie Meehan of Teagasc refers to thses landscapes as "ribbed topography".
The map below is adapted from one in which I attempted to depict the landscape from the point of view of potential for settlement in pre-historic or early historic times.
I have made no attempt to distinguish between raised and blanket bog- the category also includes cut-away bog, marshland and alluvium.
The white areas indicate poorly-drained till, either as drumlins or ground moraine, developed from both limestone and sandstone parent materials.
The green areas are where limestone outcrops or is near the surface. Drumlins or areas of till may occur, but the soils are generally well-drained.
These areas generally form uplands, and correspond with the greatest density of pre-mediaeval archaeological sites.