Summary of site:
To understand and compare the rocks of an area with their age equivalents elsewhere, geologists divide them into recognisable units and name them, usually incorporating features of local geography into the name. Such divisions, called stratotypes, are characterised by reference to a particular locality where its features are best seen. This locality is the stratotype of the Greenan Sandstone Formation, a division at the top of the Lower Carboniferous, a time around 335 million years ago.
Two streams drain the south side of Greenan Mountain and the stratotype is exposed in the more westerly. The rocks here are almost vertical, steeply inclined to the north west. Close examination of sedimentary structures within the rocks shows that they are overturned, severely disrupted by a major fault.
The succession commences with breccias - angular fragments of weathered bedrock of the time (ancient Dalradian schists in this case) - in a sandstone matrix. The breccias are 30m thick and are succeeded by 5m of red mudstones and siltstones with small lime nodules that indicate they were soils. A considerable, undefined, thickness of medium to coarse grained sandstones (the grains consisting of quartz and feldspar) follow, purple-grey on freshly broken faces but weathering to an orange-purple. Thin laminations of purple-red and green mudstones occur throughout the sandstone.
Mudstones likely to yield fossil pollen spores were recognised in the sequence and samples were gathered. Four in the lowest 100m of the section were indeed found to contain these microscopic plant remains, three of them in the stratotype. The lowest, just above the basal breccia, contained fourteen species and another around 70m above the base contained a further fifteen. These assemblages can be allocated to two miospore zones used for dating purposes; they indicate the Brigantian and Pendleian, stages spanning the boundary of the Visean and Namurian epochs, two major divisions of the Carboniferous period.
The events that created these rocks took place in a landlocked basin. Deep weathering of the ancient metamorphic basement created the basal breccias and soils formed in the mudstones that were washed into them. Considerable quantities of quartz and feldspar sand were then swept into the basin by channelised streams maintaining an equilibrium with the surrounding hills, as the basin slowly subsided. One species of a simple plant (an alga) in the sequence is an indicator of fresh or brackish water. The basin was on the southern fringe of the supercontinent of Laurentia, backed by mountains. This continent, destined to become part of North America and Northern Europe, was then drifting, with infinitesimal progress, northwards towards the equator of the time.
Stratotypes are the building blocks of geological history and those containing unequivocal dating evidence are particularly valuable. The Greenan Sandstone section is therefore of extreme importance, providing both a time reference and a record of terrestrial events that span the Visean/Namurian boundary. There are no obvious threats to the site.