Summary of site:
This area in the Tempo valley between Glengesh at the valley head and Tattinweer, 6 km to south west, contains sand and gravel deposits of two types. One takes the form of elongate hills up to 500 m long, the other, flat expanses under peat deposits. Both result from the deglaciation of this landscape. The upper limit of sands and gravels falls from around 180 m in the upper valley to about 90 m at Tattinweer. There are exposures in the linear deposits at Dooneen and Tonyglaskan and a small exposure in a flat, sub-peat spread at Brockagh, all in the upper reaches of the valley.
The largely abandoned Dooneen Pit still has 2 small exposures. One shows thick beds of fine and medium grained, cross-bedded sands with concretions (solid, discrete nodules of cemented sand) up to 10 cm long in line with the bedding. The sand beds above the concretions contain mud curls up to 10 cm long and most are aligned at right angles to the flow, rolled along by the current. Concretions are often associated with the migration of water through sandy sediments with seasonally controlled freeze and thaw but mud curls are more often a feature of hot desert environments as thin muds dry out and detach from the sediment beneath. In glacial environments circumstances for the formation of mud cracks are less common and their survival in energetic streams with mass flow is small, thus adding to the interest here.
The second exposure at Dooneen is a body of channelised medium-grained sands resulting from the steady flow of sediment into a lake along the margin of the ice sheet. These circumstances may reflect a warming climate with the melting of stagnant ice and a steady supply of transporting meltwater.
The Tonyglaskan Pit was working at the time of the survey with faces 12 m high and 50 m long exposing beds of gravels, cobbles and boulders with stratified, fine to medium-grained sands. The gravel and boulder beds are typically very variable in the size of particles, crudely cross-bedded with many debris filled channels. There are no obvious breaks in sedimentation and the sediment becomes finer in grain from west to east. The cross-bedding shows a southerly flow. The flat top of these deposits is a clear indication of a delta setting and the very variable nature of the coarse sediments, from gravel to boulder, indicates torrential water flow from the melting ice masses stranded on the Fintona Hills. The surging local currents were capable of carving large channels in the delta gravels and then filling them with cobbles and boulders. These deposits are at a higher level than those at Dooneen and probably formed before them when lake levels were higher.
A second exposure at Tonyglaskan contrasts strongly with the first. It exposes mainly laminated, thick to massive beds of medium to fine grained sands with some pebbles of local rock types. Within the sands are pods, about a metre long by 30 cm wide, of pebbles and cobbles. Gravel beds up to a metre thick are also present. These sediments indicate a much quieter environment indicated by the finer sands and the absence of cobbles and boulders; the pods reflect periodic flood events.
A third exposure at Tonyglaskan shows rhythmical beds (repeated sequences of sediments) of fine sand and silt topped by weakly bedded units of pebbles and cobbles lacking matrix. The rhythms were probably deposited by repeated pulses of sediment laden meltwater into a fairly still ice-dammed lake. The pebbles and cobbles represent more energetic pulses.
For the context of these deposits see the Tempo Valley – Overview, site 485.