Early interpretations of the glacial history of the Sperrin Mountains recognised a minor role for glacial lakes in the deposition of sand and gravel deposits throughout the area. Recent evidence however now supports the association of sand and gravel deposits in the area with deposition in lakes. There is ample evidence from the valleys of the Dunlogan and Drumderg Rivers, just west of Moneyneena, to support this contention. The sand and gravel deposits west of Moneyneena were originally deposited in a single glacial lake delta which was later dissected and eroded by the post-glacial rivers, restricting the deposit to three narrow bands flanking the present day courses of the rivers.
The sediments are thickest in the centre of the deposit, in the sections that occur between the rivers, and thin to the north and south onto the flanks of the original valley which was lined with glacial till. The top surface of the main part of the deposit is gently undulating and is inclined towards its centre line from an altitude of about 125m on the north side, at St. Eoghan's Primary School, and 138m at its southern margin, just north of the Drumderg Road. Numerous sections through the deposits are found in the deeply incised valleys of the Dunlogan and Drumderg Rivers and near vertical faces up to 20m high are common. At most sections the rivers have eroded through the glaciolacustrine deposit but it is only rarely that the underlying glacial till is exposed in the banks or bed of the rivers. The best section in the deltaic deposits, which is in the north bank of the Drumderg River [H7472 9721], exposes at least 14m of stratified clay, sand and gravel. The thickness of the stratified deposits increases to at least 20m in both banks of the river to the west of this locality.
