Morphometric Analysis of Glaciofluvial Terraces in the Esk Basin, Midlothian

Abstract

This paper describes the wastage of the last ice-sheet of the Esk basin, Midlothian, with particular reference to the dissected glaciofluvial valley-train along the course of the River North Esk. Following the formation of an icedivide to the south of the basin, glaciofluvial material was carried into the basin from the south-west and deposited in contact with a northward-retreating ice front in the central and northern parts of the basin. From mapping and measuring the heights of the fragments of the dissected valley-train, twelve stages of downcutting have been recognized, the first eleven related to northward glacial retreat and the last to post-glacial sea-level changes. Each glacial retreat stage is marked by a stream profile terminated by an ice-contact front; the heights of the terminal features correspond to the intake heights of a set of glacial meltwater channels on the eastern edge of the basin. The northward glacial retreat was interrupted by a minor readvance of ice from the north. This is indicated by the Roslin Till, a thin deposit of limited areal extent. The paper includes a detailed description of the field measurements undertaken and the height-distance method of representing glacial and post-glacial stream profiles; the errors inherent in the methods are discussed. IT HAS long been realized that east-central Scotland is a critical area for the study of glacial ice movements, as it has been affected by ice sheets emanating from both the Highlands and the Southern Uplands. This paper has developed out of a study of ice movements and of the landforms associated with glacial and deglacial phases of ice-sheet coverage. In Midlothian along the River North Esk, there is a belt of glaciofluvial sands and gravels about I6 km long, extending from south of Penicuik to near the coast north of Dalkeith. The dissected glaciofluvial surfaces are partly overlain by the most recent till of this district, the Roslin Till, but are nevertheless so well preserved that the writer was induced to attempt to measure the surfaces and relate them to the general glacial stratigraphy. The height-distance method of representing the glacial and post-glacial stream profiles is described in detail, as the errors inherent in this method are seldom explicitly stated. The account also offers an explanation for the presence of the Roslin Till that is contrary to earlier explanations. Description of the Esk Basin The Esk basin occupies about 300 km2 on the south side of the Firth of Forth (Fig. I). The western edge is marked by the Pentland Hills which extend north-east to south-west for about 30 km. The maximum elevation of these hills is just over 575 m, but there are more than I5o km2 over 300 m in height, the sandstones, tuffs, and lavas having been eroded to form domes and broad-shouldered spurs. On the eastern side, the Roman Camp ridge separates the basin from the upper Tyne valley. This ridge is a small anticline of Carboniferous rocks extending for about 13 km north-north-east to south-south-west and dropping in height from about 250 m at the southern end to about 80 m near the coast at Prestonpans. The synclinal Esk basin itself has a gently undulating surface with a steady gradient