Lennoxtown Essexite erratics train, central Scotland

Abstract

This thesis is an investigation of glacial erratics derived from two small, adjacent, distinctive outcrops of essexlte near Lennoxtown, Central Scotland. Analysis of till samples has revealed that rock fragments and sand-size particles of essexite are absent up-ice of the essexite outcrops and scarce in a down-ice direction. Using dry stone walls as random samples of glacially transported stones, classic erratics train patterns of the distribution of erratics from both outcrops have been derived. The close spacing of Joints on one outcrop has led to the rapid disappearance of its erratics in the walls with distance from the source due to glacial crushing. Crushing is also shown by a size reduction of erratics from the other outcrop in the walls in a down-ice direction. Measurement of size and morphometric properties of essexite erratics has revealed that crushing is the dominant and abrasion a subsidiary process in glacial transport, whilst abrasion is dominant for particles that have undergone fluvioglacial and beach transport. Roundness appears to be the best measure differentiating between processes acting in the glacial, fluvioglacial and beach environments. Tests of compressive strength and experimental abrasion of essexite have been carried out to aid interpretation of the results. Tlll-particle preferred orientations, striae and the long axes of ice-moulded features snow a similar direction to that of the major axis of the train. From the evidence available it is suggested that the essexite wall stones underwent englacial transport in the lower layers of the last active ice and were deposited as an ablation mantle. A position of transport at the base of the ice is invoked for essexite particles in the till. The lateral spreading of essexite erratics down-ice of the source is attributed to divergence of basal ice flow around subglacial obstacles.