Image analysis of `natural concrete samples by automated and manual procedures

Nature provide deposits with a structure and properties resembling those of man-made concrete. A tillite at Mt. Feather, McMurdo Dry Valleys, Transantarctic Mountains, Antarctica, is such a deposit. This ancient glacial deposit is of considerable importance to the early glacial history of the Antarctic Ice Sheet. The power of stereological analysis for an objective determination of the preferred direction of small clasts (<5 mm) was investigated in sediment samples from this deposit. Orthogonal sets of vertical thin sections were therefore subjected to a directed secants analysis. The operations were executed in an automated set-up and manually. The roses of intersections were analysed for preferred orientation, the results of which were combined to yield the tilt angle of the clasts. The preferred orientation signal is weak. As a result, in the automated set-up this signal is camouflaged by the effect of digitisation, hampering the assessment of a reliable estimate for the preferred orientation. The manual procedure, although basically more time-consuming, is more accurate and should be preferred, especially in the case of weak signal characterisation, as met in the present situation.