WEAR AND TEAR ON FLINT PEBBLES
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THE IMPLICATIONS OF “WEAR AND TEAR” ON FLINT PEBBLES IN SOME LOCAL PLEISTOCENE DEPOSITS HOWARD MOTTRAM Introduction We can find pebbles in Suffolk that are composed of quartz, quartzite, chalk, chert and a few other rock types but the vast majority are composed of flint. In the Norwich Crag, notably in its Westleton Beds, it is in fact difficult to find pebbles that aren’t composed of flint. It has often been said that the flint pebbles in the Norwich Crag are well rounded, a characteristic that has been used to indicate that the pebbles have been worn by lengthy and/or strong periods of attrition; but, is this quite right?
The creation of flint pebbles Flints in the Chalk would have been of variable size and shape but, invariably, they would have had curved surfaces. When the Chalk was subjected to weathering and erosion, the brunt of the attacks would have been taken up by the soft chalk matrix so releasing individual flints. Although flints are very strong, they are brittle and narrow cylindrical (rod shaped) types and those with projecting “horns” or “knobs” would have been most vulnerable to damage. Initial damage by processes such as shattering by freeze-thaw, collision with one another when falling out of a Chalk exposure, collisions in turbulent water (e.g. the highland headwaters of a river, meltwater under pressure), or shearing at the base of an ice sheet, would have resulted in fracturing that produced smaller flints with sharp edges, i.e. pebbles with angularity. In rivers If the now angular flint pebbles were deposited soon after initial damage, then they would have remained like this, largely angular. Should the flints have stayed in river systems and been rolled and bounced downstream on the bed of large rivers for sustained lengths of time, then subsequent damage would have chipped away at the angular edges of the flints and further wear would have been inflicted by the slow process of abrasion. Abrasion would have been caused by rubbing against bedrock, sand and other pebbles and it would have affected the entire surface of moveable pebbles so reducing their size. It has long been held that abrasion not only makes pebbles smaller but that it can also transform angular pebbles into rounded ones (see for example, Domokos et al., 2014). There are several factors involved in the effectiveness of abrasion so that this pair of outcomes, size reduction and rounding, is not always evident (Sherman et al., 2013, p.243; Bridgland, pers. comm.). The flints in the Kesgrave Sands & Gravels were obtained from the Chalk of the Chiltern and North Downs areas and were, therefore, transported long distances but often over relatively soft and poorly abrasive bedrocks such as the Chalk itself and the London Clay. The flints in the Kesgrave Sands & Gravels do show an increase in rounding, albeit that it is more strictly a slight reduction in angularity (see also Bridgland, 1999, p.108). Conversely, the flints in the Ingham Sands & Gravels were obtained from the Chalk in the Ingham area but were soon deposited so that they had little opportunity of being rolled into rounder pebbles (Bridgland, 1999, p.109) and they remained angular, as indicated in Table 1. Trans. Suffolk Nat. Soc. 57 (2021)