In a deformed polymictic conglomerate from the Hill End area, New South Wales, Australia, almost all the strain is confined to fine-grained, quartz-rich rock fragments. Coarse quartz and feldspar clasts generally show little evidence of intracrystalline plasticity. The most strongly elongated rock fragments are finer grained and richer in layer silicates relative to quartz and feldspar, indicating that both grain size and composition have influenced finite strain, and thus strain rate. The relationship between grain size and strain rate is broadly consistent with a dominance of grain-boundary deformation mechanisms over intracrystalline dislocation mechanisms at finer grain-sizes. However, strong quartz c-axis preferred orientations have been measured in the most highly strained clasts, implying a significant component of dislocation glide. Close examination of the microfabric, mineralogy and petrology of the fine-grained clasts shows that other factors are likely to have influenced their mechanical behaviour and led to enhancement of strain rate. These factors include the trace-element chemistry of the phases (especially quartz), the role of a mobile fluid phase, the presence of poorly-bonded mica (001) interfaces and the evolutionary nature of microfabric and mineralogy, all of which are relevant to the bulk of crustal metamorphic rocks and should be taken into account in developing mechanical models of metamorphic belts.