The characteristics of Lower Palaeozoic marine sediments, particularly those in Wales, are described. It is believed that the study of these may throw light on the mode of origin of more recent sediments. The Lower Palaeozoic sediments often show alternating bands of sand and fine material. It has been suggested that the coarse material has been transported by turbidity currents, or, alternatively, that tsunamis associated with earthquakes are responsible. Structures produced by slumping and sliding are common.