and folding produced a particularly hard variety popularly used by the Romans in their mosaics. The chalk often contains larger fossils, including ammonites, belemnites, and brachiopods. Flint occurs throughout in both nodule and tabular layers. The iridium-rich layer of sediment, found around the world at the K-Pg boundary, is missing in Dorset. This may be due to the uplift and erosion that removed the upper layers of the chalk. The sediments resting on the Dorset chalk are, for the same reason, distinctly different from those in other parts of the world, and the remains of the plants and animals within are much more recent. Early Paleogene The expansion of the North Atlantic, which first occurred during the Triassic period, now reached a point where it was affecting the Dorset landscape. Areas that were previously under water were exposed to erosion as the chalk was folded and forced upwards. The early deposits from this period, found across South-East England, do not occur in Dorset. The small amounts found in Devon, however, coupled with the many solution holes found across the Dorset chalk, suggest that erosion may have removed them. Some of the larger solution holes, known as dolines, are found on the heaths between Dorchester and Bere Regis. Near Briantspuddle is Culpepper's Dish, 86 m wide and over 21 m deep. The first deposits found from this period in Dorset are the Reading Formation clays. A subdivision of the Lambeth Group, they are visible above the chalk at Studland. Mottled, brightly coloured, and generally lacking fossils, the clays are thought to be deposited beneath lagoons. In the east of the county, however, fossilised remains of gastropods and sharks' teeth have been found alongside flint pebbles, clearly identifying these clays as marine deposits. The Bracklesham Group, found across much of the Dorset portion of the Hampshire Basin, were mainly deposited by rivers, and form the foundations of the Dorset heathland. The frequent and heavy rains that occurred during this period transported large amounts of material from northern uplands across the Purbeck and Weymouth areas. A thick layer of chert gravel from the Purbeck strata can be found on Blackdown Hill near Portesham, but whether it was carried there or is the remains of an earlier deposit is unknown. Blocks of quartzite can be found across the downlands of southern Dorset, and these sarsens have been used in some of the local prehistoric