about in thickness. The sparse fossils found in this sequence include the brachiopod Terebratulina and the echinoid Conulus. The former Upper Chalk by comparison is softer than the underlying sequence and the flint nodules it contains are far more abundant in the South of England, although in Yorkshire the underlying strata have the highest concentration of flints. It may contain ammonite and gastropod fossils in some nodular layers. The thickness of this sequence varies greatly, often averaging around . Fossils may be abundant and include the bivalve Spondylus, the brachiopods Terebratulina and Gibbithyris, the echinoids Sternotaxis, Micraster, Echinocorys, and Tylocidaris, the crinoid Marsupites, and the small sponge Porosphaera. A possible azhdarchoid pterosaur is known from Coniacian-aged rocks that form part of the Upper Chalk, making it the youngest known pterosaur discovered to date in England. The youngest beds of the sequence are found on the coast of Norfolk. Other fossils commonly found in this formation include: solitary corals (such as Parasmilia), marine worm tubes (such as Rotularia), bryozoans, scattered fragments of starfish and fish remains (including shark teeth such as Cretolamna and Squalicorax). Chalk landscapes of England The Chalk outcrops across large parts of southern and eastern England and forms a significant number of the major physiographical features. Whilst it has been postulated that a chalk cover was laid down across just about all of England and Wales during Cretaceous times, subsequent uplift and erosion has resulted in it remaining only southeast of a line drawn roughly between The Wash and Lyme Bay in Dorset and eastwards from the scarps of the Lincolnshire and Yorkshire Wolds. Gentle folding of the Mesozoic rocks of this region during the Alpine orogeny has produced the London Basin and the Weald–Artois Anticline, the Hampshire Basin and the less gentle Purbeck-Wight monocline. The broadly western margin of the Chalk outcrop is marked, from northeast to southwest, to south by the Chalk downlands of the Yorkshire Wolds, the Lincolnshire Wolds, a subdued feature through western Norfolk, including Breckland, the Chiltern Hills, the Berkshire Downs, Marlborough Downs and the western margins of Salisbury Plain and Cranborne Chase and the North and South Dorset Downs. In parts of the Thames Basin and eastern East Anglia the Chalk is concealed by later deposits, as is the case too within the Hampshire Basin. Only where the Weald–Artois Anticline has been 'unroofed' by erosion i.e. within the Weald is the Chalk