Neocomian and Aptian ages which occur in northern England. Before the deposition of the Albian beds a considerable upheaval of some parts of Britain occurred, and an unconformity separates the higher Cretaceous beds from older strata of Cretaceous and Jurassic ages, thus complicating the major phases by local changes in the characters of the strata. (ii) _The Albian and higher Cretaceous Beds._ The commencement of the deep-water phase of the third marine period may be said to occur in Albian times in Britain, reaching its maximum during the deposition of the chalk. The existence of a deeper sea towards the north of England is indicated by the characters of the Albian and newer strata. The Albian beds of gault consist of a stiff clay in southern England, replaced by coarser mechanical sediments towards the west. As one passes north from the London ridge (which exerted its influence in Albian times, after which it was finally buried in sediment) the gault thins out, and becomes gradually replaced by calcareous deposit when it is known as the red chalk which replaces the gault in northern Norfolk, Lincolnshire and Yorkshire. A local unconformity separating the chalk and gault in parts of East Anglia points to another local uplift with its accompanying complications in the characters of the strata. After the uplift had ceased, general depression must have occurred, and the various divisions of the chalk were accumulated in a fairly open sea, though, for reasons to be given presently, this was probably of no great lateral extent, save when united with the open ocean, probably in a manner similar to the connexion between the Gulf of Mexico and the Atlantic. The general variations in the lithological characters of the various members of the Cretaceous system will probably be rendered clearer by reference to the accompanying diagram (fig. 24) representing the variations when traced across England from south to north[101]. [Footnote 101: For information concerning the British Cretaceous beds, see Topley and Foster, "Geology of the Weald," _Mem. Geol. Survey_, 1875; Bristow and Strahan, "Geology of the Isle of Wight," _Mem. Geol. Survey_, 1889; Lamplugh, "On the Speeton Clay," _Q. J. G. S._, vol. XLV. p. 575, and "The Speeton Series in Yorkshire and Lincolnshire," _ibid._, vol. LII. p. 179; Barrois "Recherches sur le Terrain Crétacé supérieur de l'Angleterre et d'Irlande," Lille, 1876; and various papers by Messrs Hill and Jukes-Browne, in the _Quarterly Journal of the Geological Society_ and _Geological Magazine_ of recent years. For the Scotch deposits consult a paper by Prof. Judd, _Q. J. G. S._, vol. XXXIV. p. 736, and for those of Ireland, see Hume, _Q. J. G. S._, vol. LII. p. 540.] [Illustration: Fig. 24. Ch. Chalk. Al. Albian. Ap. Aptian. N. Neocomian. J. Jurassic. ]