CHALK FORMATION and CRETACEOUS GROUP, in Geology, consist of the upper strata of the Secondary series immediately below the Tertiary. aeries and superineumbent on the Wealden, or where that is wanting, on the Oolitic system. This group is common to Europe, and also to at least a part of Asia. It consists of chalk resting upon either an srenaeeors or argillaceous deposit. The Chalk bears a remarkably uniform mineralogical character over a surface extending from the British Islands through Norther France, Northern Gerninny, Denmark, and Sweden, into both European and Asiatic Russia (De La Beebe, ' Rea. in Theor. GcoL') The Cretaceous system is subdivided into the following strata : Upper. 1. 31cestrieht beds . . Lower. 2. Chalk with flints . .
Chalk . . Marty. 3. Chalk without flints, In which numerous species of and Chalk with marl / marine fossils are found.
Upper Greenland Greenland . Gault Lower Greensanq (Nrocotnien) The area over which this system prevails in England, and the various strata which it contains, have been well defined by geologists.
The Cretaceous Group occupies nearly the whole of the south eastern part of England. A lino drawn from Crewkerne on the Routh west to Ihnistanton on the north-east forma its western boundary ; and it extends from this line to the east cored, with the exception of a small part on the south-east. it is also found on the north of the Wash forming the Welds of Lincolnshire. It is intersected by the Ilumber and constitutes also the Wolds of Yorkshire. It termiuntes near Flamborough Head at Speeton, about six miles north of that promontory. The same formation appears to prevail in the north of France, extending to the Cretaceous district of Aix-la-Chapelle. The Chalk, or upper portion of the system, is in some parts covered with the strata of the Tertiary series ; in other pints it is denuded. The Greensand crops out round its edge, which in broken and interrupted in many places where the lower strata appear.
On the north-east the Chalk appears in a narrower belt along the cliff from near Cromer to Hunatanton. From Cromer along the coast to the mouth of the Thames, and along the north bank of that river, it is con cealed by the upper strata, which extend a considerable distance inland.
Beginning with Norfolk and proceeding southwards, the boundary line between the Chalk and the superior strata is about 4 miles east of Snettisham, 2 miles west of Dias, 8 miles cast of Mildenhall, between 4 and 5 miles west of Bury St. Edmunds, continuing to the north-east of Hertfordshire by Hertford and near St. Alban's, to 3 miles south-east of Wendover.
In the northern part of this district the Chalk has in a few places been partially denuded. On the uorth-east there is a small piece of Chalk along the banks of the Bare. Tho town of Norwich also stands urn Chalk, which, in its vicinity, extends along the banks of the are and Wensum. There is a piece of Chalk likewise at each side of small parts of the Stour and of the Orwell. The strata super incumbent on the Chalk series extend a little beyond Uxbridge on the west, and then run in a narrow tongue to the north-west, 3 miles south of Wendover, which intervening space at that point is the whole width of the denuded Chalk, Wendover being there its western limit. The Chalk becomes wider towards the south-east, taking in Maidenhead ; it is again partially covered a little to the west of that place, and continues in a very uneven line to Reading, Newbury, and a little to the cast of Marlborough. Tho Tertiary strata then run
from west to east by Kingsclere, Basingstoke, Guildford, to 3 miles north of Dorking, round by the Darent, and appear near Woolwich and Greenwich. Another detached portion appears in the north-west of Kent, taking in the Isle of Sheppey, and continues from the 'estuary of the Medway along the coast to the Isle of Thanet. The Chalk is covered on the west and north of Canterbury and at Sandwich ; the beds which cover it terminate on the coast at Deal. On the south coast, from Worthing westward, the Chalk is again concealed by the superincumbent serum, extending nearly in a straight line through Chichester as far as a little to the cast of Salisbury; it then continues in a south-west direction nearly as fax as Dorchester. Tho Chalk passes entirely across the Isle of Wight nearly from cast to west, in a narrow ridge consisting of vertical strata, from Culver Cliff to Compton Bay ; there is also a small piece on the south of the island. The limits of the Chalk on the other side, where it is bounded by the outcropping of the lower strata of the system, remain to be noticed. The cliffs of the whole of the Isle of Thanet are composed of Chalk. The chalk-cliffs again commence near Deal, and are continued past Dover to East Weare Bay, a distance of about 13 miles. As the Chalk rises from Dover towards Folketone the upper beds disappear, and the cliffs consist entirely of the lower members of that stratum. Tho rise of the Marly Chalk occurs about 14 mile to the east of the escarpment of Folkatone Hill, which is 566 feet high. Just at the rise of this bed there is a very copious and perennial spring called Lydden Spout. About 2000 paces west of this spring the cliff recedes from the sea, and the intermediate shore thence to Copt Point is occupied by a mass of ruins which has fallen from above. The sudden transition from the chalk-cliffs is very remarkable. Turning inland the chalk-range is bounded ou the south by the outcrop of the Green sand strata, which extend 4 nines north of Ashford, 3 miles north of Maidstone, and is cut by the Medway, whence it forms a line curving outward to the south as far as the Darent, where it is 5 miles north of Sevenoalca. Mersthem is on the boundary, which then continues in nearly a west direction, immediately north of Reigate to Box 11'11, near Dorking, and thence to Guildford, leaving a very narrow ridge of Chalk. From Guildford to a point about 2 miles from Farnham there is a remarkable ridge, called the Hog's Back, produced by an upthrow of the Chalk and the breaking off of the southern portion of the curve. The coast between Copt Point and Beachy Head, near these respective points, is occupied on each side by the Greensand, and the inter mediate apace by the Wcalden Clay and Hastings Sand : the two latter do not belong to this aeries, being lower denuded strata. The Sussex chalk-range, or South Downs, commences at Eastbourne, near Beachy Head, and continues thence along the coast beyond Shoreham and onwards in a west-by-north direction. The Greensand bounds it on the north, and leaves a range of Chalk varying from 8 to 3 miles in breadth. About 3 miles south-west of Peterafield the Greensand again sinks below the Chalk, which is connected with the north range by the Alton chalk-hills, running from near Farnham to near Baser Hill (917 feet).