Abstract The Purbeck-Wealden of the type areas in onshore southern Britain (Fig. 1 ) encompasses the Berria-sian-earliest Aptian stages of the Cretaceous, a time span of approximately 21 Ma. At least some of the lowest part of the succession belongs to the latest Jurassic (Portlandian); how much depends on where the Jurassic-Cretaceous boundary is placed. Ostracods are frequently diverse and abundant in the predominantly calcareous or argillaceous units (e.g. Purbeck Limestone Group, Wadhurst Clay Formation, Grinstead Clay Formation, Weald Clay Group), sometimes forming ostracod limestones, but in clays that have undergone pedogenesis and in arenaceous facies (e.g. Ashdown Beds Formation, Upper and Lower Tunbridge Wells Sand formations) they tend to be rare or poorly preserved. They are essentially non-marine faunas, with only a few convincing indicators of direct marine influence. Although their interpretation is difficult and has given rise to controversy, ostracods are undoubtedly the most useful biostratigraphical tool available in Purbeck-Wealden sequences. They have also been used with some success in correlations of the offshore ‘ Purbeck-Wealden’ facies of Portlandian-Barremian age found in the Celtic Sea and Fastnet basins between southern Ireland and SW England (Fig. 1 ).

Abstract Among Wealden towns Tunbridge Wells is comparatively new. Before the Civil Wars of the 1640s there was no village here, nor any name on a map. Chance finding of chalybeate springs a few miles south of Tunbridge (now Tonbridge ) attracted attention at Court, and even gynaecological interest. Curiously, this provides explanations both for the supposed virtues of the waters and the founding of a summer resort. By repute, the springs were discovered in 1606, though this story was already 160 years old before it first appeared in print. Verifiable facts indicate that Thomas Neale, FRS, (1641–1699) was the main agent in organizing the nascent resort’s amenities, beginning in 1676 with plans to construct a chapel or assembly room. The springs themselves issue from Lower Cretaceous Wealden beds, a few feet above the Wadhurst Clay, in a shallow valley formed by the headwaters of the River Grom. Siderite (iron carbonate or chalybite) abounds in these formations.

Abstract There are three principal types of stratigraphy, each with its own terminology: • geochronolog —units of geological time (e.g. period, epoch), with a parallel and exactly corresponding stratigraphy; • chronostratigraphy —time-rock units or time-stratigraphic units represent stratified rock successions (e.g. system, series, etc.) assigned to geological time units; • geochronometry —the measurement of absolute time in years as numerical ages (e.g. 345 Ma), principally by means of radiometric dating, but increasingly, in Phanerozoic rocks, by the numerical dating of Milankovitch cycles. This has led to some confusion. Current recommendations, led by the Geological Society ( Zalasiewicz et al. . 2004 ) are to adopt the geochronology scheme, which has been used in this chapter. Part of the geological timescale, covering the last 545 million years of earth’s history (Phanerozoic—from the greek meaning ‘visible life’), is shown in Table 6.1 ( Gradstein & Ogg 1996 ) with the youngest at the top. This shows the broadest sub-divisions of era and period (the Neogene and Palaeogene are often referred to as the Tertiary, though this name may fall into disuse). The names for the eras derive from the greek meaning ‘ancient’, ‘middle’, and ‘new’ life. Periods further sub-divide into early, middle, and late (or in some cases simply early and late), and thence into epochs and ages (refer to Appendix C). Time before the Phanerozoic is usually referred to as the Pre-Cambrian.