Abstract The Upper Cretaceous Chalk Group is renowned as a major aquifer, but the development of secondary porosity due to karstic conduits is poorly understood. Hydrogeological data and evidence from boreholes, sections, and tracer tests indicate that dissolutional conduits occur throughout the Chalk aquifer. Here, we assess the evidence for Chalk karst, and combine it with theoretical models of dissolution and cave formation to produce a conceptual model for the development of karstic conduits. Dissolution due to the mixing of saturated waters of contrasting chemistry along key lithostratigraphical inception horizons form extensive but isolated conduit networks. These form a significant proportion of the secondary porosity and enhance permeability. They prime the aquifer for the development of more integrated conduit networks formed by focused recharge of unsaturated surface derived water. However, the porous, well-fractured nature of the Chalk means that the time needed to form large integrated cave systems is often longer than timescales of landscape change. Continued landscape evolution and water table lowering halts conduit development before they can enlarge into cave systems except where geological and geomorphological settings are favourable. Groundwater models need to consider the formation of secondary karst permeability as this has a major influence on groundwater flow.

Abstract The dissolution of limestone and chalk (soluble carbonates) through geological time can lead to the creation of naturally formed cavities in the rock. The cavities can be air, water, rock or soil infilled and can occur at shallow levels within the carbonate rock surface or at deeper levels below. Depending upon the geological sequence, as the cavities break down and become unstable they can cause overlying rock strata to settle and tilt and also collapse of non-cemented strata and superficial deposits as voids migrate upwards to the surface. Natural cavities can be present in a stable or potentially unstable condition. The latter may be disturbed and triggered to cause ground instability by the action of percolating water, loading or vibration. The outcrops of various limestones and chalk occur widely across the UK, posing a significant subsidence hazard to existing and new land development and people. In addition to subsidence they can also create a variety of other problems such as slope instability, generate pathways for pollutants and soil gas to travel along and impact all manner of engineering works. Knowledge of natural cavities is essential for planning, development control and the construction of safe development.

Key geoarchaeological factors are explored with reference to the formation and potential preservation and/or loss of early Paleolithic artifact assemblages within Pleistocene fluvial deposits. The importance of these assemblages concerns the uniquely long-term perspectives that they offer to the study of early hominin occupation histories and landscape use. The factors explored (river type, bedrock type, the chronology and cycles of fluvial activity, and confluence activity) build upon the previous work of D.R. Bridgland, and A.J. Howard and M.G. Macklin with regard to terrace formation, preservation, and other fluvial activity. Particular emphasis is placed upon short-term fluvial activity (the context for assemblage formation) and the long-term potential for preservation and erosion of fluvial terraces and their archaeological contents. Case study examples are presented for the Solent River and the River Axe on the British south coast, exploring geoarchaeological issues within the context of understanding assemblage taphonomy and hominin behavior at local and regional scales. The paper concludes by assessing the potential and limitations of the approaches outlined.