Ground investigations for the A303 Stonehenge Tunnels revealed a unique and complex Chalk geology including the presence of the thickest (>20m thick), and previously unknown phosphatic chalks in England, partly filling fault controlled erosional channels. The use of natural gamma-ray borehole logs to determine the presence and thickness of the phosphatic deposits is of particular value and combined with the lithostratigraphy, macrofossil and nannofossil biostratigraphy from cores has, for the first time, accurately constrained the Coniacian to Santonian age and the lenticular geometry of such deposits. Four phosphatic chalk events between 88.5-86.5 Ma are recognised associated with synsedimentary faulting. We suggest a causal link between tectonics, subsidence and channel-formation, phosphatisation events, pulses of oceanic upwelling on a frequency of about 0.5 million years to mantle-controlled plate tectonic episodes. The implications of this geology for construction of the A303 and the archaeology of the area are discussed. Key stratigraphical control Borehole R11 nannofossil analyses and age of the phosphatic chalks Borehole R11, because it contained the most horizons of phosphatic chalks, separated by 'normal' chalks, was chosen as the standard for further detailed stratigraphical analyses using