Abstract
Summary: Pronounced time-related trends are evident in the clay mineralogy of the Speeton Clay (Lower Cretaceous): the kaolinite/illite ratio decreases consistently as the sediments get younger, and the other main mineral, probably an illitesmectite, becomes progressively more illitic. It is suggested that these changes reflect weathering conditions at the source rather than differences in actual source-rock type or weathering of the Speeton Clay itself. This conclusion is supported by the relative consistency in trace element contents, which might be expected to reflect major changes in source material.