Pebbles and cobbles of Cretaceous carbonate rocks in Eocene and Neogene fluvial conglomerates and lag deposits derived from them have patterns of silicification that indicate most silicification occurred after deposition. This interpretation is supported by oxygen isotopic values of silica and by the absence of conchoidal fracture surfaces and percussion scars that are typical of many fluvially transported chert clasts. Silicification is manifested chiefly as thin rinds on well-rounded carbonate pebbles and cobbles, although some carbonate cobbles are internally silicified in diffusion patterns (Liesegang bands) that conform to present clast shape. Silica phases are microcrystalline quartz and chalcedony (locally with beekite texture) with minor lutecite and opal-CT. Chertification was affected by groundwater rich in silica, probably derived from the hydrolysis of volcanic ash in Neogene tuff beds. The likelihood that temperatures of groundwater were considerably higher than those found in marine pore waters, plus the likelihood that hydrological conditions could not have persisted for more than a few 10 6 years, suggests that silicification was much more rapid than for deep-sea cherts. Silicification of the clasts was similar to silcretization processes.