Marlstone Rock

Early Jurassic (Pliensbachin-Toarcian)

Lithology
Sandy, shell-fragmental and ooidal ferruginous limestone interbedded with ferruginous calcareous sandstone, and generally subordinate ferruginous mudstone beds. Locally any of these lithologies may pass by increase in iron content into generally ooidal ironstone, and in places any of these may dominate. The iron content (as ooids, altered shell material or in the groundmass) is berthierine (dark green iron-rich layered silicate formed in low-oxygen marine conditions), altering to siderite. Fossil content variable throughout but locally abundant especially in limestone beds.

Upper Boundary
At upward change to mudstone/nodular limestones of the Whitby Mudstone Formation.

Lower Boundary
At downward change to mudstone/siltstone of the Dyrham Formation; base typically erosive and conglomeratic. Locally may rest on a sandstone bed at the top of the Dyrham Formation, which it may be impractical to map separately from the Marlstone Rock.

Representative Sites
Robinswood Hill

Information on lithology and boundaries taken from the British Geological Survey Lexicon of Named Rock Units