The Lower Estuarine Series is an obsolete term for a thin sequence of fine-grained clastic sediments of Jurassic age, which are now called the Grantham Formation.
The rocks of this formation form a lower part of the (Bajocian) Inferior Oolite Group, of Middle Jurassic age.
In central England, it normally lies above the Northampton sand and below the Lower Lincolnshire Limestone Member.
[1] The group is found in the East Midlands of England and was formed when the London-Brabant Island was drifting through the low northern latitudes, in conditions represented today by the Sahara Desert.
The formation is often only around 2 - 5 metres thick.