The tract of country occupied by the Oxford Clay in the neighbourhood of Cambridge, is much obscured by drift, and the only places where the clay can be examined are in brick-pits, wells and railway-cuttings. These exposures are not very numerous, and on this account the boundaries of the Oxford Clay are hard to define. Its western limit is marked by the outcrop of the Lower Oolites of East Northamptonshire, and runs in an irregular line from Bedford northwards to Peterborough. Its eastern boundary is somewhere to the east of the course of the River Ouse from Bedford to Huntingdon : the clay-pits of Sandy, St Neots, and Godmanchester are in this formation, but no exposures exist to the east of these localities. It probably underlies the Elsworth Rock, near the village of Elsworth, though it is not now seen there. Leaving Godmanchester, the next exposure is in the brick-pits of St Ives, but I have not been able to detect it at the surface further north or east.

The thickness of the Oxford Clay is pretty considerable, but owing to the paucity of sections, it cannot be determined with any degree of accuracy. In the Survey Memoir the thickness is estimated at 700 feet, but this is too great, since most of the Ampthill Clay is included in the Oxford Clay. Near St Ives Railway Station, a well was dug in this clay to a depth of 150 feet, and another at Bluntisham to 300 feet, but in neither case was the clay pierced.