Cottenham lies on a long, thin geological formation that runs diagonally south westwards from Hunstanton on the Wash, thorough the Isle of Ely, Gamlingay, Sandy and Woburn, and on to the south coast. To the west are massive beds of older rocks - Oxford clays and Oolitic limestones which run all the way to Bath - laid down in the Jurassic period. To the east are the younger rocks - chalks of the Cretaceous period - which underly most of East Anglia.
The older rocks are exposed because in more recent times the landmass has tilted and then been eroded flat. This removed the younger rocks in the west, but left them in the east where the land had sunk. Imagine a sponge cake tilted then cut through horizontally; Cottenham lies on the exposed jam and cream filling.
This narrow band of rocks comprises Kimmeridge Clay to the north and west; this is less oily and with a lower iron content than the older Oxford Clay. East of this is a stratum known as the Lower Greensand, which forms much of the low ridge on which the village of Cottenham stands. Although elsewhere this Greensand does have a greenish tinge, just to be confusing here it is brown or even reddish (sometimes known locally as the 'red land'). In the south-east of the parish a stratum known as the Gault, which underlies much of Cambridge and Waterbeach, is exposed. You can see the boundaries between these areas on one of our maps [link to geology map].
These rocks were laid down in the late Jurassic and early Cretaceous periods, some 120-150 million years ago. This was the age of the dinosaurs, but at this time the area that was to become Cottenham was actually under water. The rocks were formed by silt washed down into estuaries and shallow seas, so what fossils they contain are usually of sea creatures (and some of these actually come from earlier rocks eroded away and re-deposited). This area was then substantially further south, in a much warmer climate, but continental drift has since forced it northwards.
On top of these bands of underlying rock lie later surface deposits; silts, gravels and boulder clays, spread across the area by rivers and glaciers. These cover much of the northern parts of the parish. The newest deposit is a layer of peat, formed relatively recently by fenland vegetation rotting down in water. Cottenham never had the great swathes of peat that are found further north in the fens and today only a few small areas in the very north of the parish survive.