Many rock formations make up the varied countryside of Dorset. It is marked by a number of limestone ridges particularly the band of Cretaceous chalk forming the Dorset Downs which run North-East from Maiden Castle to join with Cranborne Chase.
The village lies on the edge of the Isle of Purbeck on a smaller chalk ridge known as the Purbeck Hills where Portland and Purbeck Limestone have long been quarried near Worth Matravers. At the coast around Lulworth Cove the compact series of east-west running strata are visible, including: Portland Stone, Lower Purbeck, Middle Purbeck, Portland Sand, Wealden Beds and Upper Greensand. To the North of the village the river Frome runs through a wide flood plain of tertiary clays into the estuary that becomes Poole Harbour.
Coombe Keynes village lies on the clay of the Reading Beds and is underlain by an iron-rich sand which is often hard enough to use as building blocks. West of the village clay has in the past been dug for making bricks. Both the sand and the clay are part of the West Park Farm Member of the London Clay, about 60 million years old and deposited as sediments on a shoreline that was building out seawards. Therefore some of the clay and sands are marine, and contain marine fossils, but others have been left high and dry to weather into soils.
To the west, the older chalk forms the landscape, while to the east are younger sediments of the London Clay. The gravel present to the south of the village, and sitting on top of the chalk, is the clay with flints which results from millions of years of weathering of the chalk. The only substance hard enough to remain solid is the flint that has been broken down into gravel.
The church and other buildings have used some of the dark brown sandstone from the village itself, as well as the local bricks. The church, in particular, has also been built of Purbeck limestone from Poxwell, where the quarries are in the oldest limestones suitable for building, known as the Cypris Freestones. ‘Cypris’ is the latin name for a small water-beetle, present in huge numbers in these limestones. Old buildings using this stone have a characteristic weathering pattern of horizontal hollow lines.