Collyweston Great Wood and Easton Hornstocks is the largest Northamptonshire remnant of the ancient Purlieu coppices of Rockingham Forest. The underlying rock is Jurassic Lower Lincolnshire Limestone overlain by Upper Estuarine ‘Series’ sands and Boulder Clay drift.
This has given rise to a range of soils of widely differing drainage and chemistry which in turn support an outstanding range of semi-natural woodland types. Little of the site has been affected by re-afforestation so that a complex mosaic of vegetation occurs, closely correlated with soil characteristics.