The bedrocks on which the Millennium Green stands are about 135 million years old. They are sedimentary rocks, and were deposited by a large river system flowing into a shallow brackish lake, at a time when dinosaurs were common in this area.
About 30 million years ago these bedrocks were uplifted from sea level to an altitude of about 300 metres by the same earth movements as formed the Alps. Since then the rocks have been eroded down to their present levels.
As a result of this erosion the soil and underlying formations now comprise what are known as Ashdown Beds. These are alternating layers (beds) of sandstones, siltstones and shale with thin ironstones that make up the land sloping down from the Millennium Green to the bottom of Town Row, and thence up through Spout Hill, Catts Hill and Yew Tree Lane.
Iron
The geological processes described above resulted in an extensive layer of sedimentary ironstone which was the basis of the Wealden iron industry. The primitive bloomery process of the Iron Age continued into the Romano British period up till around 1800 years ago. Industrial scale iron smelting returned to the Weald in the 16th and 17th century using water wheel powered blast furnaces. Iron smelting ended when the local woods could no longer supply enough charcoal. There is no evidence of iron working on the Green, the nearest bloomery site being off Chant Lane, with the nearest blast furnace at Jarvis Brook where cannon were cast for nearly100 years .
Water
Rain falling on the Ashdown beds slowly percolates down through the porous sandstones until it is forced by the underlying impervious clay layer to travel sideways until it emerges on the surface as a spring.
The spring at the top of Springfield, captured as a well, was an important source of drinking water for Rotherfield before the arrival of mains water in the late 1890s.