 The Hythe beds consist of the Upper Hythe bed, which has a four substrata identified by the letters "A" to "D", and the Lower Hythe bed, which has two strata which are identified by the letters "A" and "B".Originally that whole area was covered by the Bargate Beds, but when the Weald was uplifted, the Bargate Beds were eroded away.Most of the tunnel passes through the Upper Hythe C and D layers and the Lower Hythe A layer, which are described as "Weak, locally very weak to moderately strong, slightly clayey fine-to-medium sandstone with occasional thin beds of clayey/silty fine sand" and typically has Uniaxial Compressive Strength (UCS) values of 2–5 MPa.