The whole of the Channel Tunnel route is within the Cretaceous layers that form the north slope of the Weald-Boulonnais Anticline. In France all three tunnels start within the flinty chalks of the Lower Senonian at the portal at Frethun. Proceeding generally down sequence the tunnels pass through Turonian chalks and into the clayey chalks of the Cenomanian, which then form the main tunnelling horizon right up to the UK portal. The UK portal at Castle Hill was constructed in a mixed face of the lowest Chalk Marl, Glauconitic Marl and topmost Gault Clay. The UK portal posed specific difficulties during construction as the tunnel passed through a major landslip at the base of Castle Hill. This was continually monitored by TML during construction while major remedial works were undertaken, including toe-weighting and drainage channels, in order to sufficiently stabilise it.