The lower stratigraphic units of the chalk cliffs of Dover consist of a sequence of glauconitic marls followed by rhythmically banded limestone and marl layers. For most of its length the tunnel bores through a chalk marl stratum (layer) Geological profile along the tunnel as constructed. Glauconite is characteristic of sediments deposited in marine conditions. Glauconitic marl is marl containing pellets of glauconite, a clay mineral that gives the mark a green color. The dominant carbonate mineral in most marls is calcite, but other carbonate minerals such as aragonite or dolomite may be present. It has a blocky subconchoidal fracture, and is less fissile than shale. This is more correctly described as an earthy or impure argillaceous limestone. Marlstone is an indurated (resists crumbling or powdering) rock of about the same composition as marl. The term is today often used to describe indurated marine deposits and lacustrine (lake) sediments which more accurately should be named 'marlstone'. These typically contain 35–65% clay and 65–35% carbonate. The term was originally loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under freshwater conditions.

Marl or marlstone is a carbonate-rich mud or mudstone which contains variable amounts of clays and silt. Scheme of the transitional lithotypes from mud (or mudstone) to lime (or limestone), illustrating the definition of marl (marlstone) as a mix of calcium carbonate and clay

April markly