the reduced content of calcium carbonate versus straight lime, expressed as the calcium carbonate equivalent. Because the carbonate in marl is predominantly calcium carbonate, magnesium deficiency may be seen in crops treated with marl if they are not also supplemented with magnesium. Marl has been used in Pamlico Sound to provide a suitable artificial substrate for oysters in a reef-like environment. Portland cement Marl has been used in the manufacture of Portland cement. It is abundant and yields better physical and mechanical properties than metakaolin as a supplementary cementitious material and can be calcined at a considerably lower temperature. Civil engineering The Channel Tunnel was constructed in the West Melbury Marly Chalk, a geological formation containing marl beds. This formation was chosen because of its very low permeability, absence of chert, and lack of fissures found in overlying formations. The underlying Glauconitic Marl is easily recognizable in core samples and helped establish the right level for excavating the tunnel. Marl soil has poor engineering properties, particularly when alternately wetted and dried. The soils can be stabilized by adding pozzolan (volcanic ash) to the soil. Nuclear waste storage Some marl beds have a very low permeability and are under consideration for use in the storage of nuclear waste. One such proposed storage site is the Wellenberg in central Switzerland. Marl lakes A marl lake is a lake whose bottom sediments include large deposits of marl. They are most often found in areas of recent glaciation and are characterized by alkaline water, rich in dissolved calcium carbonate, from which carbonate minerals are deposited. Marl lakes have frequently been dredged or mined for marl, often used for manufacturing Portland cement. However, they are regarded as ecologically important, and are vulnerable to damage by silting, nutrient pollution, drainage, and invasive species. In Britain, only the marl lakes of the more remote parts of northern Scotland are likely to remain pristine into the near future. See also References Citations Bibliography Further reading Schurrenberger, D., Russell, J. and Kerry Kelts. 2003. Classification of lacustrine sediments based on sedimentary components. Journal of Paleolimnology 29: 141–154. External links Chalk of Kent by C. S. Harris Palaeoenvironmental Interpretation of the Early Postglacial Sedimentary Record of a Marl Lake Limestone Sedimentary rocks