Marl

How does it form?

Marl forms in marine or freshwater environments, often through the activity of algae. In shallow lakes with high pH and alkalinity, the stems and fruiting bodies of the macroalga Chara (stonewort) become calcified. When the alga dies, these calcified parts break down into fine carbonate particles that mingle with silt and clay to produce marl. In the marl ponds of the north-eastern United States, often kettle ponds on limestone bedrock, the skeletons of freshwater molluscs such as Sphaerium and Planorbis also accumulate in the bottom marl.

Marl lakes are found mostly in areas of recent glaciation. In the sea, alternating beds of chalk and marl are common in the Cretaceous rocks of north-western Europe. Glauconitic marl, which contains the green clay mineral glauconite, indicates deposition in marine conditions.

What is it used for?

• Agriculture: Used to correct acid soils and improve soil structure. Its more widespread use from the 16th century on contributed to the early modern agricultural revolution. Early in the 19th century it was gradually replaced by lime and imported fertilisers.
• Cement: Used as a raw material in Portland cement. As a supplementary cementitious material it gives better physical and mechanical properties than metakaolin and can be calcined at a considerably lower temperature.
• Engineering: Some very low-permeability marl beds were used for the Channel Tunnel and are being investigated for nuclear waste storage, for example at Wellenberg in Switzerland. Marl soils, however, behave poorly when alternately wetted and dried; they can be stabilised by adding pozzolan (volcanic ash).
• Aquaculture: In Pamlico Sound, USA, marl has been used to provide an artificial reef-like substrate for oysters.

Where is it found?

The lower part of the cliffs of Dover consists of glauconitic marls and rhythmically banded limestone and marl. In Hungary, the Upper Eocene Buda Marl behaves between rock and soil and is described as both “weak rock and strong soil”. Marl is the dominant rock type of the Vaca Muerta Formation in Argentina. Marl- and opal-rich beds in Spain’s Sorbas Basin have been correlated with Earth’s orbital cycles.