Marl is a sedimentary rock consisting of a mixture of lime (calcium carbonate) and clay. Geologically, a rock is called marl at roughly one quarter to three quarters clay content; with more lime it becomes limestone, with more clay it becomes mudstone or claystone.
In the Dutch province of Limburg, “mergel” means something slightly different in everyday use: the soft, yellowish Maastricht limestone quarried there in underground galleries and used to build churches, castles and farmhouses. Strictly speaking that is limestone rather than marl — but the name is firmly established.
Properties
- Soft and light — it can be cut with a hand saw, and blocks can be handled without lifting gear
- Porous — it takes up water readily, which makes it vulnerable to frost and salt damage
- Breathable — vapour-open, so it should be combined with lime mortar rather than dense cement mortar
- Weathers visibly — weathered layers shed sand; historically the surface was periodically dressed back
Uses
- Marl blocks in facades and internal walls, mainly in the south of Limburg
- Banding courses and surrounds in combination with brickwork
- Carving and ornament, thanks to how easily it works
- Agriculture — lime-rich marl as a soil improver to reduce acidity
Points to watch
- Always lay and point marl with a lime mortar; hard cement joints are stronger than the stone itself and break the block edges away
- Keep marl masonry dry at the base: splash-back and rising damp are the main causes of decay
- Avoid dense, film-forming paints or sealers that block vapour — moisture then gets trapped behind the layer
- Provide good copings and overhangs so rain does not sit on the top of the wall
Related terms
- Limestone
- Natural stone
- Lime mortar
- Sedimentary rock
- Boulder clay
- Natural stone maintenance
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