1. A continuous horizontal timber forming the lowest member of a framework or supporting structure.
2. (Geology) A flat (usually horizontal) mass of igneous rock between two layers of older sedimentary rock.
Sheet of igneous rock created by the intrusion of magma (molten rock) between layers of pre-existing rock. (A dyke, by contrast, is formed when magma cuts across layers of rock.) An example of a sill in the UK is the Great Whin Sill, which forms the ridge along which Hadrian’s Wall was built.
A sill is usually formed of diabase, a rock that is extremely resistant to erosion and weathering, and often forms ridges in the landscape or cuts across rivers to create waterfalls.