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sill  Igneous intrusions can be a variety of shapes and sizes. Laccoliths are domed circular shapes, and can be many miles across. Sills are intrusions that flow between rock layers. Pipes or necks connect the underlying magma chamber to surface volcanoes. | 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 dolerite, a rock that is extremely resistant to erosion and weathering, and often forms ridges in the landscape or cuts across rivers to create waterfalls. |
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