What are sills and how do they from?

Sills are flat, horizontal layers of igneous rock that form when magma intrudes between existing rock layers and solidifies. This process occurs when magma rises through cracks in the Earth's crust and is forced into horizontal fissures, cooling and crystallizing over time. Sills typically create a distinctive banding pattern in the surrounding rock and can vary in thickness. They are often found in areas with significant tectonic activity where magma movement is prevalent.