Sills -
These are horizontal intrusions of magma along the lines of beeding planes. Sills have vertical cooling cracks. Where erosion of valley sides has exposed sills, they often form cliffs and escarpments. Examples include the Great Whin Sill (which carries part of Hadrian's Wall) and Drumadoon on the Isle of Arran. Both sills and dykes are commonly made up of dolerite.
Dykes -
These are vertical intrusions with horizontal colloing crakcs. They cut across the bedding planes of the rocks into which they have been intruded. Dykes often occur in groups where they are knwon as dyke swarms. Many Scottish Islands, such as Mull and Skye have clusters of dykes all associated with one instrusive event.
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