Shale in general is the most abundant of stratified rocks. One of the most popular forms of shale used for building roads is called Marcellus Shale. When this clay shale is nearly pure it is known as argillaceous. When it contains a lot of sand it is arenaceous. You will find that shale varies in color, reddish, very black, or grey. Shale is used mainly for top dressing on roads. To have a good road base slate is recommended.

Shale is a sedimentary rock that can be metamorphosed into slate by "increased pressure" and heating.

sedimentary

Slate is metamorphosed shale.

No. Shale oil is oil found in shale. Oil shale is shale that contains oil.