The answer is - fjord/fiord coastline.
The type of coasts are formed by he submergence of U-shaped glacial troughs. These coasts are characterized by deep waters, apart from the mouth of the fjord when it's usually more shallow, steep high walls that often rise straight above the water, and tributary branches joining the main inlet at right angles.
This type of coasts are only found where there's fjords/fiords.
