Complex sentence: The Sierra del Escudo de Cabuérniga runs parallel to the coast, separated 15--20 km from it, which is why it has traditionally been considered the border between "La Marina" (The Coast) and "La Montaña" (The Mountain).
Split into simpler sentences: The Sierra del Escudo de Cabuérniga runs parallel to the coast, separated 15--20 km from it. It has traditionally been considered the border between "La Marina" (The Coast) and "La Montaña" (The Mountain).