Complex sentence: St Michael's Mount is one of forty-three (unbridged) tidal islands which can be walked to from mainland Britain and part of the island is designated as a Site of Special Scientific Interest in 1995 for its geology.
Split into simpler sentences: St Michael's Mount is one of forty-three (unbridged) tidal islands that one can walk to from mainland Britain. Part of the island was designated as a Site of Special Scientific Interest in 1995 for its geology.