Complex sentence: Sanday, so called because of its sandy beaches ("sand island"), is thought to have been mostly underwater at some periods of prehistory and is thought at one time to have consisted of several smaller islands which joined together when the sea level decreased.
Split into simpler sentences: Sanday, so called because of its sandy beaches ("sand island"), is thought to have been mostly underwater at some periods of prehistory. Archaeological evidence suggests that it at one time consisted of several smaller islands which joined together when the sea level decreased.