Complex sentence: They built the long barrow at St Lythans around 6,000 BP, about 1,500 years before either Stonehenge or The Egyptian Great Pyramid of Giza was completed.
Split into simpler sentences: They built the long barrow at St Lythans around 6,000 years BP. This was about 1,500 years before either Stonehenge or The Egyptian Great Pyramid of Giza was completed.