Complex sentence: Situated around the perimeter of the mound is a circle of standing stones, which most archaeologists regard as having been added later, during the Bronze Age, centuries after the original monument had been abandoned as a tomb.
Split into simpler sentences: Situated around the perimeter of the mound is a circle of standing stones. Most archaeologists say they would have been added later, during the Bronze Age, centuries after the original monument had been abandoned as a tomb.