Stonehenge evolved in several construction phases spanning at least 1,500 years. There is evidence of large-scale construction on and around the monument that perhaps extends the landscape's time frame to 6,500 years. Dating and understanding the various phases of activity is complicated by disturbance of the natural chalk by periglacial effects and animal burrowing, poor quality earlyexcavation records, and a lack of accurate, scientifically verified dates. The modern phasing most generally agreed to by archaeologists is detailed below. Features mentioned in the text are numbered and shown on the plan, right.