one, to unify two politically separate regions, or to legitimise the ancestral identity of migrants moving from one region to another". Evidence of a 110-m stone circle at Waun Mawn near Preseli, which could have contained some or all of the stones in Stonehenge, has been found, including a hole from a rock that matches the unusual cross-section of a Stonehenge bluestone "like a key in a lock". Each monolith measures around in height, between wide and around thick. What was to become known as the Altar Stone is almost certainly derived from the Senni Beds, perhaps from east of the Preseli Hills in the Brecon Beacons. The north-eastern entrance was widened at this time, with the result that it precisely matched the direction of the midsummer sunrise and midwinter sunset of the period. This phase of the monument was abandoned unfinished, however; the small standing stones were apparently removed and the Q and R holes purposefully backfilled. The Heel Stone, a Tertiary sandstone, may also have been erected outside the north-eastern entrance during this period. It cannot be accurately dated and may have been installed at any time during phase 3. At first, it was accompanied by a second stone, which is no longer visible. Two, or possibly three, large portal stones were set up just inside the north-eastern entrance, of which only one, the fallen Slaughter Stone, long, now remains. Other features, loosely dated to phase 3, include the four Station Stones, two of which stood atop mounds. The mounds are known as "barrows" although they do not contain burials. Stonehenge Avenue, a parallel pair of ditches and banks leading to the River Avon, was also added. Stonehenge 3 II (2600 BC to 2400 BC) During the next major phase of activity, 30 enormous Oligocene–Miocene sarsen stones (shown grey on the plan) were brought to the site. They came from a quarry around north of Stonehenge, in West Woods, Wiltshire. The stones were dressed and fashioned with mortise and tenon joints before 30 sarsens were erected as a diameter circle of standing stones, with a ring of 30 lintel stones resting on top. The lintels were fitted to one another using tongue and groove joints – a woodworking method, again. Each standing stone was around high, wide and weighed around 25 tons. Each had clearly been worked with the final visual effect in mind: The orthostats widen slightly