The final stage of Stonehenge was completed by 1 550 BC. The Bluestones had been reintroduced, a complete circle of Bluestones was placed between the outer sarsen circle and the inner sarsen horseshoe created by the five great Trilithons. Within this sarsen horseshoe yet another horseshoe, this time of nineteen Bluestones was added. In addition, the so called altar stone, a micaceous sandstone from Pembrokeshire, was placed inside the Bluestone horseshoe. The stone stood alone facing the sunrise at the heart of the sanctuary. Its name probably arose because, when one of the uprights and the lintel of the great trilithon toppled, they fell onto the micaceous sandstone and it now lies beneath them. Subsequent visitors who saw the micaceous sandstone prostrate on the ground assumed from their Christian heritage that they were looking at an altar. It was originally upright and not laying down as its present position suggests. More post-holes, known as the Y and Z holes, have been found, they were dug after the sarsen circle, and are outside it; their purpose is unknown.