This paper presents the results of new research on two sarsen stones, known as the Cuckoo Stone and Tor Stone, both former standing stones that lie on opposite banks of the River Avon and straddle the eastern border of the Stonehenge and Avebury World Heritage Site. Geochemical analysis indicates that both stones were probably transported to their present site from West Woods on the Marlborough Downs in north Wiltshire, a source that likely also supplied the large sarsen monoliths at Stonehenge. The paper examines the geological conditions necessary for the formation of sarsen across the site of the present-day Salisbury Plain to address the apparent absence of natural sarsen in the area. The results are integrated with those of archaeological fieldwork from nearby contemporaneous sites to suggest that the Cuckoo Stone and Tor Stone were probably introduced into the Stonehenge landscape in the early part of the Late Neolithic period, ie, contemporary with Phase 1 of Stonehenge and some 400–500 years before the construction of the principal sarsen settings at the monument. Visibility analysis indicates that the two stones were probably intervisible and likely to have formed part of a planned landscape and were positioned to create a formal portal to the Stonehenge area on either bank of the River Avon.