Stonehenge avenue and “Bluestonehenge”
Stonehenge avenue runs for almost 3km up to Stonehenge from West Amesbury henge, by the banks of the River Avon. Excavations in 2009 suggested that this location once held a circle of bluestones that were subsequently moved to Stonehenge, around the same time that the ditches of the avenue were dug.
The final stretch of the avenue leading up to Stonehenge is famously aligned upon winter solstice sunset and summer solstice sunrise, as is the axis of Stonehenge itself. An excavated section across the avenue in 2008 yielded a big surprise, however…
Between the avenue ditches were a number of deep natural fissures in the chalk running parallel to the avenue ditches, suggesting that the latter were simply an elaboration of the natural features and therefore (at first sight) that the solstitial orientation of the avenue is fortuitous. However, what is to us a coincidence of nature—the existence of natural ruts aligned upon winter solstice sunset—could actually have served to identify the top of those ruts as a place of huge sacred power and cosmic significance, and explain why it went on to become such an important focus for ritual activity, including the construction of Stonehenge itself. It is possible that this alignment was recognised long before the Neolithic.
The short (100m-long) avenue at Durrington Walls henge was also an elaboration of a natural feature—an accumulation of frost-shattered flint running along the bottom of a dry valley—and was also roughly solstitially oriented, in this case towards summer solstice sunset.
For more details see the full paper: Stonehenge's avenue and 'Bluestonehenge', by Michael J. Allen et al., published in Antiquity 90 (2016), 991–1008.