Moving further round the path, the visitor comes to a huge unworked sarsen, known as the Heel Stone, that stands close to the fence at the edge of the A344 road.

Today the Heel Stone stands in isolation, just outside the main entrance to the earthwork enclosure, surrounded by a small circular ditch. But it originally had a companion stone, the hole for which was discovered in the roadside verge in 1980. There are hints that the Hell Stone may have been found lying close to where it stands and not transported from the Marlborough Downs. From the Heel Stone the twin parallel banks and ditches of the Avenue run out across the downs, although today they are unfortunately cut off from Stonehenge by the road.

The route of the Avenue, which I interpreted as a ceremonial approach way to Stonehenge, starts on the bank of the river Avon at West Amesbury, over 2,5 km. (1,5 miles) away to the south-east. From here it curves up across the A303 road before crossing the tree- and barrow-covered ridge to the east and running down into a shallow valley. Here it turns and the final section runs straight for a distance of over 500 m. (550 yds) up to the entrance to the enclosure.

This final section of the Avenue is just visible in the field on the opposite side of the road, particularly in low sunlight. The Avenue was probably constructed at the same time as, or just after the great stone structures were completed just before 2500 BC.

Turning away from the Heel Stone back towards the centre of Stonehenge, visitors can see another stone lying in the entrance to the enclosure. This is known as the Slaughter Stone, its gruesome name a product of overactive Victorian imagination.

The Slaughter Stone originally stood upright and, like the Heel Stone, was flanked by additional stones that are now missing. The surviving stone now lies horizontal, and shallow depressions on its surface collect rain water which reacts with iron in the stone and turns a rusty red. This was thought to be evidence of sacrifice – a relic of ancient blood spilt on a stone altar – hence the stone’s lurid but highly inaccurate name.

Lichens

Even from a distance, visitors to Stonehenge can see that the surfaces of the stones do not have a uniform appearance. They are mottled with a wide variety of colours, created by different species of lichen covering virtually every exposed surface.

Every lichen consists of a fungus and a green alga (or a cynobacterium), living together in a mutually beneficial – or symbiotic- association. The algal partner contains chlorophyll and, like plants, has the ability to convert the sun’s energy into sugar. The fungal partner constitutes the body of the lichen, protecting the alga from the harsh environments in which lichens live. Most lichens grow very slowly, increasing by between 0,5 and 5 mm. (0,02 and 0,2 in.) a year, depending on the species.

Lichens can be found in every climatic zone throughout the world, from arctic tundra to tropical rainforests, though each species is adapted to a specific type of environment.

A lichen survey at Stonehenge in 2003 found that there were 77 different species growing on the stones, several of which are nationally rare or scarce. Although it is hard to date lichens, as new growth is constantly replacing old, it will have taken hundreds of years for this range of species to become established on the stones. The lichen types at Stonehenge are broadly similar to those at the nearby stone circle at Avebury, but with some interesting exceptions.

Buellia saxorum, a type of lichen that specializes in colonizing sarsen stones and which is widespread at Avebury, is totally absent from Stonehenge for no apparent reason. Equally surprising, many of the lichen species found at Stonehenge usually grow only on exposed coastlines. It is possible that the prevailing winds at Stonehenge, blowing in from the Atlantic, may have encouraged these species to grow, but again, specialists have not been able to find a convincing explanation. So not all the mysteries of Stonehenge are archaeological.