Photo credit: Mactographer 2009, Wikimedia Commons
Stonehenge is an oval shaped prehistoric monument that was built in the Neolithic period about 2500 BC . As the name suggests Stonehenge is made up of stones, but what are they made from, where did they come from and how do we know?
Two rock types can be found making up the Stonehenge monument, sedimentary and meta-igneous (metamorphosed igneous) rocks. We will look at both of these rock-types in detail in this article.
The sedimentary rocks are a type of sandstone called sarsen rock. This type of sandstone is not found within the Stonehenge site itself which is underlain by Cretaceous Chalk . Petrological studies have shown that the sarsen rock comes from the Marlborough Downs, 30km away . This sandstone was deposited ‘ over much of southern England at the beginning of the Tertiary period ~60 million years ago ’ . The climate in Britain during the early Tertiary was very warm, so the original sand that makes up the sarsen rock was deposited in a warm environment . The presence of root hollows within the sarsen rock suggests that the sand was deposited in moist conditions where plants were able to grow and have access to water . It has been suggested by the Wiltshire Geology Group that the sands were deposited ‘ around the level of the water table ’ and that the removal of water due to evaporation and use by plants meant that silica could become concentrated within the fluid between the sediments and begin to form a cement by when it precipitated in-between the sand grains of the original sediment.
The meta-igneous rocks found at Stonehenge consist of spotted dolerite, also known as bluestone. This rock has been found to have originated from the Carn Menyn Mountain within the Presli Hills, Pembrokeshire . The site where these rocks were excavated is located roughly 386km away from the Stonehenge monument .
These rocks originally formed when dolerite magma intruded into the crust in the Presli Hills area during the Ordovician  around 488-443 million years ago . These igneous rocks were later affected by low grade metamorphism which turned them into spotted dolerite by allowing the growth of large albite crystals .
Geological analysis and chemical tests were carried out on the bluestone allowing them to be matched to the same rocks found in the Presli Hills . The rock itself is also very distinctive and matches well with the bluestones exposed in Wales.
But, there is even more compelling evidence that the bluestones were transported all the way to Salisbury Plain from Wales. This evidence comes in the form of the ‘ Bowscombe Bowmen, ’ the skeletons of a family that were buried on the Bowscombe Downs . Analysis of the teeth of the Bowscombe Bowmen shows that they were all born in Wales in a radiogenic area found in the Presli Hills. It has been suggested by many archaeologists that the Bowscombe Bowmen helped to transport the bluestones on their 386km journey from the Presli Hills all the way to Salisbury Plain.
Some people think that the bluestones were transported closer to the site of Stonehenge by glaciers but there is very little evidence to support this idea . Although, ‘ two parallel ditches approximately 20-22m wide ’  run through Salisbury Plain - these could be periglacial stripes carved on the land by a glacier. However, comparing these stripes with the periglacial stripes found in the Arctic it is unlikely that the ditches at Stonehenge were produced by glaciers as they are too wide as periglacial stripes are usually only a few metres apart .
People originally thought that the two ditches in Salisbury Palin were evidence that the stones had been dragged across the land to Stonehenge, but this is not the case. It is likely that the stones were transported in rivers and ‘ water networks ’ and by ‘ hauling ’ them over land, possibly with the use of makeshift wooden mechanisms such as sledges ,. The stones may have been placed on Salisbury Plain due to the presence of the two parallel ditches, which may have been seen as something very important as the solstices of Stonehenge are aligned with these two ditches.
Photo credit: Wikimedia Commons 
First of all dolerite formed in the Ordovician 448-443 million years ago in an area which is now the Presli Hills in Permbrokeshire, Wales. These rocks were then subjected to low grade, greenschist, metamorphism, which formed spotted dolerite which is also known as bluestone.
During the early Tertiary (~60 million years ago) sands were deposited over much of southern England. These were patchily cemented by the evaporation of groundwater through the sediment, leading to the formation of the Sarsen rock.
In 2500 BC the Boscombe bowmen and other prehistoric people removed bluestones from the Presli Hills and transported them with the use of water, man power and rolling on logs and sledges , to a site on Salisbury plain ready for constructing the second phase of Stonehenge. Items found around the bluestones have a radiocarbon date of 2280-2030 years, indicating the date when the bluestones were erected .
In 2300 BC Sarsen rock was brought from the Marlborough Downs 30kms to Salisbury Plain for the third phase and completion of Stonehenge .