Stonehenge has not always been a circle of stones. In fact, the area on which Stonehenge is built was an important site long before the stones were erected. Most historians divide the construction into three stages, each marking a change in the great monument. The earliest artifacts at the site date from roughly 3500 BC, marking the beginning of the First Period of Stonehenge's development.
Very few stones were placed at Stonehenge during the First Period. A earthwork ditch was dug around the site, and there were wooden buildings placed around the center. The site probably served as some sort of trading post or religious community at this point, with protective ditches and a single entry point. The 56 Aubrey Holes (named after John Aubrey, who discovered them,) were dug at this point, and probably held wooden posts. Some experts claim that the four Station Stones were placed at this time as well (unfortunately, only two of these are still present at the site). The Station Stones form a perfect rectangle within the circle of the Aubrey Holes, and many astronomical alignments have been suggested for their positions. It has also been suggested that the Slaughter Stone and the Heel Stone were placed at this time, but that depends on which historian you ask. The Slaughter Stone is a gargantuan block just inside the ditch, which is fallen in modern times, but was standing when it was first placed. The most popular theory is that it was part of a gateway into the Circle, with a companion block that is now missing. The Heel Stone is one of the most recognizable stones at Stonehenge; it is the largest stone there, and it stands outside of the regular circle. The Slaughter Stone is 6.6 meters tall, 2.1 meters wide, and .8 meters thick, weighing 28 tons. The Heel Stone is 7.6 meters tall, 5.2 meters wide, and 2.4 meters thick, weighing 35 tons. Very little is known about Stonehenge at this stage in its development, and indeed it is unclear when this site first began to have importance to the prehistoric people who lived here.
The Second Period of construction at Stonehenge, roughly, stretched from 2700 BC to 2000 BC. This is when most of the stones were erected, and the most difficult engineering problems were dealt with.
It was during the Second Period that the Sarsen Circle was erected, as well as the grand horseshoe of Trilithons in the center of the Circle. The Sarsen Circle is the outer ring of the center part of Stonehenge, with seven-ton lintels hoisted nearly 14' in the air, forming a near-perfect circle. The sarsens (the pillars supporting the lintels) each weigh 25-30 tons, and the highest of them reach 5.5 meters high. The circle of sarsens is roughly 108 feet across, consisting of 30 standing stones (although only 17 stand today.) The five Trilithons are the most impressive parts of Stonehenge; they form the central horseshoe, and are the tallest stones in the Circle. Each Trilithon consists of two standing stones, with a crosspiece atop them. The Trilithons are graded in height, increasing in height as they move away from the entrance to the Circle-- the shortest are 6.1 meters tall, and the tallest one (called the Great Trilithon, for its size and singularly central position,) is 7.3 meters tall! It is also possible that the Heel Stone was placed during this period.
The Third Period saw the construction of two new circles of menhirs (free-standing stones.) It is during this period that Stonehenge settled into its normal place in prehistoric society, and it is basically in the same form now as it was then (although much more dilapidated, of course.)
Between the Sarsen Circle and the Trilithons, there
is a circle of "bluestones," which appear relatively small next to the
massive sarsens and Trilithons. They are made of spotted dolerite,
a stone which is rather different from the sandstone-like makeup of the
sarsens and Trilithons. Each of the bluestones is roughly 4 tons
in weight, and form a circle with perhaps as many as 60 stones. Then,
inside the Trilithons, closest to the center of the monument, there is
a horseshoe of bluestones, consisting of 19 stones.
Also, very close to the center of the circle, the Altar Stone dominates the view, originally surrounded by the Trilithons and all the rest of the circles (although, nowadays, half of the Great Trilithon has fallen on top of the Altar Stone, making it difficult to examine.) It is unknown in which period the Altar Stone was placed; it is even possible that it was among the first stones placed on cold Salisbury Plain.
One final note: it is surprising to note the degree of skill with which the sarsens and Trilithons were joined to their lintels; a joint-and-socket method holds the stones securely together (with a protruding knob at the top of each standing stone, and a socket on each end of the lintels.) This is standard in carpentry, but has not been noted in any other stone circles from the era of Stonehenge's construction.