The Structure of Stonehenge Today
The most remarkable part of Stonehenge is its larger upright stones, called sarsens. Sarsen is a particular kind of sandstone rock, and the closest source of such stone is Marlborough Downs, about 20 miles (32 kilometers) away from Stonehenge. Lintels are the long sarsen rocks that lie horizontally atop two vertical sarsens.
When they were all standing, the inner sarsens would've formed a horseshoe shape with five stand-alone trilithons, which consist of two vertical sarsens topped with a lintel sarsen. The largest trilithon (30 feet or 9 meters tall) would have stood at the bottom of the horseshoe, but it's now partially collapsed. The average weight of one of these sarsens is more than 49,800 pounds (22.6 metric tons) [source: English-Heritage.org].
The ancient Britons carefully shaped these hard stones. They carved mortise holes into the underside of the lintels that fit snugly on protruding points called tenons carved into the top of the vertical sarsens. Tenons and mortise holes are typically used only in woodworking, suggesting the feature was symbolic. The lintels along the outer circle, likewise, fit snugly together end-to-end with a tongue-and-groove connection. This outer circle is far from complete, but enough stones stand to make it clear what the planners probably intended: a complete circle about 108 feet (33 meters) across, consisting of 30 vertical sarsens and 30 lintels. Four more sarsen stones, known as the Slaughter Stone, the enormous Heel Stone and two Station Stones, sit outside the outer sarsen circle.
The smaller stones that sit within the sarsen circle are called bluestones because they look blue when wet or freshly cut, and each weighs 4,409 to 11,023 pounds (2 to 5 metric tons) [source: English-Heritage.org]. The bluestones aren't just one type of rock: 30 are basaltic dolerite, five consist of igneous rhyolite, five are other types of volcanic rock and three are sandstone [source: Lambert]. As such, they also came from different sources. Geological analysis suggests that at least 11 came from the Preseli Hills in western Wales (140 miles or 225 kilometers away), but others possibly came from more local sources [source: Lambert].
Many of these bluestones sit in a circle between the horseshoe of trilithons and the outer circle, while another set is arranged in an oval shape within the horseshoe. The "Altar Stone," made of Welsh sandstone (different from the sarsens), weighs more than 17,600 pounds (8 metric tons) and lies underneath the fallen sarsen of the biggest trilithon [source: Pearson].