Bluestones are a category of rock identified by a distinctive bluish-gray color when freshly broken or wet. This descriptive name is applied to various stones used in architecture and landscaping globally. The term gains its most historical weight from the prehistoric monument of Stonehenge in England, where a collection of foreign stones are collectively known as bluestones. This article explores the specific geological nature of the Stonehenge bluestones, their importance at the monument, and their distant origins.
The Geological Identity of Bluestones
The bluestones at Stonehenge are an assortment of different igneous and volcanic materials foreign to the Salisbury Plain. The most numerous is the Preseli spotted dolerite, an altered igneous rock known for its dark appearance flecked with white or pink spots. Secondary minerals create this unique look on the base of medium-grained, heavy dolerite.
The spotted appearance results from low-grade metamorphism, altering original plagioclase feldspar crystals into mineral clusters like clinozoisite and albite. This process gives the stone a distinctive texture that is harder and more resistant than granite. The bluestone assemblage also includes fine-grained volcanic rhyolites and volcanic tuffs composed of ash and fragments.
These stones are grouped as bluestones to distinguish them from the much larger, locally sourced Sarsen stones. The blue color is most pronounced when the rock is wet or when a new face is revealed, otherwise appearing gray or greenish-gray. The geological diversity of the group, which includes dolerites, rhyolites, and the Altar Stone sandstone, confirms the term is an archaeological label.
The Archaeological Significance at Stonehenge
The bluestones formed the inner structures of the Stonehenge monument. They were erected around 2300 BC in the third major phase of construction, predating the massive Sarsen stones that form the outer ring. The bluestones were arranged into an inner circle and an inner horseshoe structure, though only 43 of the estimated 80 original stones remain.
Bluestones are significantly smaller than the Sarsen stones, typically weighing between two and three tonnes. Their presence, having been transported a considerable distance, suggests a profound significance beyond simple construction material. This smaller size and exotic origin imply a special ritualistic value, possibly linked to their source landscape.
Recent research suggests the bluestones may have been selected for their acoustic properties. Studies indicate the stone arrangement amplified sounds, such as the human voice, by approximately four decibels inside the circle. This acoustic effect contained sound within the ritual space while dampening noise from the outside. Furthermore, the hard, igneous dolerite produces a loud, distinctive, almost metallic sound when struck, which may have influenced their selection.
Tracing the Origin of the Stonehenge Bluestones
The bluestones’ geological makeup proved they did not originate on the Salisbury Plain, prompting a long search for their source. In the 1920s, petrographic analysis first linked the spotted dolerite to the Preseli Hills in Pembrokeshire, Wales, over 140 miles away. Modern geochemical techniques have since confirmed and refined these locations, pinpointing exact quarries.
The majority of the spotted dolerite bluestones have been matched to the Carn Goedog outcrop in the Preseli Hills using trace element analysis. A type of rhyolite stone was also sourced to the Craig Rhos-y-felin outcrop, located north of the main Preseli ridge. This precise matching to specific Welsh outcrops was achieved by analyzing the unique chemical signatures of microscopic crystals like zircon.
The discovery of the source locations shifted the debate to how the stones were transported across the vast distance to Wiltshire. The human transport theory proposes that Neolithic people quarried and moved the megaliths by dragging them overland or floating them on rafts down the Bristol Channel. Conversely, the glacial transport theory suggests that the stones were naturally carried to the Salisbury Plain hundreds of thousands of years ago by a massive Pleistocene ice sheet. While archaeological discoveries of quarrying evidence support the human effort model, the glacial theory continues to be debated, with some geologists pointing to the existence of glacial erratics from Pembrokeshire found along the Bristol Channel.
Regional and Commercial Uses of the Term
Outside of Stonehenge, “bluestone” is a commercial or regional term applied to various building stones sharing a blue-gray appearance. In the northeastern United States, it refers to a feldspathic sandstone primarily quarried in Pennsylvania and New York. This sedimentary rock is popular for paving, patios, and walkways due to its durability and ability to be cut into thin slabs.
Across the globe in Australia, the term “bluestone” describes a different rock, typically a basalt or olivine basalt. This volcanic rock was a prominent building material in cities like Melbourne during the 19th century. It was used extensively for sidewalks and civic structures. The non-specific nature of the name means that a consumer purchasing bluestone in the US receives sandstone, while a purchase in Australia yields an igneous basalt.