I live in a building that is just a few years shy of being 200 years old. It’s also in a part of the city known as the ‘New Town.’ An oxymoron? Not really. In the late 18th and early 19th century, when Edinburgh’s New Town was constructed, it was, quite literally, new. The name stuck, and is still in use two centuries later.
As old as the New Town buildings are from a human perspective, the stones they are made from are much older – about 340 million years older. And the individual grains that make up these stones are older still. I love that idea of time layering: the buildings exist in the present; the stone from which they’re made was quarried and worked by stonemasons two centuries ago; the rocks themselves were laid down in a tropical river delta 340 million years before that; the mineral grains that make up the rocks – they are mostly small crystals of quartz – were eroded out of some ancient mountain range at an unknown but even earlier time; and the mountain range from which the grains were eroded formed long before that.
Most of the houses in Edinburgh’s New Town, including the one I live in, were built from sandstone. Much of it was quarried locally and brought into the city on horse-drawn carts. Individual loads could weigh many tons. The finest varieties of sandstone – whitish to light grey, fine-grained, and very hard – were cut into blocks of uniform size and polished to present beautiful exteriors for the Georgian buildings. The hard sandstone has aged well, as you can see from the accompanying pictures. It will probably not look vastly different after another two hundred years.
What is sandstone, exactly? In the simplest terms, it is sand turned into rock. Sandstone is a sedimentary rock and it comes in a variety of colors and textures, depending on how it formed. It might originate as dunes in a desert, or sand on a beach, or grains deposited in a river delta, like the sandstone of my house. But how do you get from a sand dune to the solid rock that is sandstone? The secret, as for most sedimentary rocks, is burial (often under many kilometers of other sedimentary layers), compaction (because of the pressure of the overlying material), and usually some degree of heating. Those conditions cause the edges of the grains in the nascent sandstone to begin to dissolve in the fluid – water – that surrounds them, and then, when conditions change again, the dissolved substances precipitate as a cement, holding the grains together in a solid rock. If you were to take a very thin slice of the sandstone from the buildings shown in the accompanying photos and look at it through a microscope, you’d see rounded grains of quartz jammed together and cemented by solid silica – the same material as the grains themselves.
Sandstone is marvelous for many reasons, not just because it makes beautiful building stones. Careful study of sandstone outcrops can reveal clues to the environmental conditions when the rocks formed, and thus help to unravel the earth’s complex history. Did sandstone found today in a cold, high-latitude location originate in desert dunes, signifying a vastly different hot, dry climate in the past? Or was the sand part of a tropical beach on the edge of an ancient continent? Are the grains in the sandstone smooth and rounded and of uniform size, indicating transport over great distances and a long period of sorting and winnowing? Answers to such questions can tell us much about the geological history of a region.
And sometimes rare grains contained in sandstone hold clues to events that occurred long before the sandstone itself formed. Zircon crystals, which originate by crystallization in igneous rocks like granite, are hard, inert, and capable of surviving most things our planet can throw at them. They are not completely indestructible, but generally when they get eroded out of their original host rocks, tumbled around by wind and waves and rivers and transported over long distances, they are little changed. Because of this robustness, they often end up in sandstones – although usually in extremely small quantities. Fortunately, zircons are also ideal for geological age determinations. Such measurements don’t say anything about the age of the sandstone, but they do tell us when the zircon crystal itself formed. Geochemists have laboriously separated zircon grains from sandstones in many parts of the world in order to date them and find out something about their origins, and about the origins of other grains that accompany them in the sedimentary rock. One such study of sandstone from Western Australia turned up something quite amazing: a zircon grain that is 4.4 billion years old, the oldest material ever dated from our planet. The implications of that age measurement are profound and to explore them properly would take much more space than I have here. Suffice it to say that this tiny zircon grain tells us that our planet had started to evolve a continental crust like the one that is familiar to us today less than 200 million years after the earth itself formed. That is much earlier than most geologists had expected.