Stone origins: American Bluestone
When you ask someone about bluestone, depending on where they’re from, you’re likely to get some vastly different answers. The single name ‘bluestone’ refers to a number of different stones through the world, from basalt to dolerite.
In this case, we’ll be talking about what’s known as American bluestone, a type of dense, feldspathic sandstone often used for the purpose of architecture. American bluestone is often conflated with Pennsylvania bluestone, its most well-known variety. We’ll also be talking about the other type of American bluestone: Its limestone variety, which has its own unique set of origins.
Just how these particular types of stone came to be, and what that means for the issue of surface cleaning and protection, is the subject of this edition of ‘Stone origins’.
Bluestone: The sandstone variety
First, we’ll look at the formation of the sandstone version, often synonymous with Pennsylvania bluestone. Sandstone is a sedimentary rock, which means it was formed over long periods of tiny bits of rock being deposited in one place.
In the case of Pennsylvania bluestone, this is exactly how it happened. Around 400 million years ago in what’s today known as the Catskill Mountain region, as oceans changed course and rivers receded, what was left over were creatures like clams trapped in deposited sediment, formed from the erosion of what are today the Appalachian Mountains.
As these creatures fossilized and the sediment was cemented, sheets of blue – not to mention durable and temperature resistant – sandstone were left.
Bluestone: The limestone variety
The limestone strain, most famously found in the Shenandoah Valley, similarly follows the typical sedimentary formation process. While the sandstone variety was formed in the Paleozoic era, limestone was formed around 450-500 million years ago in the Ordovician era.
As calcium-carbonate micro-crystals form in water, they sink to the bottom, becoming buried and forming mud. When they recrystallized, limestone was formed. At the same time, parts of various sea creatures also got caught in the buried mud, becoming part of the calcite cocktail that is limestone.
Because of their different origins, sandstone and limestone are different, and it’s important to note which type of bluestone you’re dealing with. Limestone is generally softer and more prone to scratching and acid etching, for instance.
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