You can just make out the thin needles of a type of carbonate mineral called aragonite in the upper right ooid. Each one has a central core (called a nucleus) made of a grain of sand, mud, or bits of shell, then outer layers (called cortex) made of carbonate mud. Fig 3: The Inferior Oolite as viewed under a polarizing microscope. A lot of the most decorative uses of the Oolite are chosen because the densely packed fossil grains are so attractive to look at.
#OOLITE WALL FULL#
Rocks like the Oolite are therefore known to be ‘bioclastic’ – full of fossils. We call these sediment grains made from skeletal remains ‘bioclasts’. Those durable skeletons form a good fraction of the grains in rocks like the Oolite. Animals have been making carbonate skeletons for over 500 million years. You will also find little discs, or stacks of those discs, which are the skeleton of creatures called crinoids – like a star fish with a stalk, also called sea lilies. This block is ~50 cm wide.Ī closer look at a bit of Oolite reveals that many of these carbonate grains are fragments of shell, mostly from mollusks like clams and oysters, but there are lots of bits of sea urchin in there too. The size and shape of these cross beds tell us that there were strong currents and waves moving the sediment, so the sea was probably only a few tens of meters deep. We call these structures cross bedding and they would have formed ripples on the ancient sea floor just like in the modern seas.
Only the front surface of the dune is preserved and these run from top right to bottom left in a gentle curve. Here the ooids and shell fragments have been arranged into dunes on the seabed.
But where did all this calcium carbonate come from? Fig 2: Inferior Oolite in a building near St Cross College, Magdalen street, Oxford, UK. For a start, most of them are made of calcium carbonate, not silicate. The grains in the Oolite are something entirely different. A regular sandstone is made of silicate grains, usually the minerals quartz and feldspar, which have been eroded from pre-existing rocks. At first, the Oolite might be mistaken for sandstone: it’s full of nice round sand-sized grains and it’s that same pale warm yellow we associate with sandy beaches and deserts. A quick side note, it’s not called the ‘Inferior’ Oolite because there is anything wrong with it, that’s just Victorian geologist speak for ‘lower’, because it lies beneath the ‘Greater’ (upper) Oolite. The Inferior Oolite has been a popular building stone for hundreds of years, especially for grand or official buildings. The largest shell fragment is ~2 cm long.Īnyone who has visited southern England, and many of the larger cities across the UK will no doubt have already seen the Inferior Oolite, even if they didn’t know it. The pale grey chunks are larger shell fragments, many have been dumped on a single bed surface by a strong current, probably during a storm. You can make out the tiny dark pits were ooids have been weathered out. The main beige matrix is made of thousands of tiny ooids and shell fragments. This is a building stone in the wall of Green Templeton College, Woodstock Road, Oxford, UK. The good news for those based further afield, is that many of the same types of rocks are found in buildings all over the world, even if it’s not the exact same rock unit.
#OOLITE WALL SERIES#
In this blog series I will focus on the geology of the common building stones found around the UK. There is in fact a lot of geology here on our doorsteps within the towns and cities that many of us live in. When most folks think of geology and rocks, they imagine rugged landscapes and remote locations.
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