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Geology / Silurian Reefs

Just as you associate coal with rocks of Carboniferous age, many geologists will associate the Silurian with limestone. Much of the limestone was laid down in large reef systems or bioherms in similar environments to the huge coral barrier reefs of today. After the end of the Ordovician sea levels rose - probably due to release of water from the melting ice caps. The shallow water conditions over wide areas in Europe and North America provide ideal conditions for reef development. The other conditions - clear mud-free water and warmth were also largely present.

Limestone is important as a road stone, in the production of steel and as an impermeable rock to form the traps for oil and gas underground reservoirs.

Made up primarily of Calcium Carbonate (CaCO3) but sometimes dolomite [Calcium Magnesium Carbonate (CaMg(CO3)2)], limestone is the bye-product of organisms. In the earliest formations, bacteria and single-celled plants produced the lime by extracting the minerals from sea-water. At the start of the Cambrian System many different groups of animals started depositing their shelly skeletons into rocks. The vast quantities of skeletons then formed lime muds that under heat, pressure and vast periods of time turned into limestone rock.

One of the most famous exposures of Silurian limestone must be at the Niagara Falls. Here the hard limestone rock forms the layer over which the river falls onto the softer underlying rocks.

Reefs form as 'barriers' in shallower waters and as isolated 'pinnacles' in deeper water. The reefs are formed by a whole collection of different organisms performing different tasks in the reef building process. The principles of the reef building process are the same today as in the Silurian 400 million years ago.


Constructors Build the basic structure of the reef. In the Silurian, stromatoporoids built domes and tabulate corals built domes and chains up to two feet in size.
Binders Form stabilising sheets over loose material. Stromatoporoids, tabulate corals and bryozans performed this function in the Siurian.
Bafflers Slow down the water flow over the reef by forming branching structures over the reef surface. Tabulate and rugose corals can perform this function.
Reef Dwellers The great majorities of organisms (about 90%) make use of the reef structure. Although not building the reef in life, their skeletal remains contribute greatly to the bulk of the reef. Organisms such as sponges, rugose corals, crinoids, brachiopods, cephalopods and bryozoans are in this category.

Brachiopods were very numerous and diverse perhaps accounting for one quarter of reef dwellers. They filtered food from the water and were anchored to the reef or nestled in soft mud.

Trilobites crawled the reefs in search of food on their many feet and using their well developed eyes and antennae to find their way around.

Crinoids resemble underwater 'tree ferns' or 'sea lilies' opening up scaly, feathery arms to catch microscopic animals in the water. The stems are often preserved as small columnar pieces and can make up a significant proportion of a limestone.

Dolomite is not formed directly, it is original deposited as Calcium Carbonate and afterwards the deposit is then permeated by magnesium rich waters that gradually replace half the calcium with magnesium. Dolomite is harder and heavier than the original rock.