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Many people consider Scotland to be divided into five geological regions by four major faults which run across the entire width of the country.
These faults were formed when tectonic movements caused the crust to fracture and move from its original position.
In some cases, the ages of the rocks to either side of the fault are separated by many millions of years. An example of this is the Moine Thrust, where Lewisian Gneiss units have been thrust over Ordovician Sedimentary rocks. Given that the Ordovician rocks are estimated to be "only" around 500 million years old compared to the 2600 million year old Lewisian Metamorphic units, this would amount to a 2100 million year age difference. It is also believed that the Protoliths of the Lewisian units could be as old as 3 billion years (www.scottishgeology.com).
The rock units encountered in each of the regions vary greatly but the general trend is that the units increase in age in a northwesterly direction.
©2004, Andrew Rogie