Age of the Rocks
Current research has dated the formation of the rocks at Brimham to 320 million years ago, during a period of relatively stable sea-levels starting 321.5 million years ago and ending 319.5 million years ago.
Age of the Rocks
Current research has dated the formation of the rocks at Brimham to 320 million years ago, during a period of relatively stable sea-levels starting 321.5 million years ago and ending 319.5 million years ago.
The Boat Rock with very visible bedding planes which in this case are all part of the same rock strata.
The sandstone found at Brimham is Lower Brimham Grit, a coarse stone made up of medium grained sand particles and is found overlaying the Brewerley Shales and beneath the Brimham Shale. These lay within the strata that form the Hebden Formation, itself part of the Carboniferous Millstone Grit Group. These carboniferous rocks are found across much of northern England and were laid down during the Namurian stage during cycles of delta formations and subsequent river plain flooding. The characteristic composition and sand grain structures within the rocks allows this lithostratigraphic grouping which can then be correlated with comparable strata containing characteristic fossils.
An artist’s impression of a goniatite, similar to Reticuloceras reticulatum.
Biostratigraphy relies on index fossils, also known as guide or indicator fossils, which are used to define and identify geological periods or faunal stages. Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends.The index fossils for the periods during which the rocks at Brimham were deposited are types of primitive ammonites, known as goniatites, belonging to the subclass Ammonoidea which includes, in particular, the key species Reticuloceras reticulatum and R. coreticulatum. These index fossils are present in the ammenoid zone characterised by R. reticulatum.The correlation of the index fossil species in the Lower Brimham Grit places the rock strata within the Kinderscoutian chronostratigraphic substage with an age of 320 million years.
A close-up view of one of Brimham’s rocks showing the quartz crystals that are characteristic of this type of sandstone.
The absolute age of the rocks can be estimated using chronostratigraphy to date the rocks which uses combinations of radioactive isotope measurements, chemical analysis of rocks and other physical properties. It remains very difficult to isotopically date most fossils or sedimentary rocks directly, and so inferences must be made in order to arrive at an age date which reflects the beginning of the period. The age of intrusive igneous rocks can be measured by establishing the ratios of isotopes that are produced by the radioactive decay of, for example, uranium to lead which can give a precision of 2 percent in rocks up to 4.5 billion years old. Other radioactive elements can be used for radiometric dating, in particular the decay of potassium to argon, or samarium to neodymium. Dating based on measuring the number of fission tracks within material ejected from volcanoes can be used on rocks up to 400 million years old. Quartz crystals themselves can be aged by optically stimulated luminescence techniques, though this does not give the age that the quartz was incorporated in the sandstone itself. With a calculated age for an igneous rock strata, and knowing that the principal of superposition implies that strata below the igneous rock are older and those above the rock are younger, then the relationships of the strata provide assurances that the dating of the sediments is as accurate as possible.
The Elephant’s Head now known to have been eroded out of rocks dated to 320 miilion years ago.
Following the International Commission on Stratigraphy's geological timescale, the Lower Brimham Grit falls within the Bashkirian stage, the oldest stage of the Pennsylvanian series, itself part of the Craboniferous system. In the Western European classification, Brimham Grit is within the Kinderscoutian regional substage which forms part of the Namurian stage.So Brimham can be classified as both Bashkirian, Pennsylvanian, Kinderscoutian and Namurian with an age of 320 million years.
