Bacup lies in Rossendale, an area famous for its quarries and slipper industry. Millions of years ago, during the Carboniferous period, a time when the rocks that lie underneath the valleys and hills of Rossendale were formed, the whole of what is now the North of England was covered by huge river deltas and lagoons. Sediments, mainly sands, silts and muds, were eroded from hills in an area that now includes Scandinavia and Greenland and were swept into vast river deltas and lagoons in a central basin in a position now occupied by the Pennines. The sediment settled to the bottom as the water slowed down in the deltas and lagoons. The nearest equivalent sediments of today are forming in huge river deltas such as the Mississippi delta. All Rossendale’s rocks are layered or “stratified”. This is because over the years conditions changed leaving different layers in the sediment. Sometimes the sediment would be mud, whilst at other times it would be sand. There were even times when pebbles were washed down to form a layer of pebbles mixed with sand. The different layers eventually turned into rocks with different properties. Coarse grained sedimentary rocks, perhaps containing pebbles, are known as gritstones, but in the past have been called ‘grits’ leading to the name Millstone Grit; medium grains equal ‘sandstone’; finer grains give rise to ‘siltstones’. The finest grained sedimentary rocks were once mud and are often dark coloured. Formerly known as ‘shale’ they are now referred to as ‘clayrock’. Typically the rock sequence is alternating layers of strong brown sandstones, softer dark shales (clayrock) and occasional gritstones.
However here we have something different, and no its not an erratic, there are enough of those in parks. So what is it? Well to the untrained eye, it is an memorial stone with a plaque on it, but look beyond that, and you encounter an age which was very different from the carboniferous. Looking at the rock, there are two colours, grey and pink, one side which is smooth, one rough, what we have here is granite.
What is Granite?
Granite is a common type of igneous rock. Igneous rock is one of the three main rock types , the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallisation either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. Granite is granular can be described as phaneritic and felsic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. By definition, granite is an igneous rock with at least 20% quartz and up to 65% alkali feldspar by volume. Phaneritic is a term usually used to refer to igneous rock grain size. It means that the size of matrix grains in the rock is large enough to be distinguished with the unaided eye as opposed to aphanitic (which are too small to be seen with the naked eye). This texture forms by the slow cooling of magma deep underground in the plutonic environment.
Granitic rocks mainly consist of feldspar, quartz, mica and amphibole minerals, which form an interlocking, somewhat euuigranular matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter color minerals. Occasionally some individual crystals ( phenocrysts) are larger than others, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is known as a granite porphyry. Granitoid is a general, descriptive field term for lighter-colored, coarse-grained igneous rocks.
Phenocryst
This is a relatively large and usually conspicuous crystal distinctly larger than the grains of the rock of an igneous rock. Such rocks that have a distinct difference in the size of the crystals are called porphyries, and the adjective porphyritic is used to describe them. Phenocrysts often have euhedral forms, either due to early growth within a magma, or by post-emplacement recrystallization. Euhedral crystals are those that are well-formed with sharp, easily recognised faces. The opposite is anhedral: A rock with an anhedral texture is composed of mineral grains that have no well formed crystal faces or cross-section shape in thin section. Anhedral crystal growth occurs in a competitive environment with no free space for the formation of crystal faces. An intermediate texture with some crystal face formation is termed subhedral.
Normally, crystals do not form smooth faces or sharp crystal outlines. Many crystals grow from cooling liquid magma. As magma cools, the crystals grow and eventually touch each other, preventing crystal faces from forming properly or at all.
Normally the term phenocryst is not used unless the crystals are directly observable, which is sometimes stated as greater than .5 millimeter in diameter. Phenocrysts below this level, but still larger than the groundmass crystals, are termed microphenocrysts. Very large phenocrysts are termed megaphenocrysts. Some rocks contain both microphenocrysts and megaphenocrysts.
Classification by phenocryst. Rocks can be classified according to the nature, size and abundance of phenocrysts, and the presence or absence of phenocrysts is often noted when a rock name is determined. Aphric is a term used to describe rocks that have no phenocrysts, or more commonly where the phenocrysts consist of less than 1% phenocrysts (by volume); while the adjective phyric is sometimes used instead of the term porphyritic to indicate the presence of phenocrysts.
This being an earthcache there are questions to answer in order for you to log, please do not include them in your log.
(1) Please describe the stone, what does it feel and look like?
(2) What colours are there?
(3) Are the crystals Euhedral, anhedral or subhedral?
(4) Can you see any phenocrysts?
(5) How tall does the rock stand?
(6) Is it a native rock to Rossendale, what is the rationale for your answer?