DO NOT APPROACH THIS EARTHCACHE FROM ABOVE!!!
THERE IS NO DIRECT PATH TO THE EARTHCACHE SITE, BUT YOU ARE BEST APPROACHING FROM THE WEST
THE ROUTE TO THIS EARTHCACHE IS ROUGH UNDERFOOT. BE CAREFUL
THE CO-ORDINATES TAKE YOU NEXT TO BACKSTONE BECK
If you look at the rocks on Ilkley Moor, they seem to be dominated by Carboniferous sandstones which were laid down at a time when the north of England was covered by a vast river delta, down which was carried and deposited immense amounts of sediment in the form of sand, pebbles and rocks. Here at this site there is evidence of a different geological process.
Here at the EarthCache site, look up at the cliff in front of you.
You can see stratified beds. Layers of rock are known as strata, each layer is known as a bed. They are stacked up on top of each other, a bit like a geological Victoria Sandwich cake, it has layers, well so do rocks. The layers you can see here are made up of a variety of sedimentary rocks.
Wharfedale has four main sedimentary rocks.
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.
(1)Shale is a type of sedimentary rock called clastic rock, which forms when "clasts" -- that is, fragments of other rocks or minerals -- are deeply buried and compacted in a depression. In the case of shale , the clasts are silt and clay particles. Over time, the buried sediment becomes cemented and forms sedimentary rock. In the case of this location, some of these shale layers were laid down during a marine environment. This has been evidenced by the finding of goniatites in amongst the layers. Goniatites are an extinct marine animal which belonged to a group known as amminoids. These were types of mollucs, and when connected to our modern day marine life are said to be closley related to octupus and squid. I have never found any of these type of fossils here, and as the moor is an SSSI, please do not break up any rocks.
(2) Mudstone is a fine-grained sedimentary rock. It is usually black or dark grey-brown and is often soft and crumbly. Mudstones form when very fine-grained clay particles are deposited in water. They tiny particles settle to the bottom of oceans, lake floors or lagoons or even in quiet stretches of rivers. As the mud is buried and compacted by overlying sediment, the water is squeezed out and it turns into mudstone. You will notice that some of these rocks have lines on them, these lines are evidence of sedimentation, which the natural process in which material (such as stones and sand) is carried to the bottom of a body of water and forms a solid layer. Some of the layers are thicker than others, though with mudstone they tend to be very thin.
The only difference between mudstone and shale is that mudstones break into blocky pieces whereas shales break into thin chips with roughly parallel tops and bottoms.
(3)Sandstone (sometimes known as arenite), is a clastic sedimentary rock composed mainly of sand sized minerals or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are: tan, brown, yellow, red, grey, pink, white, and black. Since sandstone beds often form highly visible cliffs and other features, certain colors of sandstone have been strongly identified with certain regions. The formation of sandstone involves two principal stages. First, a layer or layers of sand accumulates as the result of sedimentation,either from water (as in a stream, lake, or sea) or from air (as in a desert). Typically, sedimentation occurs by the sand settling out from suspension; i.e., ceasing to be rolled or bounced along the bottom of a body of water or ground surface (e.g., in a desert). Finally, once it has accumulated, the sand becomes sandstone when it is compacted by pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains.
(4)Limestone, this type of sedimentary rock is composed of the accumulation of the shells of ancient sea creatures, which once lived in an ancient shallow tropical sea. As they died they settled on the bottom of the shallow sea. It tends to me white / grey in colour, with a fine grain. As it is composed of the remains of ancient creatures, it may have fossils in it.
So how did marine animals end up on Ilkley Moor?
At times during the Carboniferous period, sea level changed at times, because of glaciations in the southern hemisphere. So at times the river delta was covered by sea water. At the sea level changed, up and down, the shaly rock was deposited on top of the sandstone which had been laid down by the river delta, and at times sandstone was deposited on top of the shaly rock. The different layers of shale and sandstone indicate many alternating periods of river delta and sea.
So at one time if you were alive during the Carboniferous period you would have been standing under water, and at times amongst a river delta.
This being an EarthCache, in order to log it, I ask that you answer some questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.
1, How high is the cliff you are stood infront off.
2. Cross the beck ( if it is not in spate!), look at the strata, please measure the thinest later you can see, and measure the thickest layer you can see.
3. Please tell me which of the above Wharfedale rocks form the strata making up this cliff, what is your rationale for your decision?
4, Please describe the colour, and feel of the rocks forming the strata.