Smearsett. 🚨DO NOT APPROACH FROM ABOVE 🚨 EarthCache

This EarthCache involves getting to an exposed ledge on Smearsett Scar.  

🚨🚨DO NOT APPROACH FROM ABOVE 🚨🚨

The photograph shows where you need to be.

So, what are we here to look at? Well, I was up here with another EarthCacher, and we had an academic discussion about a specific band of limestone that is here. 

'It's just limestone ', was an expression. Then as the discussion developed we talked about the chemical composition 'something has been replaced here '. Then over a brew, and a home made ginger biscuit, we talked about deposition environments, 'there was a difference in water conditions '. 

As with  all good EarthCaches, this one arose out of that discussion.  You may just want to have a read, a brew and a biscuit when reading this, but that is up to you, and is not a logging requirement.  

So, the geology. You will notice that where you are, there is a distinct exposure of limestone, then as you look to the west towards Pot Scar, that the terrain and geology changes. This is due in part to failt action, namely the Feizor Fault.  This EarthCache is about a specific type of limestone, but the fault plays a role or reference, though there are other EarthCaches about Fault action. So lets concentrate on the limestone.  To the side of the ledge there is a distinct band of a different type of limestone. 

So, what is limestone?

Limestone is a sedimentary rock.

Sedimentary rocks are types of rock  that are formed by the deposition and subsequent cementation of that material at the earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic  particles to settle in place. The particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, the sediment was formed by weathering and erosion  from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers. Sedimentation may also occur as minerals precipitate from water solution or shells of aquatic creatures settle out of suspension.

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. Most limestone is composed of skeletal fragments of marine organisms.

We are here to look at Dolomitic limestone.  Now, limestone has the chemical composition of calcium and carbonate, making calcium carbonate and thus limestone.  Dolomitic limestone is different, it has a high concentration, more specifically greater than 50% of magnesium.  Magnesium is an element. So, how did it get involved with the limestone?

Dolomitic is the term for a limestone that has been altered by geological processes that involve the replacement of the calcium ions by magnesium ions that is called dolomitization. There are several mechanisms by which this dolomitization can take place including evaporative, seepage-reflux, mixing zone, burial and seawater dolomitization processes. The general name of the rock changes from being limestone to being dolomitic limestone also known as dolomite.

Evaporative process of dolomitisation. This is when there is a shallow lagoon type of environment, where evaporation of water, leads to a more concentrated collection of minerals in the water, this includes a higher concentration of magnesium, which then seeps into the calcite mud at the bottom of the lagoon. The magnesium ions then replace partially the calcium ions in the mud, which in time becomes limestone.  

Seepage-reflux process of dolomisation . This is when what is known as hypersaline water filters through the permeable rock, which in this case is limestone.  Hypersaline means a high concentration of salt, which includes magnesium. Limestone is permeable, this means that water can easily flow through the rock. Sn example of this is in the Dales nearby. The limestone here means that there is little surface water as it filters through the rock, but go to Helwith Bridge, and there is the Swarth Moor bog, where the underlying geology of Horton Formation Siltstone is less permeable, so water builds up. 

Mixing zone process of dolomisation.  This is in an environment where fresh water and sea water mix,  this creates a marine environment which is rich in certain salts such a magnesium. This magnesium rich fluid then permeates the limestone and lime rich mud , with calcium ions being replaced by magnesium.  

Burial process of dolomisation.  This occurs when magnesium rich fluid circulates or passes through the limestone, this can occur at shallow or deep depth. Increase in heat and pressure, which  can occur during fault action, namely a process called metasomatism. At times the magnesium rich fluid can come from fluid which has been expelled from other rock types during metamorphic action. This basically is where heat and pressure alters the composition of a rock. 

Seawater process of dolomisation.  This is when Seawater us very concentrated, and has a high concentration of magnesium.  There is also a high rate of deposition of calcite occurring, basically meaning that there is a high rate of lime rich mud being formed.  The high concentration of magnesium displaces partially the calcium in the limestone forming sediment.  

So, what does dolomitic limestone look like in the Dales? Well from afsr, it all looks the same, but in the Dales, it tends to be darker and a brown / tan colour. 

This being an EarthCache, in order to log it, I ask that you complete the above  tasks. Please send the answers  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. Please describe the height of the band of dolomitic limestone here in cm.

2. Please tell me which element dolomitic limestone is rich in. 

3. Using the above information, how has the dolomitic limestone formed?