Monday, July 25, 2016

Moher, Moher, Moher!

Well it's been more than a week for this current blog post, but it will be worth the wait, I promise!

Continuing west in Ireland from our previous blog on limestone lies a famous tourist attraction that is evidence of years and years of geological formation. The Cliffs of Moher are an amazing feature along the west coast of Ireland in County Clare. The story behind these cliffs is incredibly complex and demonstrates what a bit of geomorphological and geological processes can achieve given a lot of time. Intrigued? Allow me to explain, but first let me to introduce a visual of this stunning area!


Beautiful, eh? Well, what you are looking at is essentially many years of deposition along a river delta.  A river delta is where the end of the river meets a "non-moving" body of water like sea or a lake. At this location all the sediment that traveled with the river deposits are the current that transported the material stops. Typically we think of river deltas as being very flat areas found at sea level. In the case of the Cliffs of Moher, repeated sediment deposition (5 different major events) in conjunction with sea level changes and tectonic uplift created this unique geological feature. The Cliffs formed during the Upper Carboniferous period approximately 300 million years ago. The 5 mile stretch of coastline reaches 700 feet above sea level at its highest point just north of O'Brien's Tower.

O'Brien's Tower built in 1835 along the Cliffs of Moher.

So getting back to the sediment deposition at the Cliffs, striations or layers of rocks can be seen along the exposed walls. What you are seeing pictured below are sandstones overlaying, siltstones on top of mudstones. The darkest layers are the mudstones. These layers are what you can see from the edges of the Cliffs. Below these rocks are the Clare Shales and yes, evenutally the limestone we talked about previously is located below the shale.

The Clare Shale Formation is approximately 40-50 feet thick and contains phosphate, carbonate nodules, and chert (silica based rocks lacking a typical repeating pattern of mineralogy). Fossils can be seen in these rocks as well. This time period had a very slow deposition of clays at the bottom of a deep basin (keep in mind the continents were still moving around during this time and what we presently know as Ireland was located near the equator ). Above the Clare Shale begin the sedimentary rocks that we know as the Cliffs of Moher. As mentioned there were five repeated deposition time periods where mudstone, siltstone, and sandstone layers were deposited. However, only two of these time periods or "cyclothems" can be seen in the area. Each mudstone layer is 23-60 feet deep followed by 115-260 feet of siltstone and sandstone. The sandstone and siltstone contain fossils,  water ripples suggesting waves in the basin, and tunnels from creatures living in the material before it hardened into rock. The siltstone and sandstone come from river flooding events depositing eroded materials from the land. The mudstone is evidence of sea level change from glacio-eustatic events (melting and freezing of polar caps and glaciers) as the mudstone is marine.

The Cliffs of Moher contain rocks that are susceptible to weathering and eroding. Overtime the sandstone will be a bit more resistant to the elements than the mudstone, which can cause collapse and further destruction of the Cliffs. It is hypothesized that the Cliffs were located as much as 4 miles further west. This rate is based on the coastal recession rates of the nearby Aran Islands, that is receding at approximately 16 inches per year! It's a great time to go visit Ireland, so take in this amazing place before it's gone've got some time!


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