The Great Glen Fault It is difficult to look at Scotland from above without noticing a major geologi

The Great Glen Fault It is difficult to look at Scotland from above without noticing a major geologic feature; there’s a giant line! The line you see from above can be found on the surface as well; it is represented by a series of lowlands and valleys that cut entirely across the country and extend offshore. Several lakes, including the famous Loch Ness, also sit completely within this valley, known as the Great Glen. That depression is the remnants of tectonic upheaval eons ago; the valley you see today marks the location of the Great Glen Fault. The fault is mostly a strike-slip fault, where rocks moved past each other horizontally. This fault was initially a sinistral or left-lateral fault, meaning if you look across the fault, the rocks on the other side would have moved left relative to where you stood. This fault was first formed at least 400 million years ago during what is called the Caledonian orogeny. At that time, a piece of land known as Baltica, which includes northern Scotland collided with Laurentia, which today composes much of North America. When continents collide, they typically build mountain ranges, but oftentimes continents don’t move perfectly at each other, they also move side-to-side. If the continents are moving together but at an angle, building mountains doesn’t accommodate all the motion; a strike slip fault must form as well to take up that component of movement. The Great Glen fault was that fault. This fault was reactivated later during the Carboniferous and Cretaceous, but in those times the fault moved the opposite direction; right-lateral or dextral. As the mountain ranges shifted and broke apart, the stresses changed and the rocks found it easier to use the previously-formed fault zone than to break in a new place, so the fault was reactivated at those times. The valley formed today occurs because of how the rocks were treated in the fault zone. They were crushed and broken, forming what we call “cataclasite”. Battered, broken rocks are easily eroded, leaving a great valley across the Scottish highlands today. -JBB Image credit:NASAhttp://en.wikipedia.org/wiki/File:Scotland_from_satellite.jpg -- source link

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