Fairburn fortificationsThe state gemstone of South Dakota is one of the most beautiful agates on Ear
Fairburn fortificationsThe state gemstone of South Dakota is one of the most beautiful agates on Earth, with lovely banding patterns and (in the best specimens) bright red, orange and brown colours due to iron oxides. Named after the town nearest to where it occurs, it is a target for rockhounds across the country (see http://on.fb.me/16fsEEt). They were first discovered in the late 19th century, though native peoples had known of them since time immemorial.It formed as nodules in limestones in the nearby Black Hills dating from the Carboniferous some 300 million years ago, and been weathered out and dispersed around the area since the Oligocene, when the uplift of the west increased riverine erosion rates. The limestone precipitated in a shallow sea that transgressed onto the Pangaean supercontinental shores and the silica was both washed off the continental rocks in solution and came from the shells of tiny marine organisms called diatoms. During the long process of diagenesis, whereby the sediments turned into rocks, the silica was remobilised and precipitated in cavities and burrows. When the Black Hills started to rise some 70 million years ago, they were exposed to erosion and their precious contents spread around the area, rounding many of the nodules along the way. At the bottom of the hills they lay buried for some 30 million more years in newly forming sedimentary rocks.At the type locality they are found in stream conglomerates (aka pudding stone) that drained out of the hills during the Tertiary. Since the area is a sedimentary stack of these river sediments, they are found at several levels, whenever the streams had sufficient energy to mobilise larger chunks of rock in the endless interaction between geology and climate. They are characterised by sharp, well defined, bands. Common hues include red, blue, purple, brown, white, black, yellow, pink and orange.LozImage credit: Captain Tennealhttp://www.geocaching.com/geocache/GC1WHNH_fairburn-agate-beds?guid=16fc4a47-a602-440d-9c9f-e3cd8487b499http://www.netstate.com/states/symb/gemstones/sd_fairburn_agate.htmhttp://www.agatehunter.com/fairburns.htmhttp://www3.northern.edu/natsource/EARTH/Fairbu1.htmhttp://www.mindat.org/min-1441.htmlhttp://csmsgeologypost.blogspot.com/2012/06/fairburn-agates-gem-of-south-dakota.html -- source link
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