The Umm Ishrin FormationThe Umm Ishrin formation is a fascinating and famous geological unit, althou

The Umm Ishrin FormationThe Umm Ishrin formation is a fascinating and famous geological unit, although it is likely most famous because of the way people have cut it. The ancient city of Petra in Jordan, a UNESCO World Heritage site visited by about a million people per year, is cut into this sandstone.The sandstone tells a story of the Cambrian environment on the north side of the Gondwana supercontinent. Gondwana was fully linked together at that time, with the area that is now the Arabian plate hooked to the northeastern corner of Africa. Gondwana was likely put together as pieces of igneous rock collided with one another over billions of years and this region was no exception. The sandstone buries metamorphic, igneous rocks formed during the Precambrian that are exposed in the mountains outside the city.The rocks of the Umm Ishrin Formation are part of a larger sequence called the Ram Group. During the middle Cambrian, about 520 million years ago, ocean levels rose enough to cover this whole part of the continent. Over 100 meters of fine-grained sediment are deposited on the igneous, crystalline basement rocks. Those sediments gradually become coarser as they get thicker, indicating that the water levels were receding and the shoreline was approaching.Those sediments are then topped by the Umm Ishrin formation, a massive, 500+ meter thick, cross-bedded sandstone. That sandstone likely represents river-deposition conditions, but it’s not quite like any river we know today.Today, when a river develops, plants take over the river. They occupy the edges, hold the rivers together, and even borrow some of the water in return. During the Cambrian, there were no land plants present to stabilize the river, so the water was able to carry sediments and migrate as it wanted. Huge rivers carried a gigantic amount of sand from deeper in Gondwana and deposited it in this area, close to the edge of the continent, with little mixing of fine-grained components as we find today in floodplains. Those sand grains were then buried by additional sediment and gradually lithified into stronger rocks.Sandstones like these are generally strong rocks, hard to erode, but over thousands of years eventually the water will win. A thin canyon called a Siq, a narrow gorge carved by rapidly flowing water during flash floods, is the main access to Petra. The same flash floods that allowed access to the area also threaten it today. The combination of earthquakes toppling the construction, weathering, flash floods, and hundreds of thousands of human visitors per year is gradually wearing away at the rocks. Eventually erosion will remove this city; care while visiting can help delay that and preserve it for future generations.Image credit: Dennis Jarvishttps://flic.kr/p/4nY62ERead more:http://www.geotimes.org/june04/feature_petra.htmlhttp://bit.ly/1K2PTUphttp://bit.ly/1TyutA6 -- source link

More pics about