The Medusae Fossae Formation at Eumenides Dorsum – wind at work -
The Eumenides Dorsum mountain range is located west of the Tharsis region and is part of the Medusae Fossae Formation (MFF). The MFF itself is a very extensive geological formation found along the Martian highland-lowland boundary between the Tharsis and Elysium volcanic centers. The formation appears easily erodible and extends discontinuously for more than 5000 km, covering a region about the size of India.
It is thought, that the MFF consists of ash that was deposited by pyroclastic flows or volcanic air falls from the Tharsis volcanos or from Olympus Mons starting in Hesperian times, some 3.8-3.0 billion years ago. In general, the surface of the formation appears smooth and gently undulating as seen in the upper left portion of the image (southwest). Other regions are wind-sculptured into kilometer long linear ridges and grooves like in the center and lower left part of the image (southeast). The ridges are called yardangs and generally point in direction of the prevailing winds that carved them.
Many crescent shaped depressions can be spotted all over the low right part of the image. These hollows are apparently carved into the sand by wind and are called blowouts. A blowout is a saucer or trough-shaped depression created by wind erosion on a preexisting sand deposit and formed together with an adjoining sand accumulation called depositional lobe or blowout dune. The process of creating the blowout is rather simple: The wind transports the sand and erodes the smooth surface. When it hits a buried object, like for example a rock or just a more resistant portion of the sediment, the wind with the sand is forced around the object, creating an eddy at the face of the object. The wind is then forced downward, around and then up, carving out a hollow while lifting the sand from the base of the obstacle and depositing it behind it.
Only very few craters can be found on the surface. This indicates that that wind erosion has been the latest stage of erosional processes acting here. In the upper right part of the image, some few impact craters are visible, possibly showing the underlying older rocks that were draped by the MFF. The MFF is believed to be the largest sedimentary deposits on the planet and also the largest single dust source on Mars.
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