Samstag, 4. April 2009

Mima mounds and hump meadows

Mima mounds (the name derives from the Mima Prairie in Washington state), also known as Hogwallows, are uniformly distributed mounds of soil some 1-3 meters tall that occur in various places in the world, coining the name fort he landscape where they are occurring of „pimpled prairies“. Their age or origin is still unknown. Various theories have been proposed: that they are Amerindian or pre-historic peoples' burial mounds, remnants of glacial or periglacial activity, interactions of the soil with earthquake vibrations, or the cumulative action of gophers or other burrowing animals over a long period of time. But every of this theories deals with different lacks of evidence, traces of gophers or their activity were not found in all areas with hogwallows, also they occur in regions not glaciated or reached by glaciers in the past, and so on.
Excavations made into the Washington mounds show that underneath a blanket of prairie grass lies a mixture of loose sand, fine gravel, and decayed plants up to 2m thick.

Similar geomorphological features – small mounds superimposed on slopes and plains- are known from the austrian Alps and Bavaria. They are called „Buckelwiesen“ – (hump meadows) and prevail on carbonat rich sediments or morain deposits.

Four main theorys to explain their formation were developed since first research attemps between 1940-1960:

- the mounds represent the remanents of a carst-landscape, where different rates of dissolution of carbonatic rocks caused the development of troughs, enhancing the erosion betwen the single „humps“. But this theroy implies a primarly landscape with first differences for disolution processes.

- they formed by melting ice – wedges or / and by cryoturbation, pronounced freeze-thaw cycles can produce mixture of different grain-sized and ice-bearing sediments by difference in the specific weigth and thermal conductivity – producing a pronounced relief. This theory seems also supported by the fact that the forming sediments was deposited during the last glacial, and experience periglacial conditions.

- they rappresent places where once stands trees. The trees protected the underlying soil from erosion, only in the space between trees erosion and dissolution take place, then finally humans cut down the trees to use the resulting meadows for pasture.

- they are primary features of sediment depositon in a particular environment.

Based on new geomorphological and computer aided mapping, dissolution test in the sediments and excavation of some of the mounds, a research team of the Universtity of Vienna now propose a new theory:

The mounds and troughs are disposed more or less paralell, and possess a general oval form, with proportions between length to width of 1,7. Excavations carried out showed in some mounds cavities filled with humus material, interpreted as stump remains, and deep troughs in calcareous material filled with organic rich sediment. Dating showed a young age, of only 720 to 880 years, partly in accordance of older published dates ranging between 1.000 to 7000. This results esclude a direct implications of ice in the formations of the mounds. The new explanation propose that trees, thrown down by wind (this explains the subparalell observed pattern), caused a first mound or deposition of material, and then subsequentely this material and the rotting tree itprotected the underlying calcareous material from precipitation and dissolution. With time, ditches develop between the single trees, where water can even more effectely remove sediments and calcareous sediment.
Finally humans have cut in the last centurys the trees for pasture use, and exposed the "hump meadows".

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