Mysterious boulder ?

The term „Geschiebe“ in german is unfortunately ambiguous, or at least used so. It can indicate sediment transported by a river, or by a glacier too. Geologists use the name traditionally for glacially transported sediments, limnologist and water engineers use it in the second meaning of the word, confusion – even indicating if the term is used in the glacial or fluvial sense, is secured.

But what´s now the mysterious boulder? It was found in an outcrop that at first looks seems a diamict with no apparent internal structure, single larger polimict clasts in a loamy to sandy matrix. A morain?


But a more precise observations of the larger clasts shows a distribution in single clusters, and an suggested imbrication (note granite-clasts, ca. in the middle and extreme right of the picture).


Also the landscape is very flat, or has only a gentle dip, not so typical for morain deposits, but commonly found in fluvial or limnic deposits deposited in large braided river systems.

So I tend to attribute this deposits to a fluvial system of high energy, with sporadic floods capable to transport even large boulders. The granite was first transported surely by glaciers (the actual catchment area don´t possess outcrops of this kind of lithology), but the final deposition occured by a river.
The disaggradation of the rocks must be happened after this second deposition – the boulder couldn´t possibly survive a transport in this conditions. The lack of color alteration of the feldspat, and the „freshness“ of the mica, let´s assume mechanical weathering, maybe in a colder climate with strong thaw and freeze cycles.

So a proglacial sediment of a glacier feed braided river system still seems possible to explain this sediment..