These large (hammer is 20cm long) vertical structures are found in lacustrine sediments (homogeneous fine-grained sand with dropstones, left of picture) of the Alpine last glacial maximum.
They show a sharp contact to the surrounding sediments, seem to be layered vertically - note however that the longest axis of the pebbles seem to be oriented horizontally - origin: unknown.
They show a sharp contact to the surrounding sediments, seem to be layered vertically - note however that the longest axis of the pebbles seem to be oriented horizontally - origin: unknown.
This look like a fallen block. If you look carefully at the center of the block, you will see deformation structures that indidace polarity (from left to right at the photo), that talks us about the original sedimentation position. This block seems to be turned right 90 degrees. It could be a fallen block or a erratic block, transpoerted by the glaciar (I think the fisrt one).
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Best hypothesis I could come up with is that these are fluid escape structures.
ReplyDeleteAs the pressure from new layers increases, the older sediments settle and compact, and the water berween the grains escapes through vertical fractures. There also seems to be some thinner ones on the left side of the exposure - assuming these are vertical "dykes" and not tubes, there shold be a local extensional stress field. Maybe these sediments were draped over an underlying high? That could be enough to explain the preferential orientation.
It looks like this, isn't it?
ReplyDeleteMiage glacier
It's sediments from an old lake above the Miage glacier in the Mont-Blanc range, italian side. The sand flow with the rain in a crack...