It seems to be sand dune week in the geoblogosphere. Here is my contribution.
There are weird landforms in the NE corner of Estonia which are called kriiva in Estonia. Most of them look like dunes and they are composed of fine sand but there are some difficulties.
First researchers almost 100 years ago thought that these landforms are marginal moraines because their NE-SW orientation match the orientation of the continental glaciers margin there some 12,000 years ago. However, we see no sign of deformations which should be there if we assume that these landforms are the result of a bulldozing work of an advancing glacier. This hypothesis is largely rejected now.
What is the problem then? First of all, there seems to be at least two morphologically different types of kriivas. First ones are straight and the other ones are curved. I visited several of these landforms about a month ago with some fellow geologists.
We first visited one of the straight kriivas and thought: what a heck, this is like a railway dam running straight through the forest. I never thought that a sand dune might look like this. Both sides of the landform had similar steepness. We made some excavations to look for a cross bedding but found only very subtle hints of it. The layers were mostly parallel and composed of fine well sorted sand. My belief in the dune hypothesis was quickly waning although I couldn’t figure it out what else it might be. All right, let’s assume we had a marginal crack in the glacier where the sediments were accumulating. But the sediments are too well sorted. Glaciers carry all kinds of material from clay particles to large boulders but there were only fine sand.
Then came the curved ones which were different. They are morphologically clearly resembling dunes (one side steeper than the other) and there were some cross bedding in the upper parts of the dune. However, interestingly the lower part of the dune had parallel alternating layers of silt and sand which is a characteristic to ice lake sediments (somewhat similar to varved clay sequences) and definitely not to dunes.
Well, our preliminary conclusion was that maybe these dunes were not dunes at the beginning. Maybe they were indeed some sort of “lake” sediments formed in a narrow crack? When the glacier retreated, some of these straight sandwalls were reworked and became curved dunes and some for some reason remained intact? I really don’t know the answer but if any one of you have experienced something similar, I and one of my friends who is writing his bachelor thesis on these dunes would appreciate it.
Google Maps is not very competent in this region but here is a link to that area.