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.
Here is an overview of the area taken from the Estonian LIDAR relief map. S — straight dune, C — curved dune. This map is a courtesy of Estonian land Board.
There are many small and mostly abandoned sand quarries which made our job a lot easier. This is the cross section of a straight kriiva. It almost seems to be like a manmade landform.
Curved kriiva. These dunes are composed of many smaller dunes that are sitting one on top of the other.
Alternating layers of silt (darker) and sand (lighter) in the lower part of one of the curved dunes.
Here we see clearly that the layers (slip side of the dune) are inclined. I measured the maximum true dip which is 28 degrees. The dip of the same dunes windward side was approximately 8 degrees. These are quite characteristic numbers for a dune.
Here is an example of the material these dunes are made of. It is fine sand but surprisingly not too well sorted. Note how large are some quartz grains compared to the rest. Dune sands are generally very well sorted so it may be a sign that in this case the transport route of the grains have been very short. It is compositionally typical continental sand. Quartz and K-feldspar are the most important constituents. The width of the view is 5 mm.