Half a billion year old clay

My home country Estonia has been geologically quiet place for a loooooong time.

I usually think its a bad thing. No volcanoes, no mountains, little mineralogical and lithological versatility, no real structures — everything is more or less parallel and layered. The sedimentary layers that cover the crystalline basement have never been buried very deep. Maximum perhaps 1 km or so. Therefore, these sedimentary rocks are only weakly altered by diagenetic processes. Because of that we have something rather remarkable. We have a layer (up to 70 meters in thickness) of bluish gray clay in the bedrock that is half a billion years old (from the Cambrian). I really mean clay, not claystone. This clay is still quite soft and becomes muddy when wet.

Here is a photo of this clay with mud cracks. The mud cracks are fresh but the clay itself is really old. The photo is taken in a clay quarry. The clay is used to make cement.

Recent mud cracks in Cambrian clay.

http://picasaweb.google.com/107509377372007544953/Rocks#5786990837301660770
The same clay before getting wet. The width of the sample from Kunda is 11 cm.