olelog

I am back from a week or so in the Dresden area (Eastern Germany). Dresden was completely destroyed in World War II, mainly by half a million bombs in the night from 13 to 14 February 1945. Most of the architectonically interesting buildings in the city centre have now been rebuilt, but some restoration is still going on. Most of these historic building were made of Elbe Sandstone. An example is the Frauenkirche (Church of Our Lady). Here is a link to an image of the heap of stones it still looked like in 1990. After the German reunification in October 1990 money was found/collected to bring it back to its former glory, a work that was fulfilled in 2005, that is now two years ago, and here is a link to what it now looks like.

Elbe Sandstone was deposited in Late Cretaceous (100 - 65 million years ago), mainly around 90 million years ago in the Turonian (ca. 93.5 - 89.3 million years ago). At that time most of northern Europe was covered by sea, with no high mountains in the neighbourhood. In fact one-third of Earth's present land area was submerged at the peak of the Cretaceous transgression (sea level rise). With a low erosion rate only little material was brought to the sea, which therefor had very clear water, with excellent living conditions for micro-organisms with calcium carbonate shells. Thick layers of chalk were formed on the bottom of the “Chalk Sea”. And today the Cretaceous chalk cliffs stand out at the coast of England, Northern France, Germany, and Denmark. Only closer to the coast and islands sand (and clay) was deposited, as was the case South of Dresden in what is now the Elbe Valley in the region known as Swiss Saxony (Sächsische Schweiz) with spectacular sheer sandstone cliffs.

The sandstone deposit is 400-600 metre thick. The sandstone beds still lay practically horizontal, with here and there a weaker clayish layer, joints cross vertically through the sandstone in two directions perpendicular to each other. Erosion therefor forms quadrangular blocks and pillars and table mountains. Because of this angular appearance they are called quader sandstone in the regional geological literature.

The fresh sandstone surface is usually bright yellowish, sometimes with a golden or orange glow. Erosional surfaces become dark grey, however, over time - and with todays pollution in towns like Dresden they become blackish. The erosion process in the Elbe valley is of course still going on. My photo shows where a piece of rock of around 400 m³ loosened and tumbled down into the valley in November 2000. It gives us the opportunity to compare a relatively fresh surface with the usual erosion surface. In the reconstructed Church of Our Lady it is also easy to distinguish the few old stones tn the church wall from all the new stones.