what is the geological reason for the boulders in fontainbleau, france?

Question

there are thousands of boulders or rocks in fontainbleau, which is quite flat. how come? also what is distinctive about the type of rock.

Answer 1

OK, I'm unfamiliar with the exact geology of Fontainebleau,but they definitely are not glacial "erratic" boulders (the glaciers in the ice age didn't come this far south).

Actually, they are not "boulders" but are erosive remanents. They are of Oligocene age (quite young, geologically) and are quite close to Paris, so could have been deposited by a palaeo-river Seine (I'm guessing here!). A fall in sea level / base level of the river results in down-cutting of the river into its own deposits, eventually eroding most of them away with the exception of stranded, isolated terraces and remanents, typically low hills. You see this effect in London too, where the palaeo-Thames eroded its earlier deposits away, leaving isolated hills such as around Harrow, only in this case the deposits were gravels, not sands.

The boulder-like appearance may have been accentuated by localized mineralization (silicification) of the sandstones, forming rounded concretions or large nodules that were more resistant to erosion. These concretions,which can be very large, are typically rounded, and bear no relation to the naturalbedding of the sandstones. Hence they appear to look like boulders!

Answer 2

The boulders were left there by the melting glaciers during their recession at the ending of the last Ice Age.