5.
Marl, ‘blockworkers’ *
and petrified life
About 65 to 70 millions years ago, during the Cretaceous,
this region was a warm, shallow sea inhabited by
billions of small animals, many of them having
a calcareous skeleton. The numerous microscopic
skeletons,
along with bigger ones like sea shells and crustaceans
sank to the bottom and built up a layer of limestone
or marl, in some places 60 metres thick. This is
unique in the Netherlands, that‘s why we call
this region the “Marl District”.
Geological processes caused the limestone to come
to the surface at some places, but mostly it was
covered under thick layers of sand and clay.
The limestone or marl contains many fossils of
sea shells and other animals – even giant animals
like the Mosasaurus, a reptile of up to 20 meters – and
flint. In the Stone Age people dug up the flint for
their tools. In some places they worked in real underground
flint mines!
Since Roman times the marl was used
as building material for farms, houses and churches.
In this way they made
a network of underground passages: the ‘caves’ as
we call them. In Valkenburg you can visit several of
them, like the Gemeentegrot (Town’s Cave) and
the Fluweelen Grot (Velvet Cave). The castle next to
the garden is a good example of a house built in the
local limestone
* These ‘blockworkers’ were working underground to gain the limestone.
They sawed it into blocks. That’s why they are called block-workers
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