Task Site 3
Task Site 3, where the beds of a rock called the Ffynnon Limestone can be seen. This is a kind of limestone, forming today in the soils of semi-arid regions. That suggests a similar environment here around 400 million years ago.
The upper slopes (generally above a height of 400m), consist of massively bedded sandstones called the Senni Formation, which we will examine at Location 4. Here, just below it, is a rock layer called the Ffynnon Limestone. It is a calcrete (chemical limestone), and it represents a distinctive marker horizon throughout much of the valley.
Crossing the Ffynnon Limestone
You can see the Ffynnon Limestone calcrete in the path here. It forms a hard knobbly step in the hillside.
The calcrete formed on the Devonian plains during extended periods of evaporation in the arid climate. As the ground dried out, calcite-rich groundwater was drawn up from depth and precipitated limestone just beneath the surface of the ground as it evaporated.
Pits and track
Fertiliser lime can be produced from limestones. In days gone by, local people dug the calcrete from this hillside and used it as a cheap fertiliser. You can see pits on the sides of the hill here and an old track leading to them. The pits follow the line of the calcrete for quite a distance around the side of the hill.
Cat’s Back profile
The Cat’s Back ridge curves down to the valley and has a knobbly or stepped profile – rather like an arched cat’s back. The steps occur at points where there is a sandstone or a calcrete – both harder and more resistant to erosion than the mudstones. Although the overall profile gives the ridge its name, do you think it might have been these calcretes that close up made people think of knobbly vertebrae bones?
Task Site 3 Questions
Where did the calcite/lime come from to form calcretes?
a) Rain water
b) Ground water
c) River water
How many calcrete layers can you see in the path?