A tributary of the Neath River, Melincwrt Brook is situated in the Neath Valley just south west of the small town of Resolven and it used to be home to an iron works and blast furnace at the top end above the waterfall which were opened in 1708 and worked for about a century. The leat that directed the water to the overshoot wheel that was here can still be seen close to the road bridge. The falls have drawn visitors for at least two centuries and even Turner painted them in 1794. Today it is known as the Melincwrt Falls Nature Reserve and is owned and managed by The Wildlife Trust of South & West Wales.
The Geology
The geology of this part of Wales is known as the South Wales Coal Measures and is amongst the youngest of the rocks formed in the Carboniferous Period. More specifically this area is part of the Upper Coal Measures and is formed of sandstones, mudstones and coal seams and here we can see the Pennant Sandstone, a layer of shales and the Brithdir coal seam.
Pennant sandstone is a sedimentary rock formed in a subset of the later (younger) Carboniferous Period known as the Westphalian stage. It is a very hard sandstone which varies in colour but around here is often tinged red, and, as with most sandstones, it is not flaky although it often has small vertical fracture lines. The texture is normally quite smooth to the touch.
Shale is distinctly different to the sandstone. It is still a sedimentary rock but it is clastic in nature which means that it is made up of pre-existing fragments of rock, or clasts. It will often break along layers of weakness and can be rather friable or splintery (and therefore fairly weak in terms of durability depending on the stresses put upon it). In this situation the continual pounding of the water falling onto it would break it up over time.
So what happened here was that the water flowed over the very hard sandstone that overlies the softer shale and gradually eroded back the shale. This erosion continued and undercut the hard sandstone above which partially collapsed to form the overhang we see today and the pile of sandstone rock we see at the base of the waterfall. Over time this will continue and the waterfall will slowly migrate upstream.
Visiting
Park at the parking co-ordinates and cross the road to reach the signposted footpath that starts next to Marion Hall (trailhead waypoint) and walk to stage one (just a few metres) to answer the tasks there and then continue up the obvious (often damp) footpath to the published co-ordinates at the waterfall itself for the other tasks.
The Tasks
To log this earthcache you will need to visit the different stages, and send the answers to me to the questions below:
1) I mentioned the Westphalian Stage earlier, but when was this?
2) Head up to stage/waypoint 1 and take a look at the rock exposed on the right. Describe the rock (think of composition, texture, colour etc) and tell me what type of rock it is.
3) Still at stage 1 the rock in inclined, but which way and by how many degrees roughly. BE VERY CAREFUL HERE TO NOT STEP BACK WHILST LOOKING AND FALL DOWN THE STEEP RIVER BANK!!
4) Head up to the waterfall now at the posted co-ordinates and admire the view, but whilst you are there tell me what sort of height you think the water drops (this will give you an idea of the height of the layer of Pennant Sandstone here).
5) Describe the rock around and behind the waterfall and tell me if this is the same rock or different to that at stage 1.
6) You will see there are two overhangs - one large one two thirds of the way up and one much smaller nearer the base where the shale has been eroded back. At what height above the fallen rocks is the lower overhang at (and therefore the height of the top of the shale)?
7) OPTIONAL TASK There is a small track that goes to the right of the water which is always very wet and slippery and is attempted entirely at your own risk. Very carefully follow it round and you will find you can very nearly get behind the waterfall, look up and tell me how far out the large overhang comes out? (This will give you a rough idea of how far back the shale has eroded so far). Obviously please be very careful if you choose to do this task as you will be adjacent to the water and under the rock that has collapsed in the past!!
You do not need to wait until I reply to log your find, but I will come back to you to either confirm or clarify your answers as soon as I can. Photos are always appreciated of course but are not a condition of logging and try to make them so that you do not give away all the answers please.
***Many thanks to the South and West Wales Wildlife Trust for allowing me to place this earthcache here.***