The Sedgwick Trail – a walk along the Dent Fault

As a Yorkshire Blue Badge Tourist Guide, I was lucky enough to be invited along to a Cumbria Guides Continual Professional Development (CPD) event with a Geology lecture in Dent, followed by a guided walk along the Dent Fault from just outside Sedbergh.

Both locations had historically been in the West Riding of Yorkshire, but more recently had fallen under the administrative region of Cumbria. It is worth noting that both Sedbergh and Dent fall within the boundaries of The Yorkshire Dales National Park.

Our morning lecture was to be about the Geology around Dent, which was particularly apt as the town was where the famous geologist Adam Sedgwick had been born and our afternoon walk was to be along a trail named after him. There is a lovely memorial to Sedgwick in the town which I have previously blogged about  https://www.realyorkshireblog.com/post/adam-sedgwick-memorial-dent-a-fitting-tribute-to-the-founder-of-modern-geology .

Sedgwick has often been called “the founder of modern geology” and was the first person to notice and explain the rock types along the trail and explain the differing rocks existence along what is now the Sedgwick Trail, caused by massive upheavals that raised the Lake District 2.5km higher than the Dales rocks about 290 million years ago.

Our lecture was to take place in Dent Village Hall and we were lucky enough to have the eminent geologist Dr. Danny Clark-Lowes conducting both the lecture and the afternoon walk.

The lecture covered the basics of geology, showed geological maps of the wider area particularly along the Dent fault and then went into more detail about what was happening along the Dent fault. It was a fascinating insight into the area and how the underlying geology had affected the landscape.

After lunch we had time to explore the Dent Village Heritage Centre, which was a great little window into the past situated in an old garage. As well as a tribute to Sedgwick, the museum covered the local wildlife, farming, sheep and Dales life. There was a particularly interesting display about the “Terrible Knitters of Dent” which I plan to write about in a future blog. The museum is well worth a visit - https://www.dentvillageheritagecentre.com

After our museum visit it was time to visit the Sedgwick Trail and see at first hand all the geology we had heard so much about that morning.

We drove from Dent to Sedbergh and then continued out on the Hawes Road (A684) to Longstone Common and parked up at a particularly pot holed car park along the roadside (SD 695913). From the car park there are fantastic views over to the Howgill Fells as well as further along the River Clough and its valley.

There is also an interpretive board explaining about Adam Sedgwick and the local landscape.

The Sedgwick Trail was launched in 1985 and consisted of 12 stops, but the last few are now too dangerous to access and the new leaflet which is in production will now finish at stop 10.

The trail is easily accessed from the car park – there is a wooden sign which takes you down a lane to the River Clough and its valley. At the river we turned left along the river instead of heading over the little bridge.

It is worth noting, it had been raining when we visited and the start of the trail involved walking close to some limestone and there are also many wooden steps which were quite slippy. Walking poles and sturdy boots were recommended and our safety advice was to avoid the limestone even if it means walking on boggy grass!

The River Clough’s valley crosses the Dent Fault, at which point the landscape changes noticeably. As you travel along the river bank you can see the carboniferous limestones of the Pennines become folded and crumpled up against the much older Silurian rocks on the Lake District side (west side) of the fault. 

So we started the trail by looking at the limestone which looked to be sloping both ways on the opposite bank to the River Clough and were asked which way the rocks actually were bedded? If you take a look at the photograph – what do you think?

The limestone here was deposited approximately 330 million years ago in a period known as the Lower Carboniferous. It is a sedimentary rock which was formed by deposition in a shallow tropical sea of small sea creatures with calcium in their shells. Believe it or not, this deposition took place just south of the equator!

It turns out the rocks on the left of the picture are the actual bedding planes and what looks like sloping rocks on the right are not actually bedding planes but evidence of erosion down the many cracks that appear naturally in the limestone.

Limestone is actually Calcium Carbonate (CaCo3) and can change its character quickly and easily. It does dissolve in contact with water particularly when the water is slightly acidic, which accounts for the cracks in the rocks which have been eroded on the left hand side of the picture. This process also accounts for cave formation.

This image above is perhaps a better one for showing the sloping bedding where you can see a cross section through the beds on the far bank but also the wider bedding on the nearside bank.

As we moved further down the valley we stopped to see some of the fossils in the rocks. There were ancient shell fossils called brachiopods where we could see some of the actual shells in the rock as well as the markings from an indent where a larger shell had sat.

There was also evidence of coral fossils – here we see a cross section through the outer circular case of the coral where a small soft bodied polyp would have lived.

In the limestone we came across a shiny black glassy rock. Danny our lecturer brought out his pipette of hydrochloric acid to show how the limestone fizzes when in contact with the acid but the shiny rock was not affected.

Danny explained that this was called Chert, whose chemical name is Silicon Oxide. This usually occurs when the limestone rock is buried and becomes warmer. Here widely distributed silica in the rock is mobilised into hot percolating fluids. The silica molecules then gradually search each other out to form a crystal lattice ‘nodule’ of chert within the limestone.  

This is the source of chert in limestone, but also explains how flint is formed in chalk rocks.

As we travelled further along the river bank we came to a section of the rocks which looked a bit like a dome. In this case the rocks really did slope in different directions on each side of the dome and Danny explained that this is known as an anticline.

The rock had literally folded under pressure. Quite often you would think that the rocks would have just snapped under pressure but it was interesting hearing an analogy of how this occurred. The folding had happened about 15km beneath ground where the temperatures and pressure would have been greater and this process happened very gradually.

We were told to think about the contents of a tray of toffee. If you tried to bend it quickly it would snap, but by slowly heating it and applying gentle pressure it would bend. That’s how an anticline was formed.

To the left of the anticline some of the beds were dipping almost vertically, as further evidence of the crumpling happening at the fault due to earth movements.

Just along from here we found Tom Croft’s cave. This cave has been formed by the limestone dissolving over time but what was interesting here were the white powdery calcite from the calcium carbonate rock which had been crystallised out onto the rock’s surface within the cave.

We also came across something called Breccia where the rocks stopped being as obviously bedded. This is where limestone has been put under so much pressure that the rock has been shattered into small pieces. The mineralisation in the rock such as the quartz veins is also evidence of intense heating. The rock ends up as a sort of conglomerate, but with angular rather than rounded pebbles.

As we moved westwards we then reached a point on the trail where the rock type changed. This was at the same spot where the river widened and the Howgills came into view. This is where we came across something called The Brathay Flags for the first time.

The Brathay Flags are much older than the limestone and were deposited approximately 425 million years ago as fine grained mudstones in a period called The Silurian. These were the rocks which have been uplifted as much as 2.5km! These rocks account for the more rounded hills in the Howgills as opposed to the steeper slopes in much of the Yorkshire Dales.

Finally we stopped and walked down a quite steep slope back down to the river’s edge to see a conglomerate of rocks which had been deposited on top of the Brathay Flags at a time when there was a desert area. This is known as an unconformity.

All in all it was a fascinating day. We really lucked out with the weather for our walk along the trail and it was great to be accompanied by Dr Danny Clark-Lowes. Danny really helped bring the geology to life with simple explanations and analogies to help us understand the geological processes which defined the landscape in this particular part of the Dales, which were first explained by local Geologist Adam Sedgwick back in the 1800’s.