Hutton’s Unconformity at Siccar Point

In Geology an unconformity is a surface separating two rock types of different ages from one another. An unconformity represents a period of erosion or non-deposition in the sedimentary record, i.e. a gap or hiatus where we have no record of what happened.

Hutton's Unconformity at Siccar Point
Hutton’s Unconformity at Siccar Point

One such unconformity is Hutton’s Unconformity. The confusing part is that the name Hutton’s Unconformity is the name given to several unconformities identified by the famous 18th century Scottish geologist James Hutton. On a recent fieldtrip to Scotland we visited Hutton’s Unconformity at Siccar Point, but other unconformities identified by James Hutton can be found on Arran and near Jedburgh.

Hutton's Unconformity at Siccar Point
Hutton’s Unconformity at Siccar Point

When sediments (fragments of previous rocks) are deposited (settle before becoming a rock) they form layers of progressively younger material burying the older material. If a sedimentary rock is the correct way up the further up the rock you go the younger (closest to present day) it is.

At Siccar point we can see two distinct units rock, with the layers of rock (bedding or strata) in the upper and lower units being at different orientations from one another. This means that before the second (upper) unit was deposited the lower (older) unit was tilted and eroded, producing a gap in the geological record.

Sign at Siccar Point
Sign at Siccar Point

Unconformities are crucial to our current understanding of geology. They paved the way for us to consider deep time, that is essentially an appreciation of the fact it has taken a very long, yet quantifiable length of time for the rocks we observe around us to have reached their current situation.  For the rocks here at Siccar point that means; deposition of the older sediments, tilting of the older sediments, erosion, then the deposition of the younger sediments on top. This order of processes cannot possibly happen quickly!

Sign at Siccar Point
Sign at Siccar Point

Unconformities have also been used as evidence that the mechanisms governing the production of these rocks (and the universe) have, and always will operate in an assumption  known as uniformitarianism. Uniformitarianism is a counter (and more accepted view) than the opposing catastrophism, which implies that the Earth was created in a series of sudden short lived events.

View from Siccar Point
View from Siccar Point

The Great Orme: Lower Pen Trwyn

Apologies for the multiple posts today but there’s lots to post about since returning from Wales!

I hadn’t climbed on Lower Pen Trwyn (LPT) before but would certainly go back. In terms of the Climbing LPT has some really good routes across the grades and has the feel of sport climbing elsewhere in Europe, especially if the sun is out! Could The Orme be the Welsh answer to Kalymnos?

Approach to Lower Pen Trwyn
Approach to Lower Pen Trwyn

The approach is pretty simple (steep start) with some good geology to maintain interest on the way in. The following photos and thoughts are by no means an exhaustive study, and were captured in a passing by type manner with the intention of inspiring climbers to look around them on the way in to LPT.

Sediments onto of the limestone on the approach to Lower Pen Trwyn. Presumably these are recent sediments sourced from the limestone above on the Orme, but this does not explain why there is such rapid variation in roundness and sorting between layers. Ideas?
Sediments ontop of the limestone on the approach to Lower Pen Trwyn.

Presumably the sediments above the limestone are relatively recent, and are sourced from the limestone above on the Orme? This short transportation distance however does surely not provide a satisfactory explanation as to why there is such rapid variation observable in roundness and sorting between beds?

Faults in the limestone on the approach to Lower Pen Trwyn
Several apparent faults in the limestone on the approach to Lower Pen Trwyn
Fossils at Lower Pen Trwyn. I'm not a fossil expert so any help with identification would be appreciated?
Fossils at Lower Pen Trwyn. I’m not a fossil expert so any help with identification would be appreciated?

Climbing wise Under the Boardwalk stands out as a real classic of the crag, proving to be a tricky little number requiring a sustained effort right till the end, but the crag contains abundant quality climbs. At the far right end there are several ‘easier’ sport routes in the F6a(ish) range with Beauty is Only and Skin Game being good fun.

Matt Keeves on Under the Boardwalk, Lower Pen Trwyn
Matt Keeves on Under the Boardwalk, Lower Pen Trwyn

Oh and remember that LPT is tidal, and accessible apart from about three hours either side of high tide. Its also worth noting that the 1st bolt on most climbs is pretty high (due to the tides).

UKC logbook page for Lower Pen Trwyn

The slate

I have always loved the Welsh slate (in terms of climbing and geology) so no trip to North Wales would have been complete without a substantial amount of time spent here. This is not intended as a in depth review of either the geology or the climbing, more like a few photos, words and thoughts. A really great article on the climbing can be found on UKC.

The Dinorwic Quarries looking towards the Lanberris Pass
The Dinorwic Quarries looking towards the Lanberris Pass. The shear size of these quarries is very impressive, and I’m told that the quarries which are still active at Bethesda are even bigger?

So what is slate? Slate is a metamorphic rock (meaning that it was previously another rock which was heated up and pressurised without melting causing physical and chemical changes). The rock which made up the slate previously was probably a fine grained sedimentary rock (such as a shale).

Pull my Daisy (E2 5b), Rainbow Slab, Dinorwic Quarries
Alex Peace on Pull my Daisy (E2 5b), Rainbow Slab, Dinorwic Quarries

Good things about climbing on Welsh slate:

  • Quick drying: this is wales we are talking about so it will rain and with some routes climbable only 15mins after a shower it can save a damp days climbing!
  • Unique routes

Good things about the geology of the Welsh slate:

Pull my Daisy (E2 5b), Rainbow Slab, Dinorwic Quarries
Alex Peace on Pull my Daisy (E2 5b), Rainbow Slab, Dinorwic Quarries
Pull my Daisy (E2 5b), Rainbow Slab, Dinorwic Quarries
Alex Peace on Pull my Daisy (E2 5b), Rainbow Slab, Dinorwic Quarries
Bus Stop, Dinorwic Quarries
Jessie Harris, Bus Stop, Dinorwic Quarries
Quite a strange route (even for slate) at Bus Stop Quarry
Quite a strange route (even for slate) at Bus Stop Quarry
Pulling shapes!
Pulling shapes!
Not really sure what these are, any ideas? Something leaching into the slate?
Not really sure what these are, any ideas? Something leaching into the slate?

Overall if you have never visited the abandoned quarries near Llanberis it is well worth the effort to explore the unique landscape found here.