Wednesday, 29 November 2017

On the very edge of the Central Complex

Had about half an hour at lunchtime when in Salen so I decided to have a stroll up part of Cruach Torr an Lochan, the prominent hill between Salen and Gruline. There is a path that leads up the hill, muddy in places but straightforward. Only had time to go a short distance.

At this point we are right on the edge of the Mull Central Complex. If you look at the geology map you will see a really complicated bit in the middle and lots of simpler geology to the north. The complicated bit is the Mull Central Complex - the stuff to the north is mainly basalt lavas and minor intrusions. By walking up this path, you are actually right on the boundary between the two, near enough.

Also in this area are two quarries that are mentioned in the GA Guide that was published in the late 60s. The GA Guide refers to them as part of the Knock / Loch Ba excursion. I decided to have a look

First quarry:

Quarry is in basalt lavas and gets used for storage these days. The quarry face has a lot of calcite veining running through it. Many of the veins are over 3 cm thick, full of massive calcite, running vertcal to sub-vertical through the face.

The calcite is the white mineral and it has broken off in large pieces.

On the way to the second quarry we pass this sign:
The path continues in a loop and comes out on the main Salen - Craignure road. On the way you pass a great example of a composite dyke. It is in a small quarry. I didnt visit it today but here is a pic from last year:

This one is a beauty. The interior part (the cream coloured bit) is a felsite of some sort - very altered. The edges are mafic, very fine grained basaltic looking material.The dyke can be seen on the other side of the quarry as well.. It trends NW-SE  - a fairly typical orientation! Its worth a look  - easy to get to as it is only a short distance from the main road.

The second quarry was dug into what is called the Toll Doire Granophyre. Toll Doire is the placename where the quarry is found.. The rock is cream coloured like many of these granophyres and quite different from the basalt lavas which are grey to black. The quarry is now seriously overgrown but the rock can be seen on the path beside it

The granophyre quarry, overgrown

Granophyre in situ, on the path that leads up the hill through the trees. Not the most exciting picture! This particular granophyre has been studied for some research papers. Walsh et al in a 1979 paper "Geochemistry and Petrogenesis of Tertiary Granitic Rocks from the Island of Mull, Northwest Scotland" looked at it along with others.Easy to get good samples as there are still a few quarried blocks lying around.

Up on the hill above the trees the views are great. Cruach Torr an Lochain looms up ahaead and the views in all directions are nice. Esp on today when  a rainbow put in an appearance

Cruach Torr an Lochain with the sun to the right

Cruach Torr an Lochain , going by the BGS map, is mainly composed of a mass of cone sheets cutting lavas.

On the way down I noticed an outcrop of rock on the path that is quite different from the rest. I have seen this before - it looks like a psammite (quartzite) but the field relations are hard to make out. It is just above the granophyre and below the basalt. Is this Moine psammite? It might of course just be a large block that is not in situ.

Psammite on the path: It looks very simialr to the psammites you see at Gribun or near Scoor.  The BGS map indicates an outcrop of Jurassic age rocks at Glac a Chlaonnain which is a short distance away, but there is no indication of any older metamorphics. - there is nothing on the map to indicate what this stuff actually is. An odd one.

So that was today's short stroll. If anyone has any ideas on that psammite, let me know!

Saturday, 25 November 2017

Into the caldera

Weather today was pretty wild and wintry   a day for staying down low! Took a walk along the track beside Loch Ba as far as Glen Clachaig - there is some great mountain scenery in the area and of course it is the location for the world famous Loch Ba Ring dyke. I wanted to have a look at the hills with a bit of snow on them as it often brings out detail that is hard to see otherwise. The pictures below give an idea. Click on the images to see them larger!

The Loch Ba Ring Dyke (LBRD) follows the wintry skyline. Seen from near Gruline

Moving on to the track by the loch, the weather started to clear but was still quite wild:

View up the loch!

Moving on this prominent hill is on the RHS:

The LBRD runs through this un-named hill down to the loch side. The exposed rock sene at the top is the felsite with basic inclusions that makes up most of the dyke. There are also some noticeable parallel gullies running across the hill. I havent been to investigate these, but they might be dykes of the Mull Swarm that cut the LBRD. As the LBRD is one of the last igneous features in Mull, then these other dykes that cut across must be represent some of the last igneous activity in the area. One of the dykes further up the hill has weathered out into a deep chasm

Looking across the loch:

The snow covered peak (not very clear) is called Na Bacannan and is the end of a ridge. The LBRD runs over this

This hill is called Sron nam Boc - the LBRD runs straight over this hill. Although the LBRD appears like a wall of rock on the SW side of the loch, on the other side, the BE side, it isnt so obvious. After this point on the track we are effectively inside the volcanic caldera. The LBRD itself has been described as "the most perfect example of a ring dyke known to science” (Bailey et al in the Mull Memoir, 1924)

The weather was starting to improve:

Two photos of Beinn na Duatharach. The dark crags are of gabbro. There doesnt appear to be any studies done on this gabbro as far as I can find. A search on Google Scholar only throws up an ancient paper by Prof Judd (1899) in which he refers to the hill and gives an alternative name "Beinn Uaig" His 1899 paper refers to another dating from 1874. I need to check the Mull Memoir of 1824 to see if there is anything. I dont think there is,

Moving on:

The very snowy hill is Ben Talla at the head of Gen Forsa. The LBRD runs across the foot of this hill but is really hard to find. 

Most of the rock that surrounds Loch Ba is the Glen Cannel Granophyre - on the BGS map it is really obvious and covers a big area. Appears to be the largest body of granophyre in Mull (by area anyway). Despite this, it is really hard to find in situ near the loch - there are only a couple of places where it can be sen without going up the hill. Near the cottage of Clachaig, it is exposed in a stream near a bridge:

Granophyre (the cream coloured rock) in the stream bed cut by a small zig-zagging dyke

This was the furthest point out I went to - heading back in the weather improved but was still pretty wild.

Looking up at the ridge of Beinn a Ghraig - the LBRD is the prominent rocky ridge on the LHS. The snow pics out the contact at the base quite well.

A final look back up the loch.

Sunday, 19 November 2017

Lighthouse Path Walk 19 Nov 2017

Took a stroll along the newly repaired Lighthouse path that goes to Rubha nan Gall. Great views over to Morvern, up Loch Sunart and over to Kilchoan. I wanted to have a look at the geology esp since if there had been some digging going on during the path upgrade, then there might be some goodies to see.  Here in pictures is what you can see on the path. Of historic interest, this is the location where Prof Heddle discovered the mineral Tobermorite.

The new gate on the path

A bit further on, there are some neat dykes on the shore at Rubha na Leip:

The smaller of the 2 dykes, about 0.5m across

The larger of the dykes - about 3m in width and showing very pronounced columnar jointing.Both dykes run NW-SE - Classic "Mull Swarm" stuff.

Further along the path there are some interesting amygdales to be seen in the cliff face above the path. Some of the rock here has been recently excavated so is quite fresh. 50p for scale. It was in amygdales that Prof Heddle found Tobermorite, but nothing I saw in the cliff matches the description given in his 19th century paper in Mineralogical Magazine ( link at the end)

Further along the path some very obvious red bole material can be seen - it is all a bit mixed up with the lavas. No nice clear obvious horizons that stretch laterally for any great distance. 

Some of the reddened material that is visible at path level. 50p for scale

Where the vegetation has been cut, there are now great views out to sea. Lighting conditions werent ideal but the pics give you an idea of the views. Rum could be seen in the distance with snow on it.

Looking over to Loch Sunart.

Beinn Hiant 

The view south - Calve Islad Ben Talla and Dun Da Ghaoithe.

The lavas beside the path as well as showing all the classic stuff like red boles, amygdales and other secondary minerals in veins also have some good spheroidal weathering on display. A couple of pics to show this:

Basically all the common features that you see elsewhere in the lava pile of North Mull can be seen beside the path.

The lighthouse at Rubha nan Gall came into view:

The walkway that forms the approach to the lighthouse is made of a red Jurassic sandstone that was quarried just to the north at Bloody Bay. The quarry can be visited from Ardmore Forest but is a bit of a fight with the vegetation. One of the few places in North Mull where the underlying Mesozoic rocks can be seen. The Doirlinn at Calve is another as is the coast below the Guala Dhubh. I think sandstones of similar age can be seen behind the Co-op in Tobermory!

Looking north from the Lighthouse. A little bit further north of here are a couple of exposures of rocks attributed to Blue Lias / Breakish Formation (Lower Jurassic). Limestones and calcareous sandstones - very hard and hackly. I didnt have time on this trip to go further to see these rocks but here are pictures from a few years ago!

The sedimentary rocks north of Rubha nan Gall. You can see the lighthouse in the background

Final thing to note at this location is an obvious dyke running up the hill slope. The white lichen growing on it makes it really obvious. The guys on the CalMac ferry were telling me that from certain angles it looks like smoke from the chimney of the house!

Dyke of the "Mull Swarm" runs up the hill from the house

Final picture of the day is back at the Tobermory end of the path - behind the gate in the garden of the Western Isles Hotel there is an obvious feature in the cliff. Somebody told me a long time ago that there was a fossil tree there. Assuming this is the same thing, looking at it from a distance ( couldnt get closer as the gate is padlocked) it looks a bit like a dyke. Need to do further investigation!

So that was today - loads to see on a great walk on the repaired path. Recommended!

Some links: