MacCulloch’s Fossil Tree

The Ardmeanach Peninsula with Loch Scridain on the left.

I went on my own up to Mull for a few days at the end of March. On my first full day there I climbed Ben More, 966 metres tall and the island’s only Munro - my seventh. On my third day there I took the ferry across to Iona and visited the ruined nunnery and restored abbey, and the next day I had an eleven-hour drive back to Sheffield. On my second full day on Mull – the 31st of March - I went for a walk to see MacCulloch’s fossil tree.

I left the car at the National Trust car-park just past Tiroran on the Ardmeanach Peninsula and set off west along a Landrover track. It was a beautiful day, warm enough for shorts and t-shirt for the most part. The previous day I’d climbed Ben More in dense fog, relying on map and compass to reach the summit and descending in chilly rain. The day after, on Iona, the weather was miserably overcast and drizzly. But the day I chose to visit the fossil tree was absolutely lovely. Lucky me!

Looking back at the farmhouse at Burg, the last inhabited house on the peninsula and home to the only person I saw on my whole six-hour walk – an old man who stood looking after me once I’d passed by. Today this farmhouse and a small bothy are about all that’s left here, but over fifty people lived at Burg before the Highland clearances in the 1840s. East of Burg, there were also settlements at Culliemore and Salachry, but these too were cleared in the 1800s for sheep-farming. I saw a lot of ruins of small buildings along the track.

This 19th-century monument stands in the centre of a ruined iron age fort – you can see the thick, curved wall in the right of the picture. This “probable D-shaped semibroch or a sub-oval dun” is known as Dun Bhuirg. Archaeological notes are available at the Royal Commission on the Ancient and Historic Monuments of Scotland’s website.

It was also called Castle Dare at one time. A plaque on the other side of the monument, erected by Mr. John Hamilton Turner, reads:

DAISY CHEAPE
ACCIDENTALLY DROWNED IN THIS LOCH
15TH AUGUST 1896
”BELOVED”
”CASTLE DARE” HER FAVOURITE PLACE

Puir wee lassie. According to Walking on the Isle of Mull by Terry Marsh, Daisy’s family owned the Tiroran and Carsaig Estates. She died, aged twelve, when the small boat she was in with her brothers Ronald and Leslie was overtaken by a storm as they sailed to Carsaig. The boat capsized and sank with Daisy caught in the rigging; the boatmen and her brothers survived. Her proper name was Helen Margaret Cheape. [I found this further information here.]

Looking east, back along Loch Scridain.

At one point in my walk, I rounded a corner and was surprised by this sight: two stags (red deer or roe?), a family of feral goats, and a buzzard!

This mad wheel of basaltic cooling columns is in the sea close to MacCulloch’s fossil tree. I have read that this wheel itself was formed by lava cooling around a tree – we’re seeing a horizontal cross-section of the tree and the surrounding lava, whereas MacCulloch’s tree is seen in vertical cross-section. It seemed to me that this wheel was the terrifying maw of a gigantic kraken.

The wheel and a collection of more regular vertical cooling columns.

Even closer to MacCulloch’s fossil tree, the path takes you to a rusty old ladder that leads down onto this stony beach. The ladder looked very old and seriously corroded, so it was an act of faith to climb down it. An even older and rustier ladder still hangs on beside it!

And so, finally, to MacCulloch’s fossil tree – after a four hour walk which I reduced to two hours on the return leg simply by taking next to no photographs.

The tree was probably swallowed up by a lava flow from Ben More, then an active volcano, between fifty and sixty million years ago during the Paleogene period. Although the fossil today is mainly just an imprint, at the time its discovery by John MacCulloch in 1819 the imprint was lined with a two-inch deep layer of charcoal which has since been removed by souvenir hunters and unscrupulous geologists. The remains of the stump are capped by concrete to preserve what is left.

The best and most technical description of the fossil I could find was in a paper by Marsh, B. D. &. Coleman, N. M., published in the Journal of Volcanology and Geothermal Research (volume 182, issues 1-2, pp. 76-96) in 2009, entitled ‘Magma Flow and Interaction with Waste Packages in a Geologic Repository at Yucca Mountain, Nevada’:

Perhaps a more pertinent example [of quenching] is MacCulloch’s Tree in Ardmeanach of the western Mull magmatic comples of Scotland. Here a large (~2 x 15 m) upright Eocence (~55 Ma) conifer (Taxodioxylon) has been encased in a columnar basalt lava flow. The clearly defined quenched margins are of a thickness approximately that of the radius of the tree (see Figure 7). In addition to the distinct quenched margins, also clear in this example is notable horizontal columnar jointing or fracturing due to contraction upon cooling. Columnar jointing is an indicator of the direction of cooling, with the trend of the columns being in the direction of the local strongest influence on cooling. This pattern of jointing shows the major effect of this tree in quenching massive flowing basalt.

I’ve annotated my photo to match Figure 7 in Marsh & Coleman’s paper, which they caption: ‘Upright Paleocene conifer caught in a thick basalt flow in Scotland. The distinctive quenched rinds have been noted along with the strong horizontal columnar jointing reflecting the overall effect of quenching and local rapid cooling. Also notice the man for scale. (after Emeleus and Bell, 2005).’ Emeleus and Bell are the authors of The Paleogene Volcanic Districts of Scotland. I provide the scale!


Posted in Gone for a walk + Holidays and field trips










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