Rosalind Jones (UK)
There’s a saying on the Isle of Mull – “If you come to Mull the once you return again for sure” – and it’s not an idle boast, as those who have visited and subsequently revisited this ‘geological Mecca’ will agree. Second largest of the Inner Hebrides, Mull is famous for its Tertiary igneous geology – 6,000 feet of basalt lavas intruded by a complex of concentric bodies, ringed about three igneous centres. With its unique ring dyke of mixed acid and basic magma, Tertiary granites yielding Lewisian dates, and magnetic reversals in the lavas that make compass bearings untrustworthy, Mull is an enigmatic venue for geologists.
The island’s best-known fossils are plant remains, including Ginkgo, Platinus, Corylites and Quercus, all preserved in Tertiary lake sediments deposited between lava flows. Once over collected, fossils from the famous Leaf Beds at Ardtun are now protected, as the site is an SSSI. But the biggest and most noteworthy fossil is ‘Macculloch’s Tree’. Remotely situated opposite Ardtun, on the tip of the Ardmeanach peninsula, it is a phenomenon that, if you visit Mull, you really should see.
I first came across Mull’s fossil tree as a geology student in 1966. Its location was pointed out while I was in the Ardtun Leaf Beds gully, so I scrambled over slippery rocks, past hexagonal columns of basalt and down to the shore to see. The panorama I beheld took my breath away.
The view spread out in front of me, from the northern tip of Iona, across white-crested waves to the Isle of Staffa and the Treshnish Isles, and round to Loch Scridain where layer upon layer of basalt lava, like a giant wedding cake, topped the lowest flow in which the tree stood. Atlantic rollers broke against an islet and grey seals watched me from the waves. I drank it all in and, entranced by the beauty, Mull worked its magic on me. A small white cottage further east caught my eye in the bright spring sunshine. Little did I know that one day the occupant of that lonely cottage would become a friend or that I’d write about an intriguing history that had played out within that spellbinding view.
True to Mull’s saying, four years later I returned to discover more about the island’s geological past. However, it wasn’t until 1971 that I walked the seven milesthe old chapel by the turn off at Tiroran to see ‘Macculloch’s Tree’. John Macculloch discovered it in 1819 when he was collecting information for the first geological map of Scotland that was eventually published a year after his death, in 1836. However, it was Chrissie MacGillivray, a lady then in her eighties, who I met two years later, in 1973 and who was the custodian of the tree for The National Trust for Scotland, that interested me the most. From the start, Chrissie, already ‘a legend in her own lifetime’, impressed me. The book I eventually wrote about her, which is a social history of Burg and Ardmeanach spanning two centuries, begins and ends with the fossil tree.
It was to have been called ‘Chrissie and the Tree’, but its eventual title – Tea with Chrissie – was most apt because thousands of visitors were invited to tea with her. If you visited Mull before her death in 1989 then maybe you were too. Visitors to Mull would always be asked, “Have you seen Chrissie and the tree?”If not, they would be encouraged to do so, despite a long walk. Burg was the home that Chrissie had shared with her parents and seven siblings, and finally just her brother Duncan.
It was the MacGillivray’s ancestral home that, in 1936, became a virtually un-endowed property of The National Trust for Scotland. Although the NTS inherited a ‘pig in a poke’, they were very excited when they discovered they also had an enormous fossil, uniquely preserved, within Mull’s basalt Staffa Suite. With the average chances of fossilisation in sediment being one in a million, preservation within an igneous rock is rare in the extreme.
My story about Chrissie and the history of Burg starts when the tall redwood (which has been assigned to the genus Cupressinoxylon) was about to become fossilised. Imagining how Mull must have been 55Ma, when the super-continent Laurasia split to form the North Atlantic Ocean, my opening chapter describes these events, leading up to the present day:
‘Land destined to become the British Isles existed on the edge of the new European plate, whilst nearby North America and Greenland separated as the Atlantic slowly widened. Volcanoes, created as a result of ocean floor spreading erupted ash and cinders, flooded Europe’s northwestern margin with successive lava flows. Mull occupied sub-tropical latitudes, a landscape very different from today. Instead of mountains and moorland, forests of tall conifers and exotic deciduous trees rooted in rich red volcanic soil carpeted a low landscape shaken by earthquakes. Long before the forests, volcanic fissures opened and erupted ash. Plants colonized the sterile ash and Mull greened with redwoods, magnolias, and ginkgo trees. One tall redwood with a thick trunk was destined to become an anachronism. Visualize what happened that fateful day when violent volcanic activity shook Mull’s ancient foundations.’
‘Lit by pyrotechnics, lava fountains spouted golden, then red from deep fissures. Ash clouds billowed, polluting Mull’s air. Under the crust a magma chamber, engorged with molten rock risen from deep in the mantle, was about to erupt. Mammals and birds fled, but the forest was rooted. An insignificant entity relative to Earth’s rock cycles, the redwood was to have no living thing near it for millennia. Its branches were coated in ash, its needles ablaze when the lava erupted. Enormous volumes of white-hot basalt spewed forth, flowing quickly over the ash-covered land.
Trees fell smothered by molten lava, but Burg’s tree had a stout trunk that withstood the searing onslaught. Charred and branchless its scorched trunk remained upright as fiery rock engulfed it and flowed on to cover 13 square miles. In a natural life cycle the tree should have died, decomposed and enriched the soil. Instead it carbonised and its tissues, imprisoned in gradually cooling solidifying basalt, were preserved. Sappy tissues in the tree presented a surface against which the lava chilled and as it solidified the lava contracted, forming joints at right angles to the tree trunk, creating shapes bizarrely akin to branches. Fossilised and entombed, over time successive lava flows submerged the redwood under thousands of feet of basalt.’
‘Volcanic activity eventually ended and a time of intrusive activity followed, creating Mull’s central igneous complex. In time this too ceased and gradually Mull’s thick volcanic landscape wore down. Global cooling created colder winters and during the cool summers un-melted snow compressed, becoming ice that gradually covered Mull’s landscape. V-shaped river valleys accumulated glacier ice that flowed and cut deeper U-shaped valleys. Thousands of years elapsed during the Pleistocene Ice Ages that shaped Mull’s landscape into harsh peaks and deeply scoured troughs.
Eventually the climate warmed and when the last ice melted 10,000 years ago, no longer depressed by the weight of ice, Mull gradually rose up. Slowly, landform processes during the ensuing Holocene softened the harsh glacial features, forming today’s lochs and glens and the ice-scoured lava flows weathered into giant steps known as ‘traps’. Mull’s basement rocks, depressed below sea level imperceptively rose up creating beaches, cliffs, stacks, and sea caves well above sea level. Glen More’s glacier carved Loch Scridain’s trough, and by the time the first people arrived 6,000 years ago, Atlantic breakers were eroding Mull’s western shores.’
‘Ardmeanach’s coastline eroded under this bombardment. Basalt forming the tree’s tomb was washed away, a final rock-fall releasing the petrified trunk from its straitjacket. The tree found itself a lonely sentinel on Burg’s rocky headland – an anachronism reborn. First hunter-gatherer strand-loopers collecting shellfish, later Iron Age people builders of the fort at Burg gazed on the strange rock column. Celtic missionary Columba, a Prince of Ireland, exploring Mull’s coast may have stopped and wondered at this mystery of Creation. Later, Viking hordes raiding holy Iona’s monastery sailed by, their thoughts on plunder not wonder and during centuries of ignorance when Lords of the Isles claimed Burg, the tree became an unknown and un-remarked quantity.’
‘Throughout its exposure salt spray and winter rain crystallised in its crevices. Over time the massive carbonised trunk weathered and the sea removed fragments. By the 15th century the Ardmeanach Peninsula was well populated, Burg supporting a tenant community owing allegiance to Maclaine of Loch Buie. By now only a 6-foot high stump and a 40-foot cast of the tree remained. These clansfolk might have wondered about the strange rock formation as they collected whelks to eat. At the time that John Maculloch discovered it Chrissie’s forebears lived at Burg together with many other families. Just as the once inviolate tree was eroded by waves and weather, so their traditional lifestyle was threatened. Yet somehow tradition hung on at Burg, due to its isolation and the tenacity of the MacGillivrays, especially Chrissie.’
From Tea with Chrissie – The Story of Burg and Ardmeanach on the Isle of Mull.
While researching the book, I learnt that when gathering sheep around the headland (known as The Wilderness), Chrissie’s father, Malcolm, and later his sons, especially Duncan (who later managed Burg), often passed the cobbled cove where the fossil tree was locked in its ‘straitjacket’ of lava. With their minds fixed on gathering half wild sheep, the tree was unappreciated until the NTS inherited it. Later, first Duncan and then Chrissie, became its custodian. The NTS capped the remaining six feet of trunk with concrete to protect it from the elements (and collectors), so it is the 40 foot high cast of a noble redwood, 55 million years old, that visitors trek to see. And the walk to the tree itself is enough to make anyone want to return to Mull again.
I cover this walk in the next part of this two-part article: Mull’s famous fossil tree (Part 2): Walking to the site.
Scottish Fossils, by Nigel H Trewin, Brocken Dunedin Academic Press Ltd, Edinburgh (2013), 118 pages (hardback), ISBN: 978-1-780460-019-2