On the morning of 8 July 1917 the Canadian painter Tom Thomson set out in a distinctive birch-bark canoe from his room at Mowat Lodge on Canoe Lake in Ontario’s Algonquin Park. He had lived there in every season except winter for the previous three years, in the course of which he had undergone one of the greatest artistic transformations in the history of modern art: from modest Sunday painter to an artist of profound originality who, particularly in his hundreds of vividly coloured sketches, was touched by genius.

It is not clear where Thomson was heading, but it was not unusual for him just to take off into the bush to fish, to camp, to paint. Since the spring he had been engaged on a series of daily sketches he intended would document, with his keen naturalist’s eye, the change of seasons in Algonquin. It is clear from these passionate images, which at times veer towards abstraction, that Thomson had big ambitions for his art. He would never get to realise them.

The following day his upturned canoe was seen floating on the lake, but there was no sign of Thomson. With everyone among the small community at Mowat fearing the worst, a search ensued, without success. Then, a week later, Thomson’s body floated up from the bottom of the lake, and so began a century of hand-wringing speculation among Canadians over how this accomplished canoeist could have died on a calm summer’s day on a lake he knew so well. But whether murder, manslaughter or a tragic accident – and theories abound – at the age of thirty-nine he was gone, and it was left to his friends and fellow painters to take inspiration from his work and continue his legacy.

The same inspiration of Thomson’s genius and the brilliance of his artist friends, collectively known as the Group of Seven, is what took me to

Canada in late summer to visit the places they depicted so memorably on wood panel and canvas. Since early September I have been writing a blog of my journey for the major exhibition of their work that is showing at London’s Dulwich Picture Gallery.

My father was Canadian, but it was not until I saw some of these wonderful images a few years ago at the McMichael Collection of Canadian Art, just north of Toronto, that I began properly to appreciate the untameable beauty of Canada’s vast northern wilderness. Previously, I had known only a small area around Toronto, where my father grew up and his siblings still live: a mainly pastoral landscape redolent of the picturesque European scenes these painters were trying to outgrow.

Thomson it was who showed them the beauty of ‘the north’ and, in the words of the artist Lawren Harris, ‘made us partners in his devotion to it’. His death still haunts the imagination of Canadians, but the presence I felt most strongly at Canoe Lake was the spirit of his sketches, though the landscape itself is quite changed. A century ago, the loggers had been and gone, leaving behind them a decimated region with views to the lake through sparse stands of birches where once had stood virgin forest of pine and spruce. Thomson’s friend, A. Y. Jackson, called it a ‘ragged country’, but it is obvious from images like Evening, Canoe Lake or Winter Thaw in the Woods that Thomson found it beautiful.

Today the pines have returned and Canoe Lake is again the centre of tourist activity in the park, though most of the buildings from Thomson’s time, including Mowat Lodge, have disappeared, almost without trace. But the memorial cairn erected after his death, inscribed with a eulogy by his friend J. E. H. MacDonald, still stands at Hayhurst Point, his favourite camping spot on Canoe Lake and the site from where his canvas Spring Ice was sketched.

The sensitive MacDonald was hit hard by Thomson’s death. His former supervisor at the Toronto design firm of Grip Ltd., where Thomson had worked in the last years of the previous decade, he was one of the few to remain in Toronto throughout the First World War. Arthur Lismer, another Grip old boy, went east to Halifax where, in mid-August, I began my journey. He was eventually drafted into Lord Beaverbrook’s Canadian War Memorials Fund, spending much of 1918 painting the troopships and minesweepers that docked at Canada’s largest Atlantic port.

A. Y. Jackson fought on the Western Front, got injured, and then returned to France, also as a war artist. In the final months of 1918 he was joined by F. H. Varley. Between them they created some of the finest paintings to emerge from the conflict. Images such as Varley’s For What? and Jackson’s House of Ypres, which directly address the futility of war, must have been shocking in their day and rival Paul Nash’s famous paintings in their quiet, inconsolable horror.

Before the war, Jackson, Varley and several others from the future Group of Seven had studied at various centres around Europe, including both French and German capitals. The modern approach to colour learned from the Post-Impressionists, added to the lessons in simplified design they absorbed from Art Nouveau, would feed into their later work. But Europe was now a ravaged land, while Canada, though culturally still a very conservative place, offered endless possibilities to any painter prepared to get their hands dirty.

The country the two men came back to in 1919 had been changed by the sacrifice of young lives in the trenches – some sixty-five thousand Canadian dead. Emboldened by a new national sense of independence from the colonial master, and also by Thomson’s brilliant example, these artist-adventurers, led by Lawren Harris, struck out for Northern Ontario. Its harsh landscape is rooted in the vast Canadian Shield, a scrubby and forbidding country of granite rock, dark boreal forest and countless lakes.

Harris became the new group’s prime mover. He it was who discovered the Algoma Central Railway, which since 1914 had served a spectacular mountainous region just north of Sault Ste Marie in Northern Ontario, bringing logs and iron ore to the Great Lakes. Privately wealthy, he had the railway kit out a boxcar for four people to live in comfortably and then shunt it up and down the line to various locations, from where they would strike out into the bush to find things to paint.

There was no shortage of subjects, and Algoma today is still the same tumult of trees and shrubs depicted in their colourful paintings and sketches. In mid-September I took the same train, now popular with tourists, to impressive Agawa Canyon. The route is unchanged, passing by places inseparably associated with these painters – Mongoose Lake, Batchawana, Montreal River – through blazing images such as MacDonald’s Falls, Montreal River and The Beaver Dam.

Frank Johnston – like MacDonald, Thomson and Lismer a former Grip man – was present on those first trips. A founder member of the Group of Seven in 1920, he was nevertheless unsettled by the critical hostility to the first Group show and within a couple of years had very publicly parted company with them. By this time, too, individual temperament had inevitably begun to undermine the Thomson-inspired Group style which was a feature of those early Algoma trips.

Gradually, each man found the places he loved and his own way of painting them. Jackson was above all a gregarious soul who made friends wherever he went. In the course of his long life there is almost nowhere across this vast country that he did not visit in search of things to paint, and in some places it seems everyone has a Jackson story to tell. But the area he returned to every spring from 1921 to the late 1940s, depicted in paintings of great tenderness such as Winter, Quebec, was the farming country along the shores of the stately St. Lawrence River, particularly the small towns and villages of Charlevoix County, some fifty miles north of Quebec City. Communities such as Baie St-Paul, where Jackson often stayed, have in many ways retained their essential character. The centuries-old habitant culture of the original French settlers has given way not to industry and urbanisation but to tourism and a celebration of the qualities that made this region so attractive to Jackson and his friends and generations of painters since.

The Group’s seventh member, Franklin Carmichael, was younger than the rest and took time to find his own style. This achievement coincided with his discovery in 1929 of the La Cloche Mountains, about a hundred miles east of Algoma, along the north shore of Georgian Bay. He loved these white quartzite hills so much he had a cabin built on Cranberry Lake, now on the edge of Killarney Provincial Park, from where he would paddle and hike to favourite painting spots. One in particular, overlooking nearby Grace Lake, as blessed with natural beauty as its name suggests, has become a site as sacred to Carmichael’s aficionados as Canoe Lake is to Thomson’s. The exact location of the rock on which he is shown sitting in a 1934 photograph, sketch in hand, is a secret known only to a few devotees.

After Algoma, Harris felt the call of the north, in the first instance to the north shore of Lake Superior at what is now Neys Provincial Park. Looking out on magical Pic Island he experienced an artistic awakening and, having seen it myself, I can well understand why. If nothing else, the unearthly appearance of this large island, so improbably close to the shore, seems to have led Harris to change his painting style. The rough handling and bold colour contrasts of the early Algoma works gradually give way in his images of Pic to an almost polished smoothness and an ascetic palette of blue, grey and white.

He continued this practice in the Rockies where, from the mid-1920s, Harris came in search of subjects that helped him express his growing interest in theosophy. These he found in mountains such as massive Mount Lefroy, one of the sentinel peaks that tower over much-celebrated Lake Louise, or the pyramidal Mont des Poilus, the subject of his canvas, Isolation Peak. But if Harris could afford to pay his fare, Durham-born MacDonald, never wealthy, was grateful for the Canadian Pacific Railway Artist’s Pass that allowed him to cross the Prairies for free each September to reach these magnificent mountains.

Once MacDonald had discovered Lake O’Hara in 1924, he returned here every year until ill health prevented it. He hiked up to lonely mountain passes and sketched in all weathers, which, as I found, in the mountains sometimes can all occur in the space of a single hour. His painting, Lake O’Hara, with its awe-inducing view of the blue-green lake encircled by mountain ramparts (the same view that greets visitors upon arrival at O’Hara), is a classic image of the Rockies with a graphic quality betraying his background as a designer.

MacDonald’s best paintings, however, are usually his most intimate, depicting unlooked-for places like Lake Oesa, a bleak spot some way above the tree line and several hundred feet above O’Hara. His late masterpiece, Mountain Solitude (Lake Oesa), is imbued with thoughts of death that arise quite naturally in such a place. I arrived there as snow began falling softly, just as in the painting, and was struck by MacDonald’s careful framing of the image so as best to convey the quiet melancholy of this solitary lake.

Until the building of the railway in 1885, the Rockies were a formidable barrier to the free flow of goods and people between the east and west of Canada. British Columbia is still a long way from Ontario today, but even here the Group of Seven left an indelible mark. Fred Varley, like his friend Lismer, was born in Sheffield and immigrated to Canada as a young man. Perhaps the most talented of all these painters, his bohemian tendencies did not go down well in what was then the very conservative city of Toronto. Despite prestigious commissions for portraits – a genre in which he was the equal of his idol Augustus John – by the mid-1920s he was destitute and living with his young family in a tent in the garden of one of his few remaining friends.

The offer of a teaching post in Vancouver could not have come at a better time. The then-small west-coast city was notorious as a cultural backwater, but the new position at least gave Varley a temporary respite from poverty if not from the personal difficulties that beset him throughout his life. In the east, aside from an early but isolated masterpiece, Stormy Weather, Georgian Bay, he had done very little landscape painting. The Coast Mountains of the west, however, seem immediately to have excited him – in particular an area of volcanic ridges surrounding Garibaldi Lake in British Columbia’s Garibaldi Park, about fifty miles north of Vancouver.

The Cloud, Red Mountain is a curious picture in formal terms, depicting only the peaks of mountains and a cloud hovering transcendentally above them in a cobalt-blue sky. Looming over Garibaldi Lake, the dormant Red Mountain, now known as Mount Price, with its former vent Clinker Peak, is in fact composed of ruddy volcanic rock whose colour, I was told, looks even brighter in the alpenglow that occurs at dawn and dusk at certain times of the year. The long hike up to the view of these peaks from the lakeshore took me through soaring forests of western hemlock, past smaller icy lakes turned wondrous shades by tiny silt particles, known as ‘rock flour’, that had washed down from the glaciers over the course of the summer. By early October, when I was there, the water of Garibaldi Lake is a deep turquoise blue.

In the 1930s Harris, Jackson and even Varley would venture as far as the Arctic and, in Jackson’s case, through all the inhospitable provinces of the far north. By this time MacDonald had died and the Group of Seven was dissolved, but the aim they had set out a decade earlier – to establish a distinctly Canadian style of painting – had been triumphantly achieved. Indeed, today small battalions of landscape painters across the country persist with a similar manner of simplified representation, and even the more adventurous artists can find it hard to escape their influence.

I had no such worries. Having come to Canada inspired by their vision of the country, by the time I reached Vancouver I was convinced of two things: that these men had had the time of their lives venturing into the wilderness to paint; and that the scope and the brilliance of their collective achievement, particularly Thomson’s, makes them probably the last great landscape school in Western art. Without their images to guide me, most of these remarkable landscapes are places I would never have thought of visiting. I owe my adventure to the Group of Seven, just as they owed theirs to Thomson.