In the centre of the Sierra Leone capital, Freetown, stands the cotton tree, and so vast are the branches it spreads overhead that its trunk has had to be reinforced with concrete and steel straps, for there is a legend that if the cotton tree falls, catastrophe will come.
It is said that when the tree was planted, the first colonists danced around it to celebrate their deliverance to these shores.
Thousands of people were maimed by militiamen in Sierra Leone
For although this was a British colony, the first colonisers were not British, but African slaves, freed by the British.
The slave trade was abolished in the British Empire in the first decade of the 19th century and the Royal Navy plied the Atlantic, intercepting slave ships bound for America.
The British landed them here, on a strip of golden sand, and when their feet hit the beach, they were free subjects of His Majesty.
They called their colony Freetown, which, more than a century later, would become the capital of the west African state of Sierra Leone, which means "mountains of the lion", even if their country boasted neither mountains nor lions.
The beach is a kind of frontier - and not only between the land and the sea. At Freetown it has been the frontier between freedom and slavery.
But it is also the frontier between the known and the unknown. And it has proved an enduring frontier between civilisations.
The British crossed this frontier in the early 19th century and tried to found a new society modelled on their own - with citizenship under the crown, the rule of law, free speech, institutions of parliamentary government: pioneering European dedicated to recreating Africa in their own image.
They tried to plant the seeds of the 18th century Enlightenment in the soil of tropical West Africa.
Sierra Leone today is one of the poorest countries on the planet.
The last time I was there, veterans of the liberation generation - men and women who had risked imprisonment and death in the last years of British rule in the fight for independence - were earnestly making the case for a return to colonialism.
Blair says reducing poverty in Africa is "the challenge of our generation"
Africans asking for the British to come back and take charge.
How did it happen? How did we get from that noble vision on which the colony was founded to the despair and poverty and violence that has been the Sierra Leonean experience?
The question that needs to be answered is: Why is Africa poor? Why has it remained so underdeveloped? Look at the immensity of its natural resources.
I once flew over the great bend of the Congo River. It is a marvel. In places it is nine miles wide. In terms of the water it empties into the sea, it is the greatest river in the world.
It is the only major river that flows through both the southern and northern hemispheres, crossing the equator twice. It has a thousand species of fish. It has enough hydro-electric potential to power the entire continent and more.
And yet viewed from outer space at night, while much of the northern hemisphere is a blaze of artificial light, Africa has just three discernible glimmers - at Cairo in the north-east, Lagos in the west and the Johannesburg-Pretoria conurbation in the south.
Everywhere else Africa remains - in this narrow literal sense - the dark continent.
In an age of global communications, there are more telephone lines on the island of Manhattan than in the whole of Africa. It is also the silent continent.
So again the question. Why? Why is Africa poor? Our Victorian ancestors thought they knew the answer. It was to do with an innate inferiority.
They built an entire edifice of pseudo-science aimed at demonstrating the intellectual superiority of the white races. Rudyard Kipling wanted the United States to get involved in the "Civilising Mission".
In Sierra Leone, some people now advocate a return of the British
"Take up the White Man's Burden" he exhorted them. "Send forth the best ye breed. Go bind your sons to exile to serve your captives need. To wait in heavy harness, on fluttered folk and wild, your new-caught sullen peoples, half devil and half child."
And should those new caught sullen peoples rebel? The poet Hillaire Belloc gave European empires the reassuring answer to this anxiety: "Whatever happens we have got, the Gatling gun and they have not."
Post-colonial Africa rejected race-based answers to the question. The liberation generation blamed colonialism. When I was a child in the 1960s, we thought of Africa as a place of optimism, the coming continent.
Asia was the basket case of our imaginations. Our parents told us to eat up at mealtimes because children in China and India had no food and went hungry. It was not Africa they invoked.
Bishop Desmond Tutu, with characteristic charm, used to blame the colonialists too.
"When they arrived, we had the land and they had the Bible. They taught us to close our eyes to pray, and when we opened them again, we had the Bible and they had the land."
We are crossing that frontier again, the beach at Freetown. We are preaching good governance, mutual dependence, free trade practised fairly. Are we again trying to remake Africa in our own image?
Maybe. Let us answer the question - why is Africa poor? - honestly and without fear, and hope that this time the cotton tree will not collapse and bring catastrophe.