English writting

Wrote this text to acolege assigntment, after searching in a Grammar, I think I understood all the verb forms, but before I send it, could any one tell me if everything is ok, if not underline so I can change it to the right form.

Many thanks.

For hundreds of years its very name has been a synonym for the most remote place on earth, but today Timbuktu faces a new kind of extremity. From the 13th century onwards this forbidden city has carried on an impenetrable existence in the heart of Africa. But to its inhabitants and those merchants able to converge on this crossroads of the ancient caravan routes, the town on the fringes of the desert has been a haven from the unyielding harshness of the Sahara, a place of rich grazing for cattle and camels, a university town, a revered centre of worship and a market place of both commercial and cultural exchange.

During the last two decades, however, the desert has reclaimed Timbuktu. There has been drought here for the past 15 years; for the past four years it has been increasingly severe and this year the area has had only half as much rain as in the previous year. The sands have moved south. Every year the encroachment continues.

Now the desert sweeps around the town and surrounds it entirely. Occasionally it is possible to glimpse beneath the dust the baked clay surface of what was once a fertile loam. But that is rare.
Huge dunes of sand creep onwards, thousands of tons at the rate of 20 miles a year. The fine white dust falls imperceptibly from the heavens. There is an apocalyptic quality to living with the earth above your head for so long.

Twenty years ago it was possible to arrive in Timbuktu by boat along a canal dug from the Niger river. Today, the channel is entirely dry, its side cracked and crumble, with heaps of old rubbish on its bed. Even the mighty Niger dries up; in a good year it now flows for only seven months.