Well it's not. This is.
The hooning of clunky, laid-back, fat-tyred American cruisers down Californian mountains, quite illegally, was the big bang of mountain biking. XC came later when the Europeans got hold of mountain bikes and, incidentally, doesn't need to demand respect.
Those dreadful old bikes, with usually just the one back-pedal brake, were made by the American bike industry for 20 years. Why? Well, mainly because Americans weren't much into bikes. The idea behind the geometry was to make them look like motorbikes (the worst examples even had fake gas tanks) and the fat tyres were an echo of the low pressure balloon tyres much advertised on American cars of the time.
Their real attraction, for proto mountain bikers, was that they were cheap to buy second-hand, even free for the hauling. And that's important if your intention is to hurl them down a serious mountain.
But the geometry and fat tyres also turned out to be rather brilliant for downhilling...
Read more, only in issue 168 of MBUK, on sale from 23rd December