Once upon a time? Yes, well once upon a time the Portsmouth area was just one tiny part of a gaseous and extremely hot ball, spinning around the sun at a fair old lick - but then that's not the "once upon a time" that we're after, is it.
Rather, we need to come forward to a time when people first started appearing in the area, but just briefly, we should pause as we pass through the cretaceous era, which ended about 65 million years ago and gave way to the cenozoic era, which we're still in now, as it happens.