The Ridgeway Trail lies on a spine of chalk (limestone) that was thrust up to form a ridge about 30 million years ago, when Africa collided with Europe. It was formed from the skeletons of billions of sea creatures, when it lay horizontal at the bottom of a sea. 70 million years later, the bump from Africa tilted it up and formed the ridge.
The ancients saw it as an already-created road; perhaps holy and certainly convenient. It is mainly flat, it’s out of the way of most rivers (except for the mighty Thames, they largely go around) and so forms a natural thoroughfare. 5000 years ago, it was already in regular use.