How much history is fact?

Untitled Poem

There stands inside the vacant town a byre and a well;
  The byre holds not cattle now, the well no water hides.
The bone-dry well shall tell the tale why none do still here dwell
  But why a cow shed in the midst, with town homes on all sides?

In studies made of history
There oft remains a mystery;
Though authors guess what they don’t know
To make the story better flow,
Yet if you ask an honest wiz
You’ll find the byre simply is.