The history of Icelanders is very connected to Thingvellir. There, the parliament Althingi was established and held for a long time and major decisions were made.
Almannagja is a magnificent rift created because of the earth movement in the Mid-Atlantic Ridge that crosses Iceland.
Access to Almannagjá is more controlled today than it was 15 years ago due to the large number of tourists. At that time in history it was possible to walk to a waterfall called Öxará or Ax River that is interwoven with the history of Thingvellir. It is not high, but it is large in the Icelandic historical context.
Here is a video with aerial view of Ax River and the Almannagja rift.