The Þingvellir plain is situated on a tectonic plate boundary where North America and Europe are tearing away from each other at a rate of 1mm to 18mm per year. As a result, the plain is scarred by dramatic fissures, ponds and rivers, including the great rift Almannagjá . A path runs along the fault between the cliff-top visitors centre and the Alþingi site.
The river Öxará cuts the western plate, tumbling off its edge in a series of pretty cascades. The most impressive is Öxarárfoss , on the northern edge of the Alþingi site. The pool Drekkingarhylur was used to drown women found guilty of infanticide, adultery or other serious crimes.
There are other smaller fissures on the eastern edge of the site. During the 17th century nine men accused of witchcraft were burnt at the stake in Brennugjá (Burning Chasm). Nearby are the fissures of Flosagjá (named after a slave who jumped his way to freedom) and Nikulásargjá (after a drunken sheriff discovered dead in the water). The southern end of Nikulásargjá is known as Peningagjá (Chasm of Coins) for the thousands of coins tossed into it by visitors.
