It is believed the boulders was erected during 500-1000 AD.
The stones work as a guide to the Summer and Winter Solstice, and calender. It is an amzaing formation of rocks. We listened to the guide give a talk in Swedish, (so not much was understood), but we were able to get the gist of what he was talking about, when he was pointing to the different rocks and the sun.
For more information on the Ales Stenar go the the following website, by Bob Lind.