Mad, walk towards the 3 standing stones on the left of the screen. Place the casket for Nia's protection at the center stone of the three. The crystal on the right should disappear, leaving the road open. Walk over to it, and ride forward until the Lake of offerings. Walk forward, then right, and see a bridge under the water. Walk forward between 2 upright stone carvings and cross to the center island. Place the chest of offering on the green stone table. Now you need to go to the kingdom of the Atrebates, which should be in your destinations now.
If that's not enough to get you moving, tomorrow I'm sure that Mags will see this....
"Once you give up integrity, the rest is easy." Anonymous