The reason why the stones are so round here is that this location was once a northerly sea shore. After the last ice age, about 12,000 years ago, the sea level was about 120 metres higher than it is today and Halleberg and Hunneberg were islands in the old North Sea.
The waves crashed against the edges of the mountains and the rocks that fell were ground into the smooth, round shapes you see today. The waves carried away the smaller stones and gravel, and left behind the larger stones that now form the cobble field. This is a reminder that there was once an icy cold sea shore around the island of Halleberg.