Getting these in this game too, not always but after 2 of us ran Holjes for about 20-30 mins. Yes they are solid! ;D
(http://i68.tinypic.com/2zs68le.jpg)
Here is how the cones appeared. They appeared some seconds after the car had passed the area. I guess I remember Piboso saying that the deformation calculation does happen some seconds after the area is passed, right? So maybe some variable went out of its range and caused it?
As a note: We did quite some laps on Holjes (2 cars, each about 30 laps), the terrain had changed quite a bit. To the extent, that there was sort of a terrace between the dirt part and the asphalt part. Maybe this helps somehow.
https://www.youtube.com/v/dBk_Q7RG_D4
Btw, it was much fun to race over the terrain :)