point light on terrain problem

hi

i'v got a problem with point lights in my deferred renderer (xna4), they look ok on models, but on terrain they look like this (lightning is not evenly distributed) :

http://imageshack.us/photo/my-images/88/screen0k.jpg/
http://imageshack.us/photo/my-images/8/screen1xgx.jpg/

(bigger bounding sphere represents light range, smaller - light center)

what can possibly be a problem here?

with regards
Przemek