Judges 1:34

‘ And the Amorites forced the children of Dan into the hill country, for they would not allow them to come down to the valley.'

These Amorites were a strong group occupying, (but not solely), extensive land. Unfortunately for Dan they were in Dan's territory around Aijalon and resisted all attempts by Dan to drive them out. Dan was able to occupy the hill country but not the fruitful plains. Seemingly there were few Canaanites or Amorites in those hills, which suggests living conditions there were difficult. The lands assigned to Dan were fruitful, but for that very reason they were well populated. As we know from elsewhere their faith in Yahweh was so weak that they were disobedient and many of them deserted the territory and made a new home for themselves with their own syncretistic religion (Judges 18). From there we learn of their bent towards idolatry.