There , the sandy , open strip on which the Romans could march easily narrowed at the bottom of the hill . There was a gap of only about 100 m between the woods and the swampland at the edge of the Great Bog . There , the sandy , open strip on which the Romans could march easily was constricted by the hill , so that there was a gap of only about 100 m between the woods and the swampland at the edge of the Great Bog .