How does John Gardner use imagery in Redemption?

Several images recur throughout "Redemption." Skulls, for example, appear three times to remind Jack of David's death. At one point, Jack is alone, driving the tractor in the fields, thinking about the accident and his own guilt, his "sore hands clamped tight to the steering wheel, his shoes unsteady on the bucking axlebeam—for stones lay everywhere, yellowed in the sunlight, a field of misshapen skulls." Jack's identification of the stones with skulls is connected to his memory of his brother's crushed skull in the field. He then recalls his father's story of Lord Byron and Shelley's skulls, another indirect reference to what he saw happen to his brother's head.