Over at First Things, Peter Leithart discusses why, according to the Angelic Doctor, it was fitting for Christ to die on a wooden cross.

As we enter into the second half of Lent, let us reflect on the crucifixion.

Why did Jesus die on a cross? Thomas asks (ST III, 46, 4). He gives a variety of answers: Adam sinned at a tree, so Jesus saves at a tree; Jesus suffered in the air to purify the atmosphere; Jesus was elevated in death to show us the way of ascent; the four-cornered cross indicates that salvation spreads to the four corners of the earth.