We see, in the books both in the Old Law and in the New that the men who repented of their sins bestrewed themselves withashes and clothed their bodies with sackcloth. Now let us do this little at the beginning of our Lent that we strewashes upon our heads to signify that we ought to repent of our sins during the Lenten fast.
Father Saunders on the EWTN site has this concerning ashes in the Scriptures.
Ashes symbolized mourning, mortality and penance. For instance, in the Book of Esther, Mordecai put on sackcloth and ashes when he heard of the decree of King Ahasuerus (or Xerxes, 485-464 B.C.) of Persia to kill all of the Jewish people in the Persian Empire (Esther 4:1). Job (whose story was written between the seventh and fifth centuries BC) repented in sackcloth and ashes (Job 42:6). Prophesying the Babylonian captivity of Jerusalem, Daniel (c. 550 B.C.) wrote, "I turned to the Lord God, pleading in earnest prayer, with fasting, sackcloth, and ashes" (Daniel 9:3).
In the fifth century B.C., after Jonah's preaching of conversion and repentance, the town of Ninevah proclaimed a fast and put on sackcloth, and the king covered himself with sackcloth and sat in the ashes (Jonah 3:5-6). These Old Testament examples evidence both a recognized practice of using ashes and a common understanding of their symbolism. Jesus Himself also made reference to ashes: Referring to towns that refused to repent of sin although they had witnessed the miracles and heard the good news, our Lord said, "If the miracles worked in you had taken place in Tyre and Sidon, they would have reformed in sackcloth and ashes long ago" (Matthew 11:21).
One of the most poignant photos of ashes is one most of you will remember.