Origins of Ash Wednesday

There has been a long standing association between ashes and repentance within Christianity stretching all the way back to Tertullian not to mention various grief filled moments of repentance found in the Old Testament. Ash Wednesday marks the beginning of Lent a period of fasting, inner reflection and repentance within the church calendar. For a long time (until about the 8th century), however, the practice of sprinkling ashes or applying an ashen cross to the forehead as a mark of repentance was not used on the first day of lent. Why did this change? Was it a cultural shift? Was it an idea from the papacy or other church authority? Where did this particular practice come from? In essence then what is the origin of the Ash Wednesday ritual traditions?