Just think. The world is covered with water. Is it any wonder that this mountain is called "the finger of God", the one place that beckons the boat with all living species aboard. Surely, it must have seemed so to Noah and his family, too.
Stories about the finger of God must have gotten around in those ancient days. The magicians to the Pharaoh of Egypt referred to the finger of God (Exodus 8:19) when they were unable to turn dust into lice. The tablets of stone that Moses took with him were written with the finger of God. (Exodus 31:18) It doesn't really say that God used his finger to mark Cain (Genesis 4:15). Cain had a mark on him, nonetheless, that God had put there to protect him from being slain.
It's interesting that in John 8 where the Pharisees were plotting to trap Jesus by bringing an adulterous woman to him that the first thing he does is stoop down and write on the ground with his finger. This particular incident the Pharisees quoted the law of Moses that she should be stoned. They continued to badger him over the law to which he replies, "He that is without sin, let him cast the first stone." And what does he do but stoop down and write on the ground again. (John 8:8) A look back at Exodus 34:1 tells us that God wrote the ten commandments on the tablets a second time.
The parallel here causes one to stop and wonder. The Son of God who is God in the flesh remembers writing those tablets a second time. So do the Pharisees. And they know why they were written a second time. Jesus' actions spoke louder than words to them that day.
Surely they remember the story of the writing finger in Daniel 5 that resulted in the death of Belshazzar for his wickedness in taking the holy vessels from the temple and using them for drinking at his party. Surely they remember the mark that God put on Cain. Surely they remember the Psalm of David's that speaks of God's fingers designing creation (Psalm 8:3)
Last of all they may have remembered the visions of Amos and the chastisement of Israel. "Though they dig into hell, thence shall mine hand take them..." (Amos 9:2) Today, for us, it's Hebrews 10:31, "It is a fearful thing to fall into the hands of the living God."