The Devils Kettle is an interesting formation. The area is dominated by coherent basalts, but in the area of the creek there is a marked weakness in the strata, where as the material goes from a massive basalt to a more granulated structure dominated by centimeter sized cubes of material. This causes a relative weakness which has largely been eroded out by the creek.
If you look at the tourist manual you will have a description that states that the entire area is basalt flows, one on top of another. This is a description that does a good job of describing most of the north shore of Lake Superior, however while I was up at a cabin on the Gunflint I happened to run into a guide to Cook County geology which shows the are as a rhyolite formation. The transition between these two formations, especially if one was embayed within the other, would explain much of the weakness. I would have gladly explored this translation myself, but I was limited to staying on the trails and did not feel like getting kicked out of the park for dismantling things with a rock hammer.
Many people have tried to determine how the Devil's Kettle actually works. People have suggested that it runs down a lava tube, and many other possible explanations. After looking at it myself I would have to guess that it is actually feeding into a large aquifer that is corresponding to the zone of weakness at the contact zone between the basalt and the granulated rhyolite.
I could be completely wrong, but it is my thirty second translation.
Anyway here are some pictures!