If you go back far enough in time, it turns out that tectonic plates or sliver of a tectonic plate that Katahdin formed from was from away. This sliver shmooshed1 into what is now North America over 400 Million Years Ago (Ma). Geologic time and tectonic plate movement is hard for me to comprehend. While 400 Ma is a very long time, the moment of tectonic plates is very slow. For example many plates move 20 to 100 mm in one year. We are talking just a few millimeters a year. So this shmooshing of tectonic plates need many millions of years to occur.
Once the tectonic plates were shmooshed, exciting things began to happen, earthquakes, volcanoes. These volcanoes2 produced Traveler Mountain just to the northeast of Katahdin. It appears that the Traveler mountain eruptions of ash and magna, once covered what is now Katahdin. But over the 400 Ma between then and now, much has been eroded away, leaving Katahdin with its very tough granite standing.
This is just my uneducated snap shot of where Katahdin came from. The above booklet has much more information and of course wiki and other books can go into much more detail. But that’s as far as I am currently going to dig. Now onto glaciation.
1Shmooshed is my term, not a geologic term
2These volcanoes were most likely world class events. In other words they were powerful enough to effect the world’s climate as well as local geography.