Rocks of Cambrian(?) and middle Ordovician ages are the oldest known rocks exposed within the Munsungun anticlinorium of north-central Maine. The age of the Cambrian(?) rocks is inferred from stratigraphic position and physical correlation with rocks in Maine, Vermont, and Quebec. Ordovician rocks are dated as middle Ordovician (Caradocian) on the basis of graptolite and shelly faunas found within the southern end of the anticlinorium.

Both Cambrian(?) and Ordovician rocks were deformed during the Taconian and Acadian orogenies; the Cambrian(?) rocks had been deformed previously, during a tectonic event that occurred before deposition of middle Ordovician sediments. Comparison of the effects of the three periods of deformation indicates that the earliest was the most penetrative and that the Taconian was the least intense. These relations and evidence of pre-middle Ordovician deformation in Maine, Vermont, and Quebec reported by other workers indicate an event (or events), in at least this part of the Appalachians, late in the Cambrian or early in the Ordovician. This event occurred before the Taconian orogeny and, at least locally, was more intense.