These are rocks of the Vassalboro Formation(Silurian-Ordovician in age), seen along I-95 going south from Bangor, near Etna. They are phyllites, a metamorphic rock containing small crystals of mica which (a) give the rock a sheen when seen in the right light, and (b) which cause the rock to split easily along certain planes (=cleavage). The former bedding planes lie diagonal in this picture, bottom right to top left; the cleavage planes are almost vertical, seen as streaks in the middle of the picture. This kind of cleavage is called "slaty cleavage", and is the source for slates used in roofing, because the rock can be split into even, uniform layers. The same kind of rocks (also Vassalboro Fm) are seen between Union Street and the Mall along I-95, and in Kenduskeag Gorge.