There were four intrusive events: 1) diorite (mainly composed of feldspar, hornblende and biotite) is light grey, medium grained, and takes up most of the northern two thirds of the face; 2) this is intruded by grey-pink biotite granite, which is a bit coarser. The granite has a complex, wispy contact with the diorite, but a horizontal band of it can be traced along the face to within 30 m of the north end; it contains fragments of diorite; 3) at the south end of the face, the granite (mostly white or pink) is intruded by fine-grained pink granite, which near the contacts with granite #2 is chilled to a glassy black or dark-brown rock with feldspar crystals; 4) very dark grey diabase dykes, a few centimetres to several metres wide, cut #1 and #2 all along the face on the west side of the road, while on the east side, one of them is also chilled against granite #3. A very large dyke in the north half of the face has white feldspar crystals which are absent where the dyke is chilled against #1 and #2. Note how the granitediorite contact can be traced from block to block across the dykes that interrupt it.
Folded black shale, which was deposited as organic-rich mud during the Ordovician, is interbedded with sandstone and chert on the TCH just west of the Red Cliff (former) railway overpass, 3.4 km east of Thunder Brook. There are exposures on both sides of the highway.
The shale is both thick and thin bedded; sandstone beds are grey, but weather rusty brown. Chert beds are black to dark grey, and show typical curved "conchoidal" fractures. On the north side about one third of the way from the east end is a large fold in shale and sandstone. The shale bends easily around even small, tight folds, but the sandstone layers are brittle and tend to break into segments. This is also true across the rest of the face.
Two types of dykes cut across the shale at the east end: dark green diabase and rusty brown rhyolite. The dykes are younger than the sedimentary rocks, and because a rhyolite dyke cuts a diabase dyke, the rhyolite is known to be younger than the diabase.
Shale beds are thicker in the west end and contain pyrite, as do some of the sandstone beds. The shale there, and on the south side, contains small fossils called graptolites, which look like finely branched ferns pencilled on the rock. Look for them on the flat, split layers of the shale.
This red sandstone has herringbone crossbedding, which is formed by tides moving in and out in a shallow-marine environment. The main beds dip away from the road. Near the "Road Narrows" sign, sets of crossbeds are at an angle to the main beds. Each set of crossbeds has been cut off at its top edge by current erosion before deposition of the next set, and the fact that the cutoffs are still uppermost indicates that, although tilted, the rocks remain the original way up.
At 20 m south of the sign, near the south end of the cliff, green sandstone beds have ripple marks of different wavelengths from bed to bed; these show that the beds were deposited under different conditions. The ripples probably formed by wave action; large ones imply large waves in deeper water, and small ones small waves in shallower water.
Some of the ripples have sharper crests than troughs, which indicates the top surface of the bed, not an underside mold. Therefore the beds, though steeply dipping now, are right way up, and get younger toward the road.
A little north of centre of the outcrop is a red shale bed with lines of green sandstone running across it forming a polygonal pattern. These are mud cracks formed when red mud dried out and cracked in the sun. The cracks were filled by green sand when the next bed above it was deposited.
Look at the red shale side on and see that it breaks along a vertical cleavage which is oblique to the beds. The cleavage was formed when the rock was compressed during mountain building, and shows mainly in the shale because it has more platy minerals.
Newfoundland and Labrador Traveller's Guide to the Geology
Edited by: S. Colman-Sadd and S.A. Scott, 91 pp. + map, 1994