Fig. 9. Geologic Map of Mt. Philo State Forest Park (after C. W. Welby. 1961). Because Ogf and Oib were not definitely identified by the author of this pamphlet and for the sake of simplicity, these rock units have not been discussed in the text of the pamphlet. Some dip and strike symbols have been added to Welby’s original map.
LEGEND UPPER MIDDLE ORDOVICIAN Oib Iberville shale Osp Stony Point shale Ogf Glens Falls limestone LOWER CAMBRIAN Cm Monkton quartzite Park roads Other roads Contour line Approximate park boundary Dip and strike symbol. Layers dip 21° toward N.E. Approximate contact of rock units. Surface trace of thrust (low-angle) fault, carat on upthrown side Inferred trace of thrust (low-angle) fault, carat on upthrown side Surface trace of high-angle fault Dip and strike of cleavage Observation tower
A second type of rock is exposed in the south bank of the exit road approximately 0.7 miles from the summit area. This is the black[10] to bluish-black Stony Point Shale (see [Fig. 10]), which underlies the Monkton quartzite. This shale, or hardened limy mud, is thinbedded and shows abundant cleavage[11] parallel to the layers or beds. At this outcrop the layers strike to the northeast and dip 20 to 40 degrees toward the southeast. The dip and strike of the Monkton Quartzite (see [above]) is not similar to the dip and strike of the underlying Stony Point Shale. It follows, that the layers of the Monkton Quartzite are not parallel to those of the Stony Point Shale.
Fig. 10. View looking south of cut-bank, south side of exit road, about 0.7 miles down from the summit parking area. Here the layers dip 20 to 40 degrees toward the southeast and strike in a northeast direction. Note that the shale is thin-bedded and contains numerous cleavage planes parallel to the layering. The handle of the geologic pick is about 1 foot long.
The fact that these two units are not parallel could mean that the Stony Point Shale was deposited, hardened into rock, uplifted, folded and eroded, all prior to the deposition of the Monkton Quartzite. But, first, what is the age of the underlying Stony Point Shale? If the story is as listed above, the Stony Point Shale must be older[12] than the overlying Monkton Quartzite. From the fossil animal remains found in the Stony Point Shale, geologists have dated the Stony Point Shale as upper middle Ordovician (see Standard Geologic Time Scale, [Fig. 4]). And so, here we have older rocks (Lower Cambrian) resting on younger (upper middle Ordovician).
Fig. 11. View of Mt. Philo, looking toward the northeast. The black line approximates the position of the thrust fault. The Monkton Quartzite, which is above the line, was thrust westward over the Stony Point Shale (note the arrow). Line A-B, [Fig. 9], approximates the section.
Structural Geology
How can we explain this inverted order of rock units? The geologic evidence presented in the Park does not indicate that folding of the rocks was responsible. From the surface distribution of the two rock types (see Geologic map, [Fig. 9]) and the nature of their contact with each other, a fault relation is envisioned, in which older rocks were thrust westward over the younger and thus to rest upon them (see [Fig. 11]).