The Geology of Mount Blue State Park

Specific Objects of Geological Interest

The Mount Blue Trail

The Mount Blue Trail starts at the parking area at the west base of the mountain, at an altitude of 1380 feet. There is no exposure of bedrock near the trail until one ascends to an altitude of 2050 feet, which is approximately three-eighths of the way to the top of the mountain. Here, if one follows the sign labeled "Spring," he will find a waterfall cascading over an outcrop of metamorphic rock and pegmatite. On the north (far) side of the brook are outcrops and large loose blocks of micaceous granulite. Very little of the texture in the rock can be seen due to the buff color produced during weathering. Bedding can be seen with difficulty and appears as a layering produced by bands, each about 1/16 inch thick, which contain varying amounts of the mineral biotite. The strike of the bedding is northwest, and the dip is steep to the northeast. These two terms are used to describe the attitude of an inclined plane such as bedding. Strike is the direction of the intersection of the plane with a horizontal plane; dip is the angle at which the plane is inclined from the horizontal (see Figure 4).

The pegmatite, best seen in blocks at the foot of the falls, is in place right in the middle of the falls. It is composed of quartz, feldspar, muscovite, and much smaller amounts of biotite, apatite, and black tourmaline. The outcrops of pegmatite show a feature known as sheeting, which is a set of joints, or fractures, having a low angle of dip. In this case the sheeting dips slightly to the west, which is downstream.

Starting at an altitude of about 2800 feet, approximately four-fifths of the way to the top, one first begins to come across abundant loose blocks of the cyclically bedded rocks that are exposed on the summit. If one takes a close look at this rock, he can see that it is made up of alternating bands of two types of rock. One of these is light in color and is composed of fine-grained quartz, feldspar, and some mica. In a number of cases, within these layers of micaceous granulite, there is a banding roughly parallel to the bedding. This is produced by very thin layers, or laminae, within the rock, containing varying amounts of mica. The other, darker band of the rock is schist. It is coarser-grained than the micaceous granulite. In this layer are tiny pink crystals of garnet; narrow slit-like holes on the surface of the rock indicating places where biotite flakes have weathered out; shiny, clear, platy crystals of muscovite; and the ever present quartz and feldspar. If a fresh, unweathered piece of this rock is examined, biotite is seen as shiny, dark brown, platy crystals. In addition to these minerals the schist contains sillimanite, which usually forms in colorless, needle-like crystals too small to be seen without the aid of a magnifying glass.

At an altitude of approximately 2950 feet, or only 150 feet in altitude from the summit, the climber finds the first outcrops on the trail of the cyclically bedded rocks. These appear as thick slabs dipping into the mountain. The strike of the beds is northeast, and the dip is moderate to the southeast. Here in outcrop are all of the features that have been mentioned in the description of the loose blocks lower on the trail. In addition, a vertical joint set striking N 15o E may be noted.

Between here and the top of the mountain are numerous outcrops of cyclically bedded rocks. A number of these show well-developed folds. Very good exposures of these rocks are found on the summit of Mount Blue. At the base of the stairs to the fire tower is an excellent outcrop showing a fold in the bedrock. The dip of one of the limbs of this fold is steep, the other is quite shallow. The axis of the fold plunges, that is, makes an angle with the horizontal, toward the northeast at 25o. See Figure 5 for a sketch of a fold with all the parts of the fold labeled.

parts of a fold
Figure 5
bedding plane joints
Figure 6

The large outcrop at the summit has several joints. These are all steeply dipping, and their strikes are: N 25o E, N 25o W, East, and N 75o E. This last joint set is developed in the plane of bedding (see Figure 6).

Thirty-five feet east from the fire tower is a good exposure of a pegmatite dike in metamorphic rock. The minerals making up the dike are typical: quartz, feldspar, muscovite, garnet, and sparse crystals of black tourmaline (see Figure 7). This dike can be traced toward the fire tower and is clearly thinning in that direction. It is represented in the large exposure on the very summit only by several thin veins which die out altogether farther to the southwest. At the base of this large outcrop is another pegmatite dike, somewhat thicker than the one previously described. The former mentioned pegmatite is nearly parallel to the bedding in the metamorphic rock, and thus can almost be called a sill, this latter one very definitely cuts across the bedding (see Figure 8).

pegmatite dike
Figure 7
pegmatite dike
Figure 8

Center Hill Area

The Center Hill area of Mount Blue State Park has, in addition to a magnificent view and fine picnic facilities, many features of geologic interest. Along the twisting driveway to the picnic area, on the right (south) side of the road, about 125 feet from the parking lot, there is a large erratic of folded cyclically bedded metamorphic rock and pegmatite (see Figure 9). The pegmatite had been injected into the rock before folding took place, as the pegmatite vein itself has been folded in the same manner as the metamorphic host rock. The pegmatite is composed of quartz and feldspar. The bedded metamorphic rock contains quartz, feldspar, muscovite, biotite, garnet, and sillimanite. All but the sillimanite can clearly be seen on the surface of the rock. Looking closely at the surface of this boulder, one can see that the limbs of the various folds are symmetrically disposed to little ridges on the surface of the rock, which are made of flat crystals of muscovite oriented parallel to each other. These thin ridges, which in three dimensions, are planes, define a structure called "axial plane schistosity," because these planes are parallel to the axial plane of the folds (see Figure 5). These muscovite flakes are symmetrically oriented with respect to both limbs of folds and grew in such an orientation during the folding of the rock. The flat faces of the crystals are perpendicular to the compressive forces which were responsible for the folding.

Several outcrops can be seen on the south edge of the parking lot, they are mostly pegmatite. The glassy, slightly bluish crystals are quartz; the milky to buff crystals are feldspar; the small, spherical, pink grains are garnet; the shiny, platy, deep brown crystals are biotite; the shiny, platy crystals which are either colorless or are slightly rusty stained are muscovite. In a number of places, quartz and feldspar are seen mutually intergrown, forming what is called "graphic granite," because this effect suggested to an earlier investigator the appearance of an early Babylonian cuneiform writing.

Forty-five feet uphill (to the south) of these last outcrops is a boulder of a rock which before metamorphism was composed of thinly bedded pure limestone and micaceous sandy limestone. During metamorphism, the layers of pure limestone were converted to a light gray marble and the layers of micaceous sandy limestone into calc-silicate granulite. This latter rock has a distinct greenish color from the presence of the calcium, iron, magnesium, and silicon rich minerals, diopside and actinolite. The marble beds erode much more easily than the calc-silicate granulite, because the mineral calcite, which forms the marble, is more readily dissolved by running water than the more resistant calc-silicate minerals diopside and actinolite. This type of rock was flexible during deformation and folded rather easily, as can be seen by its characteristic tight folds. Subsequent to folding, this rock was intruded by a vein of pegmatite (see Figure 10).

The large outcrops about 150 feet uphill are composed of pegmatite. They are thoroughly covered by lichens and moss, and only in a few places can one see the minerals. To the south of this outcrop, in back of the shelter - lookout hut, is a large outcrop of cyclically bedded metamorphic rock and some pegmatite. The surface of this outcrop is also covered by vegetation, but on this surface may be noted a set of grooves and striations, which were scraped out by rocks dragged at the base of a glacier. These striations run downhill, generally in the direction of the parking area.

Were one to follow the trail which starts at the parking lot, to the "Overlook Ledges," he would be rewarded not only with a magnificent view of the whole of Lake Webb and a more than 180 degree wide vista of the mountains, but also with outcrops more free of lichens than most in this area. The rocks are folded, cyclically bedded schist and micaceous granulite, injected by numerous veins of pegmatite. As elsewhere, the pegmatite is easily identified by its light color. The metamorphic rock is indistinguishable from that found on top of Mount Blue. However, a glance at the geologic map will show that it is stratigraphically below, i.e., older, the thin unit composed of calc-silicate granulite and micaceous granulite. The rocks on top of Mount Blue are stratigraphically above, i.e., younger, than this calcareous unit. Glacial grooves and striations are prominent in the outcrop on Center Hill, especially near the highest knob (see Figure 11). Another feature which is the result of erosional, grinding action of he moving glacier is the steep wall, facing toward the southern part of Lake Webb, on a portion of the main outcrop not far from the high knoll (see Figure 12). As the glacier moved over Center Hill, from northwest toward southeast, it "pulled" on the southeast sides of outcrops, loosening blocks and plucking out pieces that were not firmly attached to the bedrock. In this case parts of the outcrop were not firmly bound to the bedrock due to a joint set. Such examples of "plucking off" of rock on sides of hills in the direction of the motion of the glacier (the lee side) are very common in once glaciated terrain.

glacial grooves and striations
Figure 11
glacial plucking
Figure 12

Introduction   Geologic History   Objects of Interest   Glossary   Geologic Map (pdf)

Last updated on January 11, 2008