The Geology of Osbornedale State Park
Rock Types Found on Main Trail
- Carrington Pond Member
rusty weathering schist and
light gray gneiss
- Harrision Gneiss (Ordovician):
Medium grained foliated gneiss
Minerals of Interest
- Quartz crystals
Interesting Geologic Features
- Parallel Fractures
- Natural Spring
Osbornedale State Park is a great place for any geology enthusiast. Not only are there several different rock types exposed in the park, but also within the park's boundaries there are geologic folds, quarries, and abandoned mines. Additionally, families can enjoy the wonderful Kellogg Environmental Center that is on the park's property
Figure 1: An example of folds found in the Park
As you enter the park on the red trail, you first encounter a large outcrop of schist, a metamorphic rock that has undergone intense heat, pressure, and the actions of hot fluids. By definition, schist contains more than 50% platy and elongate minerals such as mica and amphibole. This high percentage of platy minerals allows schist to be easily split into thin flakes or slabs. Part of the Carrington Pond Member, this schist outcrop is an inter-layered gray, rusty weathering schist. The schist outcrop contains several quartz crystals, parallel fractures, and folds. Folds are formed by tensional stress that permanently bends and deforms the rock, as opposed to breaking or fracturing the rock (Figure 1). This can only occur when the rock is deeply buried, where it experiences great heat and high pressure.
Figure 2: Folds within an outcrop of the Harrison Gneiss.
As you continue down the red trail, you will encounter a quarry. The area quarried is part of the Harrison Gneiss, a dark gray gneiss that contains biotite, quartz, and feldspar. Similar to the Carrington Pond Schist, the Harrison Gneiss shows extensive folding as well (Figure 2). Gneiss (pronounced "nice") is a high grade metamorphic rock subjected to intense heat and pressure during formation. Gneiss is easily identifiable by the segregation of light and dark minerals giving it a banded texture.
Figure 3: A wide view of the quarry.
Gneiss usually consists of mostly elongated and granular, as opposed to platy, minerals. The quarry was active at one time and still shows evidence of rocks being removed from the site (Figure 3).
Further down the red trail, there is an abrupt change in the geology. The rocks along the trail change from gneiss to basalt and then back to gneiss. This transition of rock types is due to a dike, which is a sheetlike body of igneous rock that cuts across layering or contacts in the rock into which it intrudes. The dikes located in Osbornedale State Park are all feeder dikes, which help supply the large lava flows in the Hartford Basin 200 million years ago. This particular dike is composed of diabase, a fine-grained igneous rock that cooled rapidly near the earth's surface. In Figure 4a, there is a clear contact between the basalt, which is very dark, and the gneiss, which is gray. In some areas along the trail, the basalt dike is nearly 100 feet wide. After you cross the dike and the stonewall along the trail, you will reach another outcrop of gneiss, indicating you have left the basalt dike area.
Figure 4: (a) Contact between gneiss and basalt.
(b) Intrusion of basalt into country rock.
(c) Finger pointing out contact between basalt and gneiss.
(d) Basalt on left, gneiss on right.
Following the trail down the hill, you will encounter a large pegmatite outcrop. Pegmatite is an igneous rock that formed from molten rock buried deep below the surface of the Earth. Since the molten rock was well insulated beneath the surface of the Earth, it cooled very slowly, allowing the crystals to grow very large. Pegmatite generally has grains larger than 1 cm in diameter, by definition. This pegmatite contains feldspar, quartz, and biotite. Pegmatite intrusions are of great interest to mineral collectors because they may contain a variety of rare minerals. However, this pegmatite outcrop does not contain any rare minerals (Figure 5).
Figure 5: Coarse-grained pegmatite outcrop.
After the small pond where the red trail re-enters the woods, an outcrop of gneiss is visible on your left. Likewise, there is another outcrop on the right next to the parking lot.
These exposures of gneiss look different than other exposures in the park. Here the gneiss is lighter in color and coarser in texture. After you pass this outcrop, the trail continues up a small hill.
Figure 6: Person standing in depression from mining activity.
Working your way up the hill, you will begin to notice small boulders of broken off basalt lying on the ground. The small boulders of basalt indicate that you are re-entering an area of the park through which the feeder dike to the Hartford Basin runs.
At the top of the hill along the red trail there is an area that may have once been a site for mining. There are several small depressions still visible on the ground. In fact, these small depressions have rock piles nearby which indicate mining activity probably occurred in the 1800s (Figure 6 & 7). Mining activity may have played a role in the formation of several stonewalls that line the site.
Figure 7: Rock piles associated with mining activity.
The stonewalls may have been constructed from loose boulders that were excavated during mining activity. Records from this time period, however, are not available to validate this belief. Also, close to these depressions, where the yellow trail and blue trail meet, is a beautiful stone lined native spring through which water naturally flows (Figure 8).
Figure 8: Natural stone lined spring.
As you exit the park from the blue trail, you will walk through a small valley that once may have served as a road to the mines.
For more information on the geology, biology, and historic features of Osbornedale State Park, visit the Kellogg Environmental Center located at the park entrance.