The Geology of Haystack Mountain State Park
Rock Types Found on Main Trail
- Layered gneiss (Precambrian): Gray, medium grained, well-layered gneiss
Minerals of Interest
Interesting Geologic Features
- Glacial polish
- Glacial boulders
- Differential erosion
A road goes halfway up Haystack Mountain to a parking lot with a trail to the top. You can walk up the road, and then take the trail to the top. There is also a yellow trail that starts near the beginning of the road. This geologic description assumes you walk up the road, take the trail to the top, then walk back down the same trail to the yellow trail that takes you back to the park entrance.
Figure 1. Tumbled gneiss, possibly the result of glacial plucking.
The road is a gentle uphill walk, mostly through forest with nice views of a pond. After you walk around the pond, look for a tumble of rocks on the left (Figure 1). Although the rocks appear light-colored from a distance, they are actually fine-grained, black and white banded gneiss, with lichens covering most of the exposed surfaces.
Figure 2. Black and white gneiss, dipping steeply into hillside away from the road. Gneiss consists of light and dark minerals arranged into bands, giving this rock black and white stripes. Lichens cover much of the rock, requiring close examination to see the bands.
This outcrop probably consists of glacially plucked rocks. Even when ice covered all of Connecticut, some ice melting occurred, especially in summer. The water worked its way down through ice fractures to the rocks below and into cracks in the rock. There it froze, expanding the cracks. As the ice moved, it was able to pull away these loosened blocks, leaving them strung down the hillside when the ice later melted.
Figure 3. Large grains of quartz and feldspar are visible in this outcrop of pegmatite, an igneous rock.
Just up the road there is a similar outcrop of the same gneiss.
A different rock appears at the end of the road, at the top of where the trail begins.
This rock is very coarse-grained, and light in color. It is pegmatite, an igneous rock.It was once melted, then cooled so slowly that large grains were able to grow in it (Figure 3).
Please don't try to climb this outcrop because the rocks are very unstable.
Figure 4. Glacially smoothed gneiss in the trail
Walking along the trail, you will soon come to a smooth outcrop that crosses the trail (Figure 4). Glaciers probably polished this rock smooth. As the ice moved over it, fine rock material at the bottom of the ice continually rubbed across it like giant sandpaper, giving it the polish you now see.
Continuing on toward the top of the mountain, you will pass an outcrop of black and white gneiss dipping steeply into the hillside. Although this rock is a gneiss, the same type you have been passing, it does not have obvious banding (also called foliation).
Figure 5. This outcrop of gneiss shows distinct black and white banding.
Farther along, look for a well-banded black and white gneiss in the trail (Figure 5).
Figure 6. A steeply dipping outcrop of amphibolite gneiss, covered with lichens. In the lower right corner of the picture, the black and white banding is somewhat noticeable.
Near the top of the trail, after the steps, notice a steeply dipping outcrop on the right. Although difficult to see because of the lichens, this thinly banded black and white rock is amphibolite gneiss. The black mineral grains are hornblende, a member of the group of minerals called amphiboles.
Although the hornblende seems black, it is actually very dark green (Figure 6).
Figure 7. The tower on Haystack Mountain offers wonderful views in all directions. It was built in 1929 out of black and white gneiss.
Congratulations, you have now reached the top of Haystack Mountain! The tower in the clearing was made out of local gneiss. Notice how obvious the black and white banding is, when only a few lichens cover it (Figure 7).
For an incredible view in all directions, climb the tower. It is not really as high as it looks. From the top, on a clear day, you can see into Massachusetts and New York states (Figures 8 and 9).
Figure 8. Looking toward Norfolk, and the hills beyond.
Figure 9. The view west into New York state.
After enjoying the views, head back down the trail. When you reach the yellow trail, follow it back to the beginning of the road. Do not go this way if you parked at the end of the road halfway up the mountain, because the yellow trail takes you back to the park entrance.
Figure 10. This is a boulder of schist, a flaky, mica-rich rock. The layers that appear pink in the picture are orange and red in the field. It is a glacial erratic, because it was carried here from some other area where the rocks are different from the rocks it is sitting on.
Look for a boulder on the right made of schist, a rock very rich in micas that flake off easily. This boulder has red and orange stained quartz layers in it (Figure 10).
Another trail leads northwest from the road, at the northwest end of the pond, near the water spigot. The map shows the trail with no blaze, but there were new, yellow blazes on the trail in August, 2004.
Figure 11. Lichen-covered, dark-gray gneiss. Look for areas with no lichens, to see the dark and light banding common in gneiss.
This is a pleasant, wooded trail with only a few outcrops. The first one on the left, uphill from the trail, consists of dark gray, fine-grained banded gneiss. Lichens cover most of the rock, but the banding is visible in a few places (Figure 11). The bedrock along this trail is made of the same type of rock. A short, side trail off the main trail goes over a smoothed surface of such gneiss.
Figure 12. North Brook. Notice the large boulders of a variety of rocks.
Follow the trail to its end, at a nice stream (North Brook) (Figure 12).
The boulders in the stream are not all the same dark, gray gneiss found along the trail. Many of these were boulders left behind by the glaciers as they melted.
Figure 13. This pocket beach is a mixture of sand, and larger gravel. Near the water, sand was deposited over the gravel, showing some of it through the center of the sand.
Tiny pocket beaches have formed in areas sheltered by large rocks. During higher water, the sand is moved along the bottom of the stream, then dropped in areas where the water velocity slows down (Figure 13).