Step back in time millions of years and imagine a glacial lake with prehistoric animals on this very spot. Use this field guide to interpret what you see. The history of the world is written in these rocks.
1. About 200 feet beyond the Field House on the right is an area where fill has been dug out. Here are glacial erratic boulders gneiss), and granite dropped by melting glaciers 20,000 years ago. Glacial erratics are usually rounded rocks of material, different than bedrock.
2. The bedrock of gneiss is part of the Manhattan Formation. This outcrop shows how the once-flat layers were uplifted and tilted to the west.
3. The exposed ledge in the road is part of the Manhattan Formation (gneiss) and contains lenses of pink feldspar.
4. The road coming in to your left leads to the quarries. Beneath the big white oak on your right, the pegmatite (once hot molten rock) is in contact with the Inwood Marble. The action of the hot magma caused the mineral scapolite to form. Notice how the rock is being split by the pressure of the tree.
5. The road and flat hill on your right were made of spoil from the quarries. The quarries were used for agricultural lime, bur were not very productive, and were consequently abandoned at the end of the 1800's. Uplifted Quarry One shows the jointing structure - the way the rock layers break into blocks. Calcite crystals are found along the joint cracks. The north wall exhibits differential weathering where some areas are more resistant than others and stand in relief. The dark brown spots in the marble are limonite. This type of limonite formed when pyrite crystals changed chemically into iron rust. Occasionally, agronite forms in joint cracks as very small galactic masses.
6. Go up the erosion control steps made of railroad ties and follow the high trail to the right. When your reach the wide level, there will be an outcrop of pegmatite on your right. This is of low-temperature origin and contains the minerals: microline feldspar, black tourmaline, quartz, biotite mica and gamets. Inclusions of the Manhattan Formation (gneiss) are contained in the pegmatite. This occurred when the hot, molten magma cut through the gneiss, breaking off in fragments. When the magma cooled, the gneiss remained suspended in it. The leaning rust-colored rock is a fragment of the Manhattan Formation (gneiss). Iron rust was produced by weathering. The whole hill is termed a "rock drumlin" and was rounded by glaciers moving over it. The shape of the hill was determined by the pegmatite since it is more resistant to wear than marble.
7. Just beyond the left is the rim of Drag-folded Quarry Two. This quarry was also mined in the 1800's. You will be walking into this quarry at station 13, where there is additional information about the rock formations.
8. Going back a few feet to the pegmatite outcrop, follow the narrow trail on the far side to see the Giant's Chair. This was once part of the main ledge and was formed when marble weathered away and left the more resistant pegmatite. Much of the rock falling below made a talus slope. To the right of the Giant's Chair, the marble can be seen in contact with pegmatite. A zone of actimolite is visible at the contact. Return to the main trail and cross the rim of Drag-folded Quarry Two. Notice the sandy path of calcite crystals from the weathering marble.
9. Straight ahead is a pegmatite outcrop with two dogwood trees growing out of it. The trail narrows and goes down to the left past a natural wildflower garden whre hepatica, bloodroot, columbine, Solomon's seal, Solomon's plume and anemone grow.
10. Go to the right of the fireplace and follow the trail toward the clearing. On your right is a small quarry test pit.
11. Looking out toward Danbury from the knoll is the preglacial valley which once contained the glacial Lake Housatonic. Most of the city of Danbury rests on the sediments of this lake.
12. Return to the fireplace and take the lower trail through a thick stand of "scouring rushes" (equisetum). These are prehistoric trees and once grew 30 feet tall. They are quite abrasive and colonists tied them in bundles to scour pans.
13. Looking into Drag-folded Quarry Two, you will see pronounced drag folds on the north face, which are of special interest. These were formed when the rock layers were uplifted. It took tremendous pressure to make the rock bend without breaking it. This is an excellent example of mountain building activity. You will see large boulders of pegmatite which contain rose quartz. Finally, look closely at the wall of the quarry and you will see that a rock formation in the center is in the shape of a horse.
14. Return to the white oak and the main road. Turn left and at about 50 feet on your right, you will see the entrance to Dynamite Trail which leads back to the Field House. The swamp on your left rests on sediments of blue clay, sand and peat moss which settled to the bottom of the glacial Lake Housatonic. In 1967 several ice age mollusks were found here.
15. The dynamite was stored quite a distance from where the men would be working in the quarries, in case it blew up. It did blow up at one point killing the night watchman. Remains of the old foundation are visible down the embankment near the bed in the trail.
16. As you near the Field House, the large outcrops on the right are of the Manhattan Formation. Garnets and some magnetite (an iron ore) are found in the gneissic material.
17. Before you leave Old Quarry, notice the Indian mortar near the road between the Field House and the parking lot. It was donated to the Nature Center by Walter Fanton, who discovered it at the airport where it had been dumped with a load of fill. It is a community mortar between 800 and 1000 years old. The peck marks are from pounding grain and the channel is where the meal ran down into a basket.
John A. Pawloski attended the Colorado School of Mines and holds a degree in Science Education at Western Connecticut State University. He has written a field guide of geological points of interest in Connecticut.