DEEP: Geology of Gillette Castle State Park

The Geology of Gillette Castle State Park
East Haddam
Trail Map
Rock Types Found on Main Trail
  • Igneous
    • Pegmatite
  • Metamorphic
    • Gneiss
  • Sedimentary
    • None
{Photograph of State Historic Landmark sign}
Rock Units
  • Hebron Formation (Silurian and Ordovician): Interlayered dark-gray schist with pegmatite intrusions 
Minerals of Interest
  • Tourmaline
  • Garnet
  • Diopside
Interesting Geologic Features
  • Boudinage
  • Drag Fault
  • Folds
{Photograph of Hebron Gneiss outcrop as seen from trail}
Figure 1: Hebron Gneiss outcrop as seen from trail.
The newly refurbished Gillette Castle State Park is an excellent locale to examine the Hebron formation, consisting of gneiss and pegmatite intrusions (Figure 1). Although individual trails are not marked, the spaghetti bowl setting lends nicely to exploration of the Hebron formation, the only rock formation within the Park. The coarse grained pegmatites, a member of the Hebron formation, are especially interesting to mineral lovers, as many diverse minerals can be observed throughout these pegmatites.

{Photograph of a Hebron Gneiss boulder that has fallen from an outcrop}
Figure 2: A Hebron Gneiss boulder that has fallen from an outcrop.
The Hebron formation consists of a calcite and quartz-rich gneiss. A metamorphic rock, gneiss (pronounced "nice") was subjected to intense heat and pressure during formation (Figure 2). Gneiss is easily identifiable by the segregation of light and dark minerals giving it a banded texture. Gneiss usually consists of mostly elongated and granular, as opposed to platy, minerals. At Gillette Castle State Park, the gneiss is layered and is greenish-gray in color, due in part to the green mineral diopside. In addition to diopside, quartz, plagioclase, and biotite are minerals associated with the Hebron Gneiss.
{Photograph of shale-like Hebron Gneiss outcrop with hammer for scale}
Figure 3: Shale-like Hebron Gneiss outcrop with hammer for scale.
The Hebron Gneiss at Gillette Castle State Park is very well layered. However, the thickness of these layers varies from outcrop to outcrop. For example, some outcrops are very thickly bedded, whereas others are almost shale-like in their
thickness (Figure 3).
{Photograph of black tourmaline crystals found in the pegmatite}
Figure 4: Black tourmaline crystals found in the pegmatite
Pegmatites in the Hebron formation typically are white or light-gray rocks containing reddish brown biotite, tourmaline, and garnet as accessory minerals. 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 it cooled very slowly, allowing the crystals to grow very large. The pegmatite crystals of the Hebron formation are approximately 3 cm in length. Generally small, pegmatite intrusions usually have the same composition as granite, only coarser. Furthermore, pegmatite intrusions are of great interest to mineral collectors because they may contain a variety of rare minerals. One mineral of interest, black tourmaline, is abundant near the contacts, in small prisms lying parallel to the contact surface (Figure 4). Since this is a State Park, mineral and rock collecting is prohibited.
Given that Gillette Castle State Park is located only a few miles away from the Honey Hill fault, a very large, old fault, much of the pegmatite is in the shape of lenses and layers and shows distinct pinch-and-swell structures, known as boudinage. Boudinage, which is French for sausage, is a term used to describe the way that layered rocks break up under extensional stress (Figure 5a and 5b). In addition to boudinage, other structural geology features seen at Gillette Castle State Park include folds and a drag fault (Figure 5c). Small-scale features such as small folds and faults are younger than the deformation that produced the laminar gneisses.
{Photograph of boudinage} {Photograph of boudinage} {Photograph of a fold}
Figure 5a, 5b, and 5c:   Structural features found in Gillette Castle State Park. (a) boudinage (b) boudinage (c) fold
The steps leading up to the Castle's main entrance are of another rock type, schist. Like gneiss, schist is a metamorphic rock that has undergone intense heat, pressure, and 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. This particular schist has a very high concentration of white mica, giving it a vitreous, or shiny, nature. In addition to white mica, glassy, red garnet crystals up to 5 mm in diameter are clearly visible in the steps as well (Figure 6a and 6b). The garnets are special not only because they are so large, but also since they are euhedral, meaning that their ideal crystal shape is nearly perfect. In addition, they are the Connecticut state mineral. The new visitor center has similar schist with garnets in its steps.
{Photograph of euhedral garnet crystals found in the schist steps} {Photograph of euhedral garnet crystals found in the schist steps}
Figure 6a and 6b: Euhedral garnet crystals found in the schist steps. (a) lenses cap used for scale (b) pen used for scale

Gillette Castle State Park  |  Geology of Connecticut State Parks
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