Long Hill (Turkey Hill) prospects, Haddam, Middlesex Co., Connecticut, USA
|Latitude & Longitude (WGS84):||41° 27' 4'' North , 72° 30' 20'' West|
|Latitude & Longitude (decimal):||41.45111,-72.50556|
|Köppen climate type:||Cfa : Humid subtropical climate|
Often erroneously referred to as Turkey Hill, which is further south, Long Hill hosts many very small beryl prospects in narrow, zoned pegmatites. Hall (1838) describes specimens obtained from the gneiss quarries on Long Hill, though the emerald color is a bit of an exaggeration:
The prospects are scattered widely, so the reference coordinates given below are for the summit. The following description from Barton and Goldsmith (1968) is the best and most comprehensive:
Beryl occurs in numerous small pegmatite dikes cutting both the hornblende-biotite gneisses and the small concordant quartz veins associated with the gneisses on Long Hill. The gneiss banding and the quartz veins strike N 17° W and dip 75° E. The cross-cutting pegmatite dikes strike an average N 40° E, and dip 65°-75° SE. The exposed dikes (as seen in some of the old stone quarries) are small, up to 8 feet wide by 40 feet long, and are emplaced in a typical en echelon pattern. Gem beryl (green to blue) appears to be an ubiquitous dike constituent. In some of the better exposures beryl appears to comprise 1 percent of the pegmatite mass. Most of the pegmatites, however, are poorly exposed, showing only in small rubble filled pits where prospectors set off a single dynamite charge. The pits are numerous and widely scattered and, coupled with abundant quartz-perthite pegmatite float on the ridge line of Long Hill, indicate that one or several other pegmatites may be concealed beneath overburden. However, no exploration was attempted to evolve an answer to this question.
Exposed pegmatites are fairly well zoned but zoning is not entirely consistent from one pegmatite to the next.
London (1989) notes that most of these unfoliated pegmatites strike discordantly east-west, display sharp contacts, and are muscovite and tourmaline rich with a composition mostly of microcline and quartz, with quartz rich cores. Some have accessory beryl and garnet. This is in contrast to what he maps as "syenitic feldspar pegmatite" which are concordant, weakly foliated, unzoned and contain more abundant albite but lack accessory minerals other than micas and garnet.
11 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
|Middle Ordovician - Early Ordovician|
458.4 - 485.4 Ma
Age: Ordovician (458.4 - 485.4 Ma)
Stratigraphic Name: Monson Gneiss
Description: (May be equivalent to part of Waterford Group) - Interlayered light to dark, mostly medium to coarse-grained gneiss and amphibolite; gneiss composed of plagioclase, quartz, and biotite, with hornblende in some layers and microcline in others; traces of garnet, epidote, and magnetite.
Comments: Part of Eastern Uplands; Iapetus (Oceanic) Terrane - Bronson Hill Anticlinorium. Swanzey, Pauchaug, Monson, and Fourmile Gneisses represent metamorphosed intrusive igneous rocks and yield ages from 454 to 442+3/-2 Ma. Therefore, ages range from early Late through latest Ordovician. Authors suggest that they represent the plutonic root of a calc-alkaline magmatic arc at least 250 km long that was produced partly or entirely on a continental margin in the Late Ordovician. The overlying Ammonoosuc Volcanics and Partridge Formation, rather that resting unconformably on these plagioclase-rich plutonic gneiss, are now known to have had a magmatic history overlapping with them and are approximately the same age (Tucker and Robinson, 1990) per CT012.
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052.