Known as "Quarrytown" primarily for the large "brownstone" quarries (aeolian arkosic sandstone of the Portland Formation) near the Connecticut River in the western part town, Portland is also host to the most famous pegmatite quarry in the area - the Strickland Quarry. Coordinates are for the downtown intersection of State Routes 17A and 66 in the west, which is far removed from the pegmatite quarries farther east.
The bedrock geology is split between continental sedimentary rocks in the west, part of the Mesozoic Hartford Basin of the Newark Supergroup and mostly metaplutonic, metavolcanic and metasedimentary metamorphic rocks of the Ordovician Bronson Hill terrane in the east. These terranes are separated by the Eastern Border Fault of the Hartford Basin. Much of the Bronson Hill terrane rocks in town are mixed schists, calc-silicate rocks, and minor quartzites of the Collins Hill Formation, and its metavolcanic member, which are present just east of the Eastern Border Fault. Farther east the bedrock is mostly metaplutonic Glastonbury Gneiss, which crops out as far south as Great Hill Pond. A thin sliver of Devonian Maromas Granite Gneiss is also present.
Portland lies within the Middletown Pegmatite District, and a swarm of Permian pegmatite dikes lies just east of the Eastern Border Fault, mostly within the Collins Hill Formation, but some further east in the Glastonbury Gneiss. These were heavily quarried from the north at the Glastonbury town line and just east of State Route 17 (particularly the long-lived Hale Quarry) southward onto Collins Hill, which hosted the famous Strickland Quarry and Cramer Mine, where most of Portland's diverse mineralogy originates. Most pegmatite specimens just labeled "Portland" are likely from there, although the small Walden Gem Mine and Case Quarries produced many specimens also. Sharply formed and deeply colored aquamarines hail from the small Pelton's Quarry. Unquarried pegmatites can still be easily seen along State Route 66 in the Riverdale section just north of the Connecticut River in the area known as "The Ledges".
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References
- Jarnot, Bruce. (1989): Minerals New to the Portland Area Pegmatites of Central Connecticut. Abstract from the 16th Rochester Mineralogical Symposium April 7, 1989, in Rocks and Minerals, Vol. 64, No. 12, p. 471.
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