The age and regional relations of the Glenarm Series in the central Appalachian Piedmont have been subjects of controversy for more than 60 years. The interpretation by Hopson in 1964 that the Glenarm Series is Precambrian and probably correlative with the Ocoee Series and related rocks of the southern Appalachians has become widely accepted. This interpretation was based (1) on radiometric ages of minerals from rocks interpreted as intrusive into Glenarm rocks, (2) on interpretation of the relative ages and genetic relations of these rocks and the Baltimore Gabbro, and (3) on correlation of the Wissahickon Formation of the Glenarrn Series with the Lynchburg Formation in Virginia.

Evidence suggests that many of the radio-metrically dated rocks that Hopson and later workers regarded as intrusive into the Glenarm Series and that they used to define the minimum age of the series are best regarded as either metamorphosed volcanic and volcaniclastic rocks or shallow intrusive bodies grading into and coeval with the Glenarm rocks. The radio-metric ages of these rocks probably represent the approximate “actual” age of parts of the Glenarm Series rather than the minimum age. This interpretation also casts strong doubt on the existence of two series of plutonic rocks in the Maryland Piedmont and indicates that Hopson's Baltimore Gabbro is younger, not older, than the older group of radiometrically dated rocks.

Recent work in Virginia suggests that correlation of the Wissahickon Formation with the Lynchburg Formation and with other upper Precambrian rocks is highly unlikely, and shows that the Glenarm clastic rocks are conformable and gradational with metavolcanic rocks, which are in turn conformable and gradational with the Quantico Slate (Ordovician).

Reconsideration of the Martic line and Martic Hills in southern Pennsylvania, where the Wissahickon Formation has been interpreted as overthrust onto Cambrian and Ordovician rocks, indicates that the relations are best explained as the result of superposition of folds. Reconsideration of the Peach Bottom fold and the rocks involved in it indicates that it is probably an anticline rather than a syncline, and that the Peach Bottom Slate and Cardiff Metaconglomerate are probably correlative with the Hellam Member of the Chickies Formation.

The Glenarm Series is regarded as being chiefly Cambrian and Ordovician with a maximum age of about 650 m.y. and a minimum age of Late Ordovician; it is correlated with the Evington Group in the Virginia Piedmont and with some of the rocks in the Manhattan Prong in New York. The Wissahickon Formation is probably correlative with part of the Chilhowee Group of the Blue Ridge province. Meta-volcanic rocks in the Glenarm are correlated with some metavolcanic rocks in the Carolina slate belt. In Maryland, these rocks are named the James Run Formation. A new nomenclature is proposed, in which the Glenarm Series consists of the Setters Formation, Cockeysville Marble, Peach Bottom Slate, Cardiff Meta-conglomerate, Wissahickon Formation, James Run Formation, Chopawamsic Formation, and Quantico Slate.

The Glenarm metavolcanic rocks and their correlatives record the existence of a long belt of roughly contemporaneous volcanism, an “Atlantic seaboard volcanic province,” probably an island arc, that extended at least from Georgia to New York during late Precambrian, Cambrian, and Ordovician time. The Appalachian geosyncline is regarded as an “intra-continental,” pre-continental-drift geosyncline. The island arc was near the eastern part of the pre-Atlantic continent, and the Glenarm clastic rocks were probably deposited in a basin between the island arc and a shelf area represented by the Chilhowee sedimentary rocks. The source for much of the Glenarm Series was the eastern part of the pre-Atlantic continent. The numerous similarities between the Glen-arm Series rocks and sequences in New England, New Brunswick, and Newfoundland suggest that the northern Appalachian crystalline belt need no longer be considered separate and distinct from the central and southern Appalachian belt.

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