BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 33. PP. 335-364 JUNE 30. 1924 PRE-CAMBRIAN ROCKS OF THE WESTERN PIEDMONT OF MARYLAND1 BY ANNA I. JONAS (Presented before the Society December 28, 1923) CONTENTS Page Introduction...................................................................................................................... 355 General geology of the eastern Piedmont of Maryland................................... 356 General geology of the western Piedmont of Maryland................................. 357 Previous work in the western Piedmont of Maryland..................................... 357 Recent work in the western Piedmont of Maryland......................................... 358 General statement.................................................................................................. 358 Cockeysville marble.............................................................................................. 358 Extent................................................................................................................ 358 Lithology................................................................................ •......................... 358 Volcanic rocks......................................................................................................... 358 Extent................................................................................................................ 358 Lithology........................................................................................................... 360 Relations to the marble and to the Wissahickon schist.................. 360 Wissahickon albite schist...................................................................................361 Age of Glenarm series and volcanic rocks................................................... 361 Paleozoic rocks to north of the pre-Cambrian series............................... 362 Relation of the volcanics to those of other areas..................................... 362 Summary........................................................... ............................................................... 363 I ntroduction The Piedmont of Maryland occupies a belt about 55 miles wide, ex­ tending across the central part of the State from the Pennsylvania- Maryland line to the Potomac River. Its southeastern edge is the “Fall line,” marked by such towns and cities as Elkton, Havre de Grace, Balti­ more, Laurel, and Washington, and its northwestern boundary is Catoctin Mountain. The Piedmont is for the most part a rolling upland under­ lain by pre-Cambrian crystalline rocks, and it comprises, in the western 1 Manuscript received by the Secretary of the Society February 2, 1924. Published with the permission of the State Geologist of Maryland. (355) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/2/355/3414595/BUL35_2-0355.pdf by guest on 02 October 2021 356 -A. I. JONAS— PRE-CAMBRIAN ROCKS OF MARYLAND part, the Frederick Valley, underlain by Paleozoic rocks, and the Triassic lowland which lies along the eastern edge of Catoctin Mountain. The area of the pre-Cambrian rocks of the Piedmont of Maryland is divided for geologic reasons into an eastern and a western belt by the Peach Bottom syncline, which crosses the Pennsylvania-Maryland line at its intersection with the Susquehanna River, north of Conowingo. This syncline extends southwest from Cardiff through Greystone, Balti­ more County, to Sykesville, Carroll County, whence it curves southward to the Potomac River near Washington. The eastern belt, lying south­ east of the Peach Bottom syncline, is widest in Baltimore County, but to the south, in Howard and Montgomery counties, it extends under Coastal Plain sediments east of Laurel and Washington. G e n e r a l G e o lo g y o f t h e e a s t e r n P ie d m o n t o f M a r y l a n d The stratigraphy of the eastern Maryland Piedmont2 has been worked out in detail by E. B. Knopf and the writer in a recent survey of Balti­ more County. The studies on which this paper is based were carried on in Carroll County and a part of Frederick County, in pursuance of the detailed survey of the Maryland Piedmont begun in 1919 for the Mary­ land Geological Survey. The pre-Cambrian rocks exposed in the eastern and western Piedmont of Maryland are as follows: Western Piedmont Eastern Piedmont Post-Glenarm intrusives Peters Creek schist Peters Creek schist Wissahickon albite - chlorite Wissahickon oligoclase - mica schist schist facies facies A Cockeysville marble and in­ Cockeysville marble äa terbedded volcanic rocks O and tuff Setters formation ------------------ U nconf ormity------------------ Hartley augen gneiss (intrusive) Baltimore gneiss The oldest rock of the eastern belt is the pre-Cambrian Baltimore gneiss, a light-colored, thoroughly recrystallized rock of sedimentary 2 E. B. Knopf and A. I. Jonas i Stratigraphy of the crystalline schists of Pennsyl­ vania and Maryland. Am. Jour. Sci., vol. 5, January, 1923. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/2/355/3414595/BUL35_2-0355.pdf by guest on 02 October 2021 GENERAL GEOLOGY OF EASTERN PIEDMONT 357 origin, which in some places is a heavily bedded gneiss of granite aspect, while in other places it is a thinly banded ribbon gneiss. The dominant constituents are quartz, oligoclase, microcline, and biotite. It has been intruded by a granite, the Hartley augen-gneiss, which has produced a lit-par-lit injection in the upper part of the formation. The Glenarm series, 8,000 to 10,000 feet thick, unconformably overlies the Baltimore gneiss. The series comprises, in ascending order, Setters quartzite, Cockeysville marble, Wissahickon formation, and Peters Creek schist. The lower formations of the Glenarm series are cut by large intrusions of plutonic rocks, granites, diorite, and gabbro, as well as by pyroxenites and peridotites. The Wissahickon formation throughout the Maryland and Pennsyl­ vania Piedmont develops two contemporaneous facies, the one called the oligoclase-mica schist facies, which was metamorphosed under deep- seated conditions; the other, the albite-chlorite schist, which is the result of recrystallization in the uppermost zone of metamorphism. The albite- chlorite schist facies lies on the northwestern side of the Peach Bottom syncline and is considered to be the stratigraphic equivalent of the oligoclase-mica schist facies of the Wissahickon which lies on the south­ eastern side of the Peach Bottom syncline. In the eastern Piedmont of Maryland, Baltimore gneiss is exposed in a series of anticlines, with Setters quartzite and Cockeysville marble exposed on their flanks and Wissahickon oligoclase-mica schist in the synclines. The Wissahickon schist grades upward into the Peters Creek schist, which occupies the Peach Bottom syncline in Baltimore and Carroll counties. G e n e r a l G e o lo g y o f t h e w e s t e r n P ie d m o n t o f M a r y l a n d The Peach Bottom syncline crosses the southeastern edge of Carroll County, so that most of this county and the eastern part of Frederick County lie in the western Piedmont belt. As has been noted, the albite- chlorite schist facies of the Wissahickon formation is developed on the northwest side of the syncline, and it covers a wide area in the western Piedmont. The underlying Cockeysville marble and interbedded volcanic rocks are exposed in anticlines in the schist. The age and relations of this infolded series of marbles and metamorphosed volcanic rocks of the western Piedmont of Maryland are the subject of this paper. P r e v io u s W o r k i n t h e w e s t e r n P ie d m o n t o f M a r y l a n d The area has been studied in part by Mathews and Keith, and the results of Keith obtained in the Frederick and Washington quadrangles Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/2/355/3414595/BUL35_2-0355.pdf by guest on 02 October 2021 358 A. I. JONAS----PKE-CAMBRIAN ROCKS OF MARYLAND and those of Mathews3 in the western Piedmont of Maryland were ex­ pressed by the latter before the Geological Society of America in 1905. Mathews recognized the Peach Bottom syncline and considered that it contains either a less metamorphosed portion of the Wissahickon forma­ tion or an infolded younger series, less metamorphosed than the Wissa­ hickon, which lies on either side of it. The belt of Wissahickon schist west of the syncline was described as lying east of a belt of phyllitic rocks associated with surface volcanics and resting on Shenandoah lime­ stone of Frederick Valley type. In 1910 Mathews,4 in a general discussion of the geology of the west­ ern Maryland Piedmont in “Limestones of Maryland,” recognizes the anticlinal character of the marble and superposition of the volcanics on the marble, but concludes “that either the volcanics may be in part Ordovician age, in which case the limestones may represent metamor­ phosed equivalents of the Shenandoah group, or the volcanics may be pre-Cambrian and overlie the limestones through thrust-faulting, in which case the limestones may be of Loudoun or Shenandoah age.” R e c e n t W o r k i n t h e w e s t e r n P ie d m o n t o f M a r y l a n d GENERAL STATEMENT During the last two years the writer has established the following succession of the pre-Cambrian crystalline schists in the western Pied­ mont of Maryland. COCKEYSVILLE MARBLE Extent.—The oldest rock of the area is the Cockeysville marble. It enters Maryland from York County, Pennsylvania, and extends south- westward,