BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA

VOL. 33. PP. 335-364 JUNE 30. 1924

PRE-CAMBRIAN ROCKS OF THE WESTERN OF MARYLAND1

BY ANNA I. JONAS (Presented before the Society December 28, 1923) CONTENTS Page Introduction...... 355 General geology of the eastern Piedmont of ...... 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) 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, , 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, occupying narrow valleys on either side of Dug Hill Ridge and wider valleys west of Parrs Ridge, near Westminster, New Windsor, and Union Bridge, and to the southwestward. Lithology.—The marble is a fine-grained cream to white rock, stained pink, purple, or green near the contact with volcanic rocks. It resembles in many respects the Cockeysville marble of the eastern Piedmont, but is finer grained and contains more impurities. VOLCANIC ROCKS Extent.—In the western Piedmont of Maryland the marble is asso­ ciated with metamorphosed volcanic rocks, which have been traced from 3 E. B. Mathews: Correlation of Maryland and Pennsylvania Piedmont formations. Bull. Geol. Soe. Am., vol. 16, 1905. 4 E. B. Mathews and J. S. Grasty : Limestones of Maryland. Maryland Geological Survey, 1910. p. 359.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/2/355/3414595/BUL35_2-0355.pdf by guest on 02 October 2021 359 TRIASSIC PALEOZOIC PRE-CAMBRIAN

RECENT WORK IN WESTERN PIEDMONT

F ig u r e 1. — Geologic Map of the western Piedmont of Maryland

on 02 October 2021

by guest Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/2/355/3414595/BUL35_2-0355.pdf 360 A. I. JONAS----PRE-CAMBRIAN ROCKS OF MARYLAND Pennsylvania 15 miles northeast of this area, through Carroll, Frederick, and Montgomery counties, to the Potomac Eiver west of Washington. Their greatest extent is in Carroll and Frederick counties, south of Westminster, New Windsor, and Union Bridge. Lithology.—The volcanic rocks consist of altered amygdaloidal basalt, diabase, tuffs, and doubtful rhyolite. The basalt and diabase have been completely recrystallized and are green epidotic amphibolites with the mineral composition, hornblende, chlorite, calcite, epidote, albite, quartz, and magnetite. They comprise both coarse and fine grained varieties possessing a schistosity even in massive members. -Much of the basalt is spotted with white amygdules filled with quartz, epidote, or calcite. A schistose variety occurs in which the amygdules are flattened to white or green blebs which show on cleavage planes. This variety is usually blue gray or purple and may be altered rhyolite. South of Westminster the meta-basalts are interbedded with blue and green schists, which are considered to be altered volcanic ash. These schists are fine-grained aggregates of chlorite and serieite and break into slaty fragments. They contain scanty feldspar which may have been fragments in the original tuff. The blue and green slaty schist increases in importance in the southwestern part of the area. Relations to the marble and to the Wissahickon schist.—South of Westminster, New Windsor, and Union Bridge, the marble-volcanic series is exposed in three anticlines, with the overlying Wissahickon albite schist in the intervening synclines. Contacts of marble and vol- canics show a zone of impure marble, with slaty partings usually 3 to 10 feet thick, lying between massive marble and volcanic rocks. At some localities the impure rock has the appearance of a marble breccia with the fragments of marble cemented in a green slaty matrix. One well exposed contact of volcanics and marble shows 80 to 100 feet of inter­ bedded calcareous slate, marble, and amygdular basalt above 300 to 400 feet of marble. At no observed contact has the marble surface appeared very irregular, with deep fissures filled with volcanic rock, as would be expected if lava had been outpoured on an old land surface. It seems probable that the volcanic extrusions were submarine and were accompanied by the depo­ sition of fine tuffaceous material. The period of volcanic activity was preceded and followed by the deposition of calcareous beds of the Cockeysville marble, which in turn was covered by other volcanic out­ pourings before the deposition of the argillaceous arkose that has been recrystallized with the Wissahickon schist. It seems probable that vol­ canic activity of an explosive character occurred during the deposition

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/2/355/3414595/BUL35_2-0355.pdf by guest on 02 October 2021 RECENT WORK IN WESTERN PIEDMONT 361 of Wissahickon formation, because in the area south of Union Bridge tuffaeeous slates underlie and may be interbedded with the Wissahickon albite schist. Future work in southwestern Carroll County and adjoin­ ing areas may throw more light on this subject. WISSAHICKON ALBITE SCHIST The albite schist facies of the Wissahickon formation lies on the north­ west side of the Peach Bottom syncline, which is occupied in this area by the Peters Creek formation. On the southeast side of Dug Hill Ridge the Wissahickon formation covers an upland area seven miles wide, along the line of the Tucquan anticline, which enters Carroll County from Baltimore County and extends southwest of Carroll County to the Poto­ mac Eiver west of Washington. The albite schist forms hills in synclinal areas between valleys and lower hills underlain by less resistant marble and volcanic rocks. The albite chlorite schist facies of the Wissahickon formation is a chlorite-muscovite schist in which albite occurs in porphy- roblasts with ragged boundaries and filled with inclusions. Many layers of the schist lack albites. The chlorite and perhaps finely divided mag­ netite and hematite grains impart a bluish green color to the rock. Near the base the formation contains a quartzose member which is a schistose /itreous quartzite with blue quartz pebbles and inconspicuous muscovite or biotite. AGE OF GLENARM SERIES AND VOLCANIC ROCKS The Cockeysville marble and Wissahickon albite schist are a part of the Glenarm series which unconformably overlies Baltimore gneiss, the oldest pre-Cambrian sediment of the Maryland Piedmont. From a study of the southeastern part of the Maryland Piedmont, the Glenarm series was thought to be pre-Cambrian, but proof of the sug­ gestion was not there found. In the western Piedmont of Maryland basal Cambrian quartz conglomerate and quartzite lie in a syncline called the Sugarloaf syncline. The conglomerate and quartzite extend 25 miles southwest, in interrupted outcrop, from the edge of the Triassic sand­ stone west of Union Bridge to the south of Sugarloaf Mountain, where they are again covered by Triassic sediments. West of Union Bridge basal Cambrian quartz conglomerate unconformably overlies volcanic rocks and near Libertytown rests unconformably on Cockeysville marble. The unconformable relation of basal Cambrian sediments to the volcanic rocks and the Cockeysville marble establishes the pre-Cambrian age of these rocks and of the Glenarm series.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/2/355/3414595/BUL35_2-0355.pdf by guest on 02 October 2021 362 A. I. JONAS----PRE-CAMBRIAN ROCKS OF MARYLAND PALEOZOIC BOCKS TO NORTH OF THE PRE-CAMBRIAN SERIES A considerable part of Carroll County, northwest of the area underlain by pre-Cambrian rocks, is occupied by Harpers phyllite, of Lower Cam­ brian age, and infolded blue slaty Conestoga limestone, of Ordovician age. The area is the southwestern continuation of a larger belt of the samp, rocks lying to the northeast in Pennsylvania, where the age and relations of these Paleozoic formations were first studied and determined on paleontologic and stratigraphic grounds. Previous to the present survey of Carroll County the blue slaty lime­ stone was not separated from the pre-Cambrian white marble, but was considered to represent an argillaceous facies of the marble. The blue limestone differs from the marble in lithology, association, and age; it lies in synclinal valleys in Harpers phyllite; it is not interbedded with pre-Cambrian volcanics and is lithologically similar to limestone in Penn­ sylvania which contains Ordovician fossils. These Paleozoic rocks are separated from the pre-Cambrian crystalline schists by an overthrust fault which has carried the pre-Cambrian mass northwestward over Cambrian and younger rocks. This fault has been traced southwestward 100 miles, from the region north of Philadelphia to the area under discussion. RELATION OF THE VOLCANICS TO THOSE OF OTHER AREAS As early as 1892, G. H. Williams5 recognized the volcanic character of the rocks of South Mountain, previously interpreted as sedimentary. Later Mathews6 pointed out the close resemblance of the volcanic rocks of the western Piedmont of Maryland to those of South Mountain and Catoctin Mountain. The epidotic amphibolites resemble the Catoctin schist of Keith,7 which he describes as an alteration of extrusive diabase and basalt. If glassy rhyolites were originally present in the Maryland Piedmont area, metamorphism has completely removed the glassy struc­ tures so well preserved in the South Mountain aporhyolites. Two areas of pre-Cambrian volcanic rocks not previously described have been recently discovered by G. W. Stose and the writer in the Pigeon Hills and Hellam Hills of the Pennsylvania Piedmont. The extent of volcanic rocks in the Pigeon Hills and Hellam Hills is not great, but the types present include all those found in the larger areas of the Maryland 5 G. H. Williams : Volcanic rocks of South Mountain. Am. Jour. Sci., 3d series, vol. 44, 1892, pp. 482-496. 6 E. B. Mathews and J. S. Grasty: Limestones of Maryland. Maryland Geological Survey. 1910. pp. 356-357. 7 Arthur Keith: U. S. Geological Survey Bull. Atlas No. 10.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/2/355/3414595/BUL35_2-0355.pdf by guest on 02 October 2021 SUMMARY 363 Piedmont as well as porphyritic rhyolite like that of South Mountain. The occurrence of volcanic rocks in the area of the Piedmont Plateau, extending from near the Susquehanna Eiver, in Pennsylvania, to the Potomac River, indicates that widespread volcanic action took place far east and southeast of the South Mountain and Catoctin Mountain area. The volcanic rocks of the Pigeon and Hellam Hills and of South and Catoctin Mountain are known to be pre-Cambrian, but they are not asso­ ciated with any pre-Cambrian sedimentary series, and hence whether they were extruded early or late in pre-Cambrian time has been a matter of inference. The finding of similar and probably contemporaneous vol­ canic rocks in the lower part of the Glenarm series places the volcanic outpouring in the period of deposition following the erosion of the Balti­ more gneiss basement on which the Glenarm series was laid down.

Summary 1. The oldest rocks of the western Piedmont of Maryland are brought to the surface in anticlines along the flanks of Dug Hill and Parrs Ridge and extend west and southwestward to the eastern edge of the Frederick Valley and southward to the Potomac River. 2. These rocks consist of the Cockeysville marble, interbedded with surface volcanics and tuffs and overlain by the albite schist facies of the Wissahickon. 3. The volcanics and Cockeysville marble are overlain unconformably by basal Lower Cambrian quartzites of the Sugarloaf syncline; hence the volcanics and Glenarm series are pre-Cambrian. 4. The pre-Cambrian rocks lie south of an area of Lower Cambrian arenaceous rocks in which are infolds of Conestoga blue slaty limestone of Ordovician age. The line of contact between pre-Cambrian and Paleozoic rocks is the surface trace of an extended overthrust fault. 5. The metamorphosed volcanic rocks of the region resemble pre- Cambrian volcanics of the Pigeon and Hellam Hills and of South Moun­ tain and Catoctin Mountain of Pennsylvania, Maryland, and Virginia and probably represent the southeastern part of a volcanic outpouring that was widespread to the northwest. 6. The volcanic extrusions occur in a pre-Cambrian sedimentary series which is younger than the Baltimore gneiss.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/35/2/355/3414595/BUL35_2-0355.pdf by guest on 02 October 2021