In the Piedmont Plateau the twofold character of the province is more marked geologically. On the eastern side of Parrs Ridge the ancient sediments are highly metamorphosed by a development of new textures and minerals due to the recrystallization of the material under great pressure. This division is also marked by the presence of large masses of granular igneous rock which consolidated at great depths beneath the surface of the earth. On the western side of the median ridge the sediments are less metamorphosed and less thoroughly recrystallized, although their original textures have been more or less obliterated. There is also marked lack of deep-seated igneous rocks which are here represented by smaller masses of surface volcanics, both acid and basic, which have been less thoroughly recrystallized than their analogues in the eastern district. Along the western border of this western district, between the Monocacy and the mountains, the early Paleozoics have only slightly changed, the blue limestones of the Frederick Valley resembling the contemporaneous limestones of the Hagerstown Valley farther west. Immediately east of the mountains the earlier rocks are covered with the slightly inclined unmetamorphosed red and gray sandstones and conglomerates of Triassic age and intruded by the diabase dikes of the same period. The threefold division of the Appalachian Region corresponds approximately to the threefold division in the sequence of the Paleozoic strata. The Blue Ridge and Great Valley are made up largely of Cambrian and Ordovician beds, in places so developed or eroded as to expose the associated igneous rocks; the Appalachian Mountains proper are made up of folded Silurian and Devonian strata, each easily recognized by the characteristic life-forms; while the Alleghany Pleateau is mainly composed of more gently folded later Devonian and Carboniferous deposits, the latter carrying the valuable coal seams of the Cumberland basin. THE PIEDMONT PLATEAU A clear understanding of the various formations found within the limits of the Maryland portion of the Piedmont Plateau can only be gained through a consideration of the conditions present throughout the great Piedmont area of eastern North America, extending from Alabama northward as far as New York. Throughout this region are exposed numerous highly crystalline gneisses and schists associated with crystalline limestone, quartzites, and igneous rocks here and there covered by Triassic shales and sandstones. RELATIONS OF THE EASTERN AND WESTERN DISTRICTS The division of the Piedmont Plateau into an eastern district composed of much metamorphosed, highly crystalline rocks, and a western district characterized by less metamorphosed, so-called "semi-crystalline," rocks has long been recognized. The rocks of the Piedmont of this area as a whole may be divided into several groups, the highly crystalline metamorphic sedimentaries of the eastern district, with their associated igneous rocks, their less altered equivalents of Cambro-Ordovician age in the western district, with their associated volcanic rocks, and the much younger sedimentary and igneous rocks of the Mesozoic. The various rocks which are most prominently developed will be more fully discussed in succeeding pages. THE EASTERN DISTRICT The rocks of the eastern district of the Piedmont show a clearly established sequence which may also be recognized, in part at least, at many points in the less well-known western district. FORMATIONS OF THE EASTERN PIEDMONT PLATEAU Archean or Algonkian (?) ....... Baltimore gneiss (in part). Mesozoic. Triassic Paleozoic-Archean IGNEOUS ROCKS Diabase. ..Acid volcanics (meta-rhyolite). Peridotite, pyroxenite, and serpentine. Gabbro, norite, and meta-gabbro. Baltimore gneiss (in part). Highly Metamorphosed Sedimentary Rocks The rocks lying east of Parrs Ridge and forming the eastern district of the Piedmont, with the exception of a few dikes of Mesozoic diabase, consist of metamorphosed sediments, and a diversified complex of intruded igneous rocks which have themselves been more or less metamorphosed from their original massive condition to schistose or laminated rocks. Each of the metamorphosed formations, beginning with the oldest, will be discussed in turn and then the various igneous rocks which represent one or more periods of igneous activity of a common parent magma. THE BALTIMORE GNEISS.-The oldest formation in this area is the Baltimore gneiss, which occurs in several well-defined areas between the Susquehanna and Potomac rivers. The easternmost of these Baltimore gneiss occurrences is within the area of Cecil County, east of the Susquehanna River, and extends from this point southwestward, widening to an area of five miles or more in breadth where it is overlain by Coastal Plain deposits in Harford County. This formation is limited on either side by igneous rocks. The second area of Baltimore gneiss is found in an anticlinal dome, 15 miles long and 5 miles broad, lying on either side of the Northern Central Railroad 10 miles south of the Mason and Dixon Line and 20 miles north of Baltimore. Three smaller areas occur in the vicinity of Baltimore. Two of them are portions of anticlinal domes which are either completely enclosed by overlying sediments or cut off by faults and igneous rocks, while the third, underlying the northwestern part of Baltimore City, is entirely surrounded by gabbro and other igneous masses and is overlain in great measure by the Coastal Plain deposits. The rocks in each of these areas consist of highly crystalline gneisses composed of quartz, feldspar, and mica, with accessory minerals, which are so distributed as to produce well-marked, gray banded-gneisses, the individual bands of which vary from a fraction of an inch to several feet, the average thickness, however, being quite slight. Some of these bands are highly quartzose, resembling a micaceous quartzite; others are rich in biotite or hornblende, producing dark schists, which in a hand specimen are indistinguishable from metamorphosed igneous masses. Sometimes separated by an appreciable unconformity and at other times separated by no apparent line, the Baltimore gneiss, unless bounded by igneous rocks or faults, is overlain by the next succeeding formation. THE SETTERS QUARTZITE.-The Setters quartzite occurs usually as a narrow rim on the flanks of the areas of Baltimore gneiss, but is not continuous or always present. The quartzite is a fine-grained, somewhat saccharoidal, thin-bedded rock of white or cream color in its typical development along Setters Ridge. At this point the beds are usually separated by thin films of muscovite or sericite in small sparkling flakes. On the surface between the individual beds are black tourmalines, which have been more or less disturbed, as is shown by the stretching which they have undergone. Locally, the rock may become very vitreous and massive. At other times it becomes more argillaceous, with a development of garnets, staurolite, and other accessory minerals. THE COCKEYSVILLE MARBLE. The maximum development of the Cockeysville marble is found in the synclinorium lying between the anticlines of Baltimore gneiss and quartzite about 10 miles north of Baltimore City, and on the flanks of the anticlinal dome northeast of Reisterstown. It is here found underlying the Wissahickon mica-schist, and overlying the quartzite, the various formations recurring at the surface through numerous foldings, the contact between the marble and the adjacent formations lying very close to the present surface of the country. Southwest from these larger areas of Cockeysville marble the formation may be traced with little or no interruption in well-defined valleys to the vicinity of Clarksville, in Howard County. The marble occurring in these areas is in the majority of instances rich in magnesium and should be called a dolomite. THE WISSAHICKON PHYLLITES AND SCHISTS. The position which this formation holds in the stratigraphic sequence of rocks appears very clear in the district under discussion, where in each instance it is apparently younger than the marble and consequently younger than both the Setters quartzite and the Baltimore gneiss. Highly Metamorphosed Sedimentary Rocks The rocks lying east of Parrs Ridge and forming the eastern district of the Piedmont, with the exception of a few dikes of Mesozoic diabase, consist of metamorphosed sediments, and a diversified complex of intruded igneous rocks which have themselves been more or less metamorphosed from their original massive condition to schistose or laminated rocks. Each of the metamorphosed formations, beginning with the oldest, will be discussed in turn and then the various igneous rocks which represent one or more periods of igneous activity of a common parent magma. THE BALTIMORE GNEISS.-The oldest formation in this area is the Baltimore gneiss, which occurs in several well-defined areas between the Susquehanna and Potomac rivers. The easternmost of these Baltimore gneiss occurrences is within the area of Cecil County, east of the Susquehanna River, and extends from this point southwestward, widening to an area of five miles or more in breadth where it is overlain by Coastal Plain deposits in Harford County. This formation is limited on either side by igneous rocks. The second area of Baltimore gneiss is found in an anticlinal dome, 15 miles long and 5 miles broad, lying on either side of the Northern Central Railroad 10 miles south of the Mason and Dixon Line and 20 miles north of Baltimore. Three smaller areas occur in the vicinity of Baltimore. Two of them are portions of anticlinal domes which are either completely enclosed by overlying sediments or cut off by faults and igneous rocks, while the third, underlying the northwestern part of Baltimore City, is entirely surrounded by gabbro and other igneous masses and is overlain in great measure by the Coastal Plain deposits. The rocks in each of these areas consist of highly crystalline gneisses composed of quartz, feldspar, and mica, with accessory minerals, which are so distributed as to produce well-marked, gray banded-gneisses, the individual bands of which vary from a fraction of an inch to several feet, the average thickness, however, being quite slight. Some of these bands are highly quartzose, resembling a micaceous quartzite; others are rich in biotite or hornblende, producing dark schists, which in a hand. specimen are indistinguishable from metamorphosed igneous masses. |