The Virginia Teacher Volume XVIII NOVEMBER, 1937 No. 8 GEOLOGIC PROVINCES OF gists have developed a "geologic time table" VIRGINIA in which rock formations of different ages INTRODUCTION and the major events of the earth's history THE present surface features of the are recorded in chronological order. In state, the distribution of minerals, this "time table," all known geologic time rocks, and soils, and the great and is generally divided into five major "eras varied physical changes which they have with subdivisions of each in terms of per- undergone, have been caused by the cease- iods." The major divisions from oldest to less action of geologic agents and processes. youngest are the Archeozoic, Proterozoic, Among these are rock disintegration and Paleozoic, Mesozoic, and Cenozoic eras. decay; wind action; work of streams, The earliest discernible geologic records ground water, the ocean, lakes and swamps; were made in the Archeozoic era. These the deformation of rocks by folding and major divisions and the characteristic forms faulting, mountain - making movements; of life developed during each, together with earthquakes, and the movements of molten the various "period" subdivisions, are rocks. The succession of geologic phenom- shown in Table I. The reader will find here ena has been orderly and continuous. When the relative positions of the various geo- measured in the terms of human existence, logic time intervals and geological forma- the rate of geological change has been very, tions referred to in this article. As used very slow. herein "Cretaceous formations" refers to As the result of detailed studies, made mainly during the past 100 years, of expos- beds of rock formed or deposited during ures of rocks of many different kinds and the Cretaceous division of the Mesozoic ages in various parts of the world, geolo- era. Table I. GEOLOGIC TIME TABLE (Reading upward, divisions range from oldest to youngest) Major Divisions Subdivisions Prominent Life or eras or periods Man Quaternary„ , ;( Recent Cenozoic Glacial Tertiary Mammals Cretaceous Higher plants, early Mesozoic Jurassic insects, reptiles and Triassic birds (dinosaurs, etc.) Permian Pennsylvanian Ferns and allied plants, Mississippian many invertebrates, Paleozoic Devonian and lower vertebrates Silurian (fishes and amphibians). Ordovician Cambrian Low forms of plants Proterozoic* Several and invertebrates • (pre-Cambrian)* Primitive forms of Archeozoic* Several plants and invertebrates. ♦The Archeozoic and Proterozoic eras are frequently considered together as pre-Cambrian, since the first period of Paleozoic time is designated Cambrian. 166 THE VIRGINIA TEACHER [Volume 18, No. 8 GEOLOGIC PROVINCES Ocean; (2) the Piedmont province, a gently rolling plateau extending from the Coastal Virginia lies in the Atlantic Coastal Plain Plain westward to the first continuous and the Appalachian Highlands. It extends mountain ridge; (3) the Blue Ridge, an al- from the sea westward, a maximum distance most continuous, dissected, mountain ridge of 440 miles along the southern boundary. and plateau; (4) the Appalachian Valley The greatest width is about 200 miles. The and Ridge province, better known locally, area of the state is 42,627 square miles, of which some 2,365 square miles are covered in part, as the Valley of Virginia, consist- by tidal waters. The approximate mean al- ing of a series of linear valleys and ridges; titude of the state is 950 feet. The state's and (5) the eastern escarpment of the Ap- geographic center is in Appomattox County, palachian Plateaus, or the Cumberland 11 miles southeast of Amherst. Front, which crosses a small part of south- The topography and geology of Virginia, western Virginia. The width of each prov- as revealed by the surface features and the ince varies somewhat from east to west, exposed rocks, are varied and complex. but the geologic limits are fairly well de- Virginia is divisible into five distinct geo- fined. With the exception of the Appalach- logic and geographic provinces, which are from east to west: (1) The Coastal Plain, ian Plateaus, each province extends north- a terraced plain bordering the Atlantic east across the state. (See Fig 1.) kl & E N TT/ S Ny 57 T' o o / ! wyv-C qY>), r> A V / I A / / A iX, <0 </ / 1// Wy Chester -y, , . 0 oi " / % fi/ W v> IT Qs A/ /i l/P - ^ // /»" ( ) /£/ a , /' Aedericksbur j ( O f •Charlottesville ^ A- IVY. / Vt / r ;. Richm YV O # Lynch burg v-- <v \ oanoke NewporY w.-. v^i V V- V- o Bristol -Sj 9 Nr Danv • S -T— . A Ft O L N A. is 50 150 MILES I I I I I Figure 1—Map showing physiographic divisions of Virginia and parts of adjacent states (from Virginia Geographical Survey Bulletin 34, Figure 2, 1933) l--iE i:mm§ oollese hmmme, Mem November, 1937] THE VIRGINIA TEACHER 167 Because of the marked differences in the vaded the major valleys, such as the James, character and origin of the rocks and sur- converting them into estuaries with local face features in the five natural divisions swamps. The surface of the Coastal Plain of the state and their bearing upon its eco- consists of numerous terraces, or broad nomic development, the characteristics of benches, which rise stair-like in steps above each geologic province are briefly described. the sea. They were formed by the plana- tion of streams and waves when the sea at Coastal Plain times stood higher in the geologic past. The Coastal Plain, or Tidewater province, In the southeast part of the Coastal Plain is the easternmost geologic division of the is the great Dismal Swamp, whose surface state. It is divided into two distinct parts is about 20 feet above sea level. The swamp by the Atlantic shore-line: an eastern sub- area is heavily wooded, mainly with red merged portion and an emerged land por- cedar, and in places contains relatively tion. The submerged area extends from dense growths of canebrakes. Locally cy- about 50 to as much as 75 miles eastward press trees are found growing in the water. from the present shore-line to the edge of Lake Drumraond, a picturesque, shallow the continental shelf. The emerged Coastal circular lake, about 2jd miles in diameter, Plain comprises about 11,000 square miles, is in its center. or slightly more than one-fourth of the The Coastal Plain is underlain mainly by area of the state. It extends from the shore- loose unconsolidated beds of gravel, sand, line west to the Fall Zone. The Fall Zone clay, and marl of Cretaceous, Tertiary and is a narrow zone where streams plunge by Quaternary ages.1 These beds of variable falls and rapids from the resistant rocks of thickness rest upon a floor of Piedmont the Piedmont province to the weaker rocks crystalline rocks, principally of pre-Cam- of the Coastal Plain. Some 2,365 square brian age1, and slope seaward with dips of miles of the Coastal Plain are covered by 5 to 30 feet per mile. The basement crys- the waters of Chesapeake Bay and num- talline rocks are 2,246 feet below the sur- erous tributary estuaries. Chesapeake Bay face at Fort Monroe, and 2,318 feet deep at and four tidal rivers divide the eastern part Mathews Court House, as determined by of the land area into five peninsulas which drilling for water and oil, respectively. are known as the Eastern Shore, the North- ern Neck, Middle Peninsula, the Peninsula, Piedmont Province and the Norfolk Peninsula. The Piedmont province extends from the The average width of the Coastal Plain Coastal Plain on the east to the Blue Ridge is about 100 miles, and its length from the on the west. It comprises about one-third Potomac River at Alexandria to the North of the area of the state. Its width at the Carolina line is about 160 miles. The aver- north is only about 40 miles, but the prov- age elevation does not greatly exceed 100 ince broadens southward to a width of feet, but altitudes of 300 feet or more occur nearly 165 miles along the Virginia-North along the western margin. Carolina line. The Coastal Plain is a broad region of The surface of the Piedmont province is low relief, with a gradual eastward slope of an elevated rolling plain, or low plateau, about 3 feet to the mile. The coast line is with an eastward slope of 10 to 15 feet per indented with numerous bays and coves, and mile. It descends from elevations of 800 to much of the coastal area is marshy, because 1,500 feet along its western border at the in comparatively recent geologic time the foot of the Blue Ridge, to elevations of 20° entire area has been lowered with respect to sea level. Hence, tidal waters have in- 'See Table I. [Volume 18, No. 8 168 THE VIRGINIA TEACHER to 400 feet along the Fall Zone. The max- Cambrian), but some early Paleozoic forma- imum elevation along the Fall Zone is about tions, such as the Arvonia slate of Ordovi- 520 feet. On the west the change from cian age, are present. the plateau to the Blue Ridge is rather well Blue Ridge Province marked, although numerous hills and short ridges, in part outliers of the Blue Ridge, The Blue Ridge province extends north- are found over the western Piedmont. eastward across the state, ^a distance of The province is drained southeastward about 300 miles. It is essentially a narrow into the Atlantic by the Potomac, Rappa- mountainous ridge northeast of Roanoke, hannock, James, and Roanoke rivers. The but broadens into a high, broad, triangular sources of these streams, except the Rappa- plateau south of Roanoke. As the prin- hannock, are west of the Blue Ridge.