Volume 0009 Number

JOHNS HOPKINS UNIVERSITY CI RCULARS Pub/is/ied wit/i t/ie approbation of t/ze Board of Trustees VOL. JX.—.-No. 8i.] BALTIMORE, MAY, 1890. [PRICE, 10 CENTS. SCIENTIFIC NOTES. Third Annual Geological Expedition into Southern MEsozorc.—Potomac.-—Overlying the eastern flank of the Archaean belt, Maryland and Virginia. By WILLIAM B. CLARK. topographically designated the Piedmont Plateau, is the Potomac formation, the basal member of the unaltered sediments constituting the Coastal The third annual geological expedition, under the direction of the writer, Plain. The deposits consist of sands, clays and gravels of varying compo- assisted by Mr. N. H. Darton, of the Pot omac Division of the U. S. Geo- sition, though the sands and gravels predominate in the lower and the clays lobical Survey, left Baltimore on the morning of April 2d for an excur- in the upper portion of the formation. The gravels are chiefly found as sion of ten days into Southern Maryland and Virginia. The party con- the shoreward facies of the basal horizon. sisted of Dr. W. B. Clark, Messrs. C. iR. Keyes, U. S. Grant, E. G. Conklin, The fossils of the Potomac are not abundant, consisting chiefly of leaf A. E. Bibbins, W. V. Metcalf, and M. M. Metcalf, of the Johns Hopkins impressions, with a few dinosaurian and molluscan remains. Marsh con- University; President J. Edgar, and Professor A M Miller, of Wilson siders that the dinosaurian forms establish the Jurassic age of the deposits, College,Chambersburg, Pa.; Messrs. W. H. Weed an~ N. H. Darton, of the while the presence of large numbers ofdicotyledenous leaves lead others to U. S. Geological Survey; and Dr. J. F. Clark, of the Philadelphia Hospital. assign the beds to the base of the Cretaceous. Whatever may be the taxo- Proceeding by rail to Alexandria, Va., where arrangements had been momic position ofthe formation, it is a well defined stratigraphical unit in made for a vessel of suitable size to accommodate a party of twelve, the the geological series of the Coastal Plain. expedition embarked upon the Schooner Idlewild (60 tons), Captain Kelly, Albirupean.—Unconformably overlying the Potomac formation in the area and dropped down the Potomac River the same evening to Fort Wash- visited are beds of light sands and clays that, upon structural grounds, pre- ington, Md., where the investigations of the Atlantic Coast section were to sent a well defined stratigraphical individuality. Palaeontological data are begin the following day. On the two previous expeditions the bluffs upon of little value, and almost no evidence is yet adduced in regard to the tax- the Western Shore of Chesapeake Bay had been examined, so that it was onomy of the deposits. The formation is probably Cretaceous. thought best to -vary somewhat the field of observation, especially, as the Cretaceous.—The Albirupean is unconformably overlaid by the dark sands PotomacRiver section presents more complete and accessible exposures, than and clays of the typical Cretaceous, that are often micaceous, and at times that upon the Bay shore. Moreover, the opportunities for valuable results glauconitic. In the area examined in the vicinity of the Potomac and in from a detailed study of the river bluffs were considered, and in the follow- the counties to the north, the rich molluscan fauna, consisting of the casts ing pages some facts of importance in the Coastal Plain stratigraphy are of ammonites, gastropods and bivalves, shows the close relationship of the brought out. The earlier writers upon Maryland and Virginia geology, deposits to the Lower Mans of New Jersey. among~hom Conrad, Rogers and Tyson deserve especial mention, present CENOZOIc.—TERTIARY.----The writer has employed in the following divis- us with valuable descriptions of local deposits, but generally failed to appre- ions of the Tertiarythe termsEoeene and Neocene as bestadapted on both struc- ciate their stratigraphical relationships. The later investigations of Messrs. tural and palaeontological grounds to interpret the geology of the Atlantic W. J. McGee and N. H. Darton, of the U. S. Geological Survey, and Mr. Coast deposits. They correspond with the terms upper and lower Tertiary. P. R. Ubler, of Baltimore, have done much toward solving the puzzling Eocene,—The Eocene of the Potomac section is represented by light or problems presented in the coastal strati~,raphy of this region. The writer dark colored sands (green, red or yellow), the color depending chiefly upon wishes especially to refer to the ~vorkof Mr. Darton, whose investigations the presence or absence of glauconite, and the amount of decomposition to about Washington have afforded substantial data upon which further obser- which the strata have been subjected. The deposits are often calcareous vations may be based. and at times argillaceous. Generally composed of incongruous sands, thick beds of consolidated rock are not infrequent. Certain zones are packed with GEOLOGICAL FORMATIONS OF THE COASTAL PLAIN. fossils, that do not present however a great variety of species. Such typical Before taking up the details of the trip, a brief outline of the leading Eocene forms as Cardita planicosta, !iL’arritella Mortoni, Ostrea compressirostra, geological features of the area visited is presented. The series represented (Jucullaea gigantea, Orassatella capri-eranium, and Cytherea ovata, are widely is as follows:— distributed. Neocene—The Neocene strata unconformably overlie the Eocene, and, in F Quaternary. Recent. general, present marked lithological differences. Cxxozoic. J { Pleistocene. The Neocene shows two horizons in the Potomac region; a lower com- I Neocene. posed chiefly of diatomaceous sands and clays, the latter containing many molluscan remains, among which are Perna maxillata, Isocardia fraterna, Tertiarii. ~Eocene.Cretaceous. Pecten Miadisonius, and Venus mercenaria; an upper composed of sands and Mxsozoic. { Albirupean. clays, the latter of a marked blue color when moist. In the upper division Potomac. a very rich molluscan fauna is found, together with species of foraminifera, 70 JOHNS HOPKINS [No. 81. corals, bryozoa, and crustacea. Altogether more than a hundred species river at Glymont were investigated, and found to exhibit the following have been described from this horizon. section QUATEreNARY.—Pleistocene.—Widely extended, unconformably, over the preceding deposits is a heterogeneous collection of gravels, sands and clays, Red sandy loam 5 feet. generally red or yellow in color, that upon structural grounds have been PLEISTOCENE. RedCoarseloamgravel 53 feet. 20 feet. variously differentiated and described as separate formations. Although in Gravel 4 feet. the following descriptions the Pleistocene is treated as a whole, a brief statement of the different divisions is essential. Light marl with traces of glauconite 10 feet. Darkcalcareous green-sand, with Os/rea compressi Under the name of Appomattox formation, McGee has described a series EOcENE. res/ra, Turri/e/laMor/eni, Dosinieps~s Meekii,e/c..20 feet. 60 feet. of orange-colored sands and clays that unconformably overlie the Neocene, Dark, variegated, argillaceous sand, with few in distinct casts, including TurrileltaMer/eni 20 feet. and are in turn overlain by the undoubted Pleistocene. Whether late Ter- tiary or Pleistocene we have as yet no evidence. POTOMAC. Variegated clay, locally arenaceous, upper layers 20 feet. The term Good Hope formation has been employed by Darton for the in a marked degree ferruginated. deposits of coarse gravels and sands that cap the higher bluffs and showan their topographic relief more extensive denudation thanthe later and lower Columbia formation that according to McGee exhibits two phases (1. fluvial, The Potomac reaches water-level before the mouth of Mattawoman Creek 2. interfiuvial), and consists of gravel, sand, loam and brick clay. The flu- is reached, and does not reappear on the Maryland shore south of that point. Dropped anchor the same evening off Cobb’s Island, at the mouth of Chap- vial phase found along the larger rivers is divisible into two horizons, the awamsie Creek, Va. upper consisting of fine sand, loam and brick clay; the lower of gravel, sand April 4.—The well known section at Aquia Creek (Va.) was examined and transported bowlders. The interfiuvial phase which covers the inter- in considerable detail, and found to consist of the following horizons mediate country is formed of gravel, sand and clay, that is chiefly of local origin. Recent.—The alluvial deposits, the wash of the hills, the swamp and PLEISTOCENE. Red sandy loam. 20 feet. beach accumulations, are the recent acquisitions to the geological series Light-colored sand, slightly glauconitic 20 feet. within the area visited, that modify to a greater or less extent the sections Light green-saud, with Gardi/aplanicos/a, Tssrs~i/ella exposed. Mor/eni, Oacul/aea essechela, Orassa/ella capri cranium, Gy/herea eva/a, etc 9 feet. ITINERARY OF THE EXCURSION. Soft, yellowish green-saud, with Os/rca compress’s EOCENE. rostra 15 feet. 65 feet. April 2.—Departure from Baltimore. No portion of the Coastal Plain Ledge of dark green-sand, with Tscrri/eilaMor/oni, Cardi/a planicosta, Panopaca elongata, Pesus section was examined in detail, although distant exposures of Potomac, sp., etc 3 feet. Dark green-sand, with Gy/heree ova/a, Grassalella Albirupean, Pleistocene, etc., were observed in the railroad cuts between capri-craniuos, etc 15 feet. Baltimore and Alexandria, and in the river bluffs below Alexandria. At evening anchor was dropped off Fort Washington, Prince George County, Much time was expended in making collections of fossils at this point. Maryland. Anchored off the month of Nanjemoy Creek (Md.) for the night. April 3.—The bluffs and ravines in the vicinity of Fort Washington were Aps-il 5.—At Pope’s Creek (Md.) a fine exposure of lower Neocene over- examined. The section at the mouth of Piscataway Creek, so often referred lying the Eocene was observed, as shown in the following section to in geological literature, was found to consist of the following formations PLEISTOCENE.

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