The Brandywine formation is chiefly developed as a terrace lying irregularly and unconformably on whatever older formation chances to be beneath it. These range from pre-Cambrian and Paleozoic (the metamorphic rocks of the Piedmont Plateau) up into the later beds of the Miocene series. Although the oldest of surficial deposits, the Brandywine formation lies topographically highest and at the center of a concentric border of younger terrace formations which wrap about it. It has a thickness on the average of less than 50 feet, although at some points a thickness considerably greater has been observed. THE QUATERNARY PERIOD The Quaternary deposits of Maryland and adjacent states form an extensive veneer throughout all but the highest portions of the Coastal Plain, frequently burying from view the deposits of earlier age in the interstream areas. The Quaternary is represented by both the Pleistocene and the Recent. The Pleistocene Superficially overlying most of the older formations throughout the greater part of the Coastal Plain and extending in places on to the Piedmont Plateau are beds of Pleistocene age which, with marked variations in thickness, composition and structure, extend from the glacial deposits of northern New Jersey through the south Atlantic and Gulf states to the Mexican border. The Pleistocene deposits belong to a single division known as the Columbia group. The Columbia Group The Columbian group, so called from the characteristic development of the deposits in the District of Columbia, is widely extended as surficial deposits throughout the eastern and southern counties of Maryland, over most of Delaware, as well as along the main stream channels that extend into the region of the Piedmont Plateau. These deposits form a series of terraces that wrap about the Brandywine and the higher portions of the older formations and thence extend as fluviatile deposits up the stream courses. The Columbia group has been divided into the Sunderland, Wicomico, and Talbot formations. THE SUNDERLAND FORMATION.-The Sunderland formation, so called from Sunderland in Calvert County, was formerly developed as a nearly continuous deposit of the Coastal Plain region below the Brandywine highlands, but erosion has now removed it over wide areas. Like the former it finds its greatest development in Southern Maryland, where it forms the divide of Calvert County and of Charles and St. Mary's counties west and south of the Brandywine area. Numerous outliers occur to the westward. A few of them are found within the body of the Coastal Plain region, while many others occur either on the Piedmont Plateau or on the margin between it and the Coastal Plain. The materials which compose the Sunderland formation consist of clay, loam, sand, gravel, peat, and ice-borne blocks. These as a rule do not occur in well-defined beds, but grade into each other both vertically and horizontally. The coarser materials, with the exception of the ice-borne boulders, are usually found with a cross-bedded structure, while the clays and finer materials are either developed in lenses or are scattered throughout the formation and may occur in the gravel beneath or in the loam above. There is distinguishable throughout the formation a tendency for the coarser materials to occupy the lower portion and the finer the upper portion of the formation, but the transition from one to the other is not marked by an abrupt change and coarser materials are frequently found above in the loam and finer materials below in the gravel. As a whole, the material is coarser in the Potomac and Susquehanna basins than elsewhere. The sources from which the Sunderland sea derived the materials for its deposits were principally confined to the Coastal Plain, although the rivers also brought in contributions from the Piedmont Plateau and the mountains of western Maryland. The thickness of the Sunderland formation is very variable. The average thickness probably does not exceed 25 feet, although at some points it reaches a thickness of from 60 to 80 feet. A few plant fossils have been recognized in the clay beds, but the fossiliferous localities in the Sunderland formation are much fewer than in the later deposits of Pleistocene age. THE WICOMICO FORMATION.-The Wicomico formation, so named for the Wicomico River in Southern Maryland, has been developed as a broad terrace below and fringing the Sunderland, at times completely filling and largely obliterating the bottoms of the ancient stream valleys that trenched the Sunderland surface. It has at the present time a much larger areal development than the Sunderland and has been much less dissected by erosion than the latter, with the result that the terraced surface has been far better preserved. Its largest development is on the Eastern Shore, where it forms the watershed throughout the center of the region, extending as far southward as Worcester County. The materials which constitute the Wicomico formation are similar to those found in the Sunderland and in fact many of them have been derived from that formation. They consist of clay, loam, sand, gravel, peat, and ice-borne boulders. The distribution of these materials is similar to that described in the Sunderland in that they grade one into the other both vertically and horizontally, but with the preponderance of the coarser materials at the base of the formation, while the finer deposits are largely developed toward the top. THE TALBOT FORMATION.-The Talbot formation, the name for which is suggested by Talbot County where the formation is widely developed, occupies the area between the margin of the older surficial deposits and the seashore. It wraps about the Wicomico and other terrace deposits as a border and extends up re-entrant valleys as a veneer. Erosion has attacked this terrace to such a slight extent that it may be considered as continuous, although here and there small areas have been separated from the otherwise unbroken surface. Like the Wicomico formation it finds its greatest development on the Eastern Shore and particularly in the southern portions of that area where it forms broad flats which decline lower and lower until they pass into marshes and blend imperceptibly with the beach. On the Western Shore it also has an extensive development, particularly toward the head of the Bay. The materials which compose the Talbot formation consist of clays, loam, sands, gravel, peat, and ice-borne boulders. The Recent The Recent deposits embrace chiefly those being laid down to-day over the submarine portion of the Coastal Plain and along the various estuaries and streams. To these must also be added such terrestrial deposits as talus, wind-blown sand, and humus. In short, all deposits which are being formed under water or on the land by natural agencies belong to this division of geological time. The Recent terrace now under construction along the present ocean shore-line and in the bays and estuaries is the most significant of these deposits and is the last of the series of terrace formations which began with the Brandywine, the remnants of which to-day occupy the highest levels of the Coastal Plain and which has been followed in turn by the Sunderland, Wicomico, and Talbot. Beaches, bars, spits, and other formations are built up on this terrace belt and are constantly changing their form and position with the variations in currents and winds. Along the streams flood plains are formed that in the varying heights of the water suffer changes more or less marked. On the land the higher slopes are often covered with debris produced by the action of frost and the heavy downpours of rain which form at times accumulations of large proportions known as talus and alluvial fans. An illustration of the former is seen in the Devil's Race-course on the western slope of the Blue Ridge, the heavy blocks in this instance. being separated by the action of frost and subsequently precipitated down the steep mountain side. A deposit of almost universal distribution in this climate is the humus or vegetable mold which being mixed with the loosened surface of the underlying rocks forms our agricultural soils. The intimate relationship therefore of the soils and underlying geological formations is evident. The deposit of wind-blown sands more or less important everywhere, as may be readily demonstrated at every period of high winds, is especially marked along the sea-coast in Worcester County where sand dunes of considerable dimensions have been formed. Other accumlulations in water and on land are going on about us all the time and with those already described represent the formations of Recent time. PRECIPITATION SOURCE AND AMOUNT The normal precipitation in this region, whether falling as rain, hail, sleet, or snow, ranges from 30 to 55 inches. This is greatest in the extreme western part of Maryland on the Alleghany Plateau, where the conditions favor both frequency and intensity of rainfall and snowfall. Throughout the Great Valley, that is, in eastern Allegany and Washington counties, the annual precipitation decreases rapidly; a second area of diminished precipitation is found over upper St. Mary's and the southern part of Charles County; and a third occurs over a narrow portion of the Eastern Shore bordering on the Atlantic, in Maryland and southern Delaware. In these three areas the normal annual precipitation is from 31 to 35 inches. East of the Blue Ridge the annual precipitation increases, and throughout the Piedmont Plateau area it amounts to from 40 to 45 inches, being in general near the lower figure west of Parrs Ridge and near the higher figure east of Parrs Ridge. A narrow area over which the normal annual fall is less than 40 inches lies just west of the Atlantic coast area already mentioned as one of the dry divisions, and a second limited area of this kind is found to embrace southwestern Kent County in Delaware and portions of Caroline, Talbot, Prince George's, Howard, and Baltimore counties in Maryland. With these exceptions, and that already noticed in portions of Charles and northern St. Mary's counties, Maryland, the normal annual precipitation for the Coastal Plain is from 42 to 48 inches. The bands of greatest precipitation in this latter area include southern Anne Arundel County, and from southern St. Mary's County northeastward over portions of Dorchester and Wicomico counties and southern Delaware. The normal annual precipitation is divided throughout the seasons as follows: Spring and summer will have 11.5 to 12 inches, and fall and winter 9.5 to 10 inches. |