GEOLOGY

The major portion of Baltimore County is underlain by the crystalline rocks of the Piedmont Plateau. These comprise ancient igneous rocks, such as granites, gneisses, and gabbros, and highly metamorphosed ancient sedimentary rocks, which have been altered to gneisses, quartzites, marbles, and phyllites. The Baltimore and Wissahickon gneisses cover the largest areas. These rocks have all been intensely folded in former geological times and much faulted and overthrust, so that their chronologic sequence is intricate and greatly obscured.

The Coastal Plain portion of the county is made up of the gravels, sands, and clays of the Lower Cretaceous lying on the depressed margin of the crystalline rocks and comprising in ascending order the Patuxent, Arundel, and Patapsco formations. Toward the extremity of the river necks but lying mostly below tide are found the sands and clays of the oldest Upper Cretaceous formation, the Raritan. Overlying these Cretaceous sediments and forming the surface over most of the Coastal Plain part of the county and extending into the Piedmont valleys as stream deposits are the loams, sands, and gravels laid down at four comparatively recent times when the land stood at lower levels. Each of these periods of depression, which are all Pleistocene in age, resulted in a smoothing of the surface and the deposition of a thin mantle of sediments as a marine terrace, which upon subsequent emergence became a terrace plain. These plains in order of their age are the Brandywine, Sunderland, Wicomico, and Talbot. The older and higher plains are naturally the most dissected by erosion, while the two youngest, the Wicomico and Talbot, particularly the latter, preserve almost intact their original flat surfaces.

SURFACE WATERS

The county is well supplied with streams. Those of the Coastal Plain portion, as previously mentioned, are tidal estuaries, with brackish water and with much mineral and vegetable matter in suspension, and are entirely unsuitable for domestic or municipal use. The streams of the Piedmont Plateau portion of the county are rapidly flowing, fluctuating

streams which when not contaminated by man are available as sources of municipal water supply. All are a part of the Chesapeake Bay drainage system and reach the Bay through the estuaries of the Gunpowder, Middle, Back, and Patapsco rivers. The principal Piedmont streams are the Gunpowder and its branches, Stemmers Run, Herring Run, Jones Falls, Gwynns Falls, and Patapsco River. The upper courses of these streams are variously utilized, but the lower courses are all rather thickly settled and unsanitary. Baltimore City obtains its supply from the Gunpowder at Loch Raven, where an impounding reservoir with a capacity of 2,250,000,000 gallons has recently been constructed. The water is treated chemically and filtered and its use has resulted in a marked decrease in typhoid and similar diseases. The Baltimore County Water & Electric Company obtains its water from Herring Run for the supplies northeast of the city and from Patapsco River for the district north and west of Baltimore City.

The Piedmont streams develop considerable water power which is utilized locally for small milling operations.

UNDERGROUND WATERS

Artesian Waters

Since the largest percentage of the population is suburban to Baltimore City and is largely served by water companies, and since in the more rural districts springs and shallow dug wells are largely utilized, the exploitation of the underground resources is less than might be expected. This has been further influenced by the uncertainty attending the drilling of wells in the crystalline rocks which underlie so much of the county, as well as the great cost of such drilling as compared with operations in the unconsolidated sediments of the Coastal Plain.

In crystalline rocks the underground waters occur in joint and fault planes and minute cavities in the rocks, and since such rocks are intricately folded and faulted it is impossible to predict the prospects with any degree of accuracy. Occasionally a well will fail completely, as for example, a 265-foot well at Gwynnbrook which was entirely dry. A

number of wells have been put down at Arlington to depths of from 100 to 200 feet and all reach water under a good head and pump from 20 to 100 gallons per minute of satisfactory water rather high in iron. A 125-foot well at Lutherville pumps 70 gallons per minute of hard water which heads 15 feet below the surface and is derived from the Cockeysville marble. Other and deeper wells at Lutherville yield much smaller quantities of water.

Numerous wells have been put down in the towns suburban to Baltimore, especially around Pikesville and Reisterstown. These are for the most part between 100 and 300 feet deep and all yield small amounts of siliceous water with a fair head. Some of the deeper wells in this district, as the 500-foot well at the Suburban Club, give greater yields up to 50 gallons per minute, while other deeper wells have small yields, like the 587-foot well at the Jewish Consumptive Hospital which yields but 17 gallons per minute. The Ruxton Water Company has two wells, 152 and 178 feet deep, which together pump 60 gallons per minute. The Suburban Water Company at Arlington have seven 8-inch wells ranging in depth from 70 to 175 feet and supply a daily consumption of about 300,000 gallons.

The Artesian Water Company at Howard Park have five 6-inch wells to depths of from 117 to 200 feet and supply a daily consumption of about 72,000 gallons. The Roland Park Water Company in addition to utilizing springs have twenty-five 6-inch wells from 95 to 500 feet deep and supply a daily consumption of about 250,000 gallons. Little can be said of the artesian prospects throughout the county, although small yields are usually obtained at moderate depths as is shown in the accompanying list of wells which brings together all the information obtainable at the present time.

In the Coastal Plain portion of the county, which is more fully discussed in connection with the Baltimore district, there are several artesian horizons which are available and are largely utilized from Highlandtown to Bay Shore in connection with the extensive industrial development along the north shore of the Patapsco. The most important of these lies at the contact between the sands and gravels of the Patuxent formation

and the underlying crystalline rocks and dips rapidly toward the southeast. North of Highlandtown and at Lansdowne it is about 100 feet below tide. At Walters, Hudson Heights, Highlandtown, and Baltimore Highlands it is about 200 feet below tide. At Brooks Hill and Colgate it is 300 feet below tide. On lower Back River Neck and in the vicinity of Dundalk and Turners it is about 500 feet below tide, while at Sparrows Point it is about 600 feet down.

This horizon is the most important and almost invariably yields large amounts of water. It is overlain by two or three less important water horizons at distances of from 30 to 40 feet.

Non-Artesian Waters

SPRINGS.-Along the fall line and throughout the Piedmont portion of the county springs are very numerous, as might be expected, and some of these yield very large amounts of pure water, such as the celebrated Chattolanee Springs in the Green Spring Valley. Beside furnishing part of the supply of various suburban water companies over 1,000,000 gallons of spring water are sold each year for table water in Baltimore City, and `large additional amounts go into the manufacture of soft drinks. Among the better known springs are the Chattolanee and Brooklandwood in the Green Spring Valley, the Royal Springs near Ruxton, the Caton Springs at Catonsville, the Powhatan Spring at Woodlawn, and the Rock Crystal Springs at Rognel Heights. Good springs are numerous at Bengies, Chase, and Westport, and throughout the central and northern parts of the county and are much utilized locally. In some localities springs are scarce, for example, in the region of Middle River there are only a few, and at Walters only one was noticed. These springs occur at the base of the hills or in small depressions in the surface of the terraces. Most of them have a good yield of clear, cold water, but the supplies are little used except in places where the springs are near dwellings. The amount of inorganic matter carried in solution is seldom large, though at Chase there is a noticeable quantity of iron, and there is also some sulphur.

SHALLOW WELLS.-Shallow dug wells are still largely utilized throughout the rural parts of the county. Their depths and the amount of water

which they yield are variable. They generally reach the water table in the loose materials overlying the crystalline rocks. The depth of these wells varies considerably, the shallowest being only 14 or 15 feet and the deepest about 80 feet. The source of the water is usually a white sand or gravel, but locally, as at Chase, the water bed is reported to be a red, clayey sand. There are clay beds above the water horizons at Westport, Walters, and Bengies. These clay beds are important because they exclude impure surface waters, but locally their value is impaired by their lack of continuity.

The amount of water in the dug wells differs from place to place and in some localities it varies with the rainfall. The following list gives a good idea of the variation in depth.

The quantity of water could not be determined at all these places. At Westport the wells contain 8 to 10 feet of water, at Middle River 3 to 6 feet, and at Bengies and Chase 3 to 4 feet. The quantity of water at most of these localities is not greatly affected by normal drouths, but during continued dry weather some of the wells may become dry.

The quality of water obtained from the dug wells is variable though in most places it is hard. Soft water is reported from some wells at Walters, and others near the Chesapeake and its estuaries yield brackish water. In general, the amount of mineral matter in solution is not great enough to be objectionable. At Bengies and Chase some wells supply water containing large quantities of iron, and at the latter place a few of them yield sulphur water. In general it may be said that the use of shallow wells becomes more dangerous each year as the country becomes more thickly settled. Unless the wells are situated so that they cannot be contaminated by surface drainage and the seepage of sewage their use should be discontinued.