land, the largest city in this district, obtains its water supply from Evitts Creek. The North Branch of the Potomac above Westernport, the Youghiogheny, Tonoloway, and Sideling creeks afford fine supplies of unutilized potable waters.

UNDERGROUND WATERS

Under the head of underground waters are included supplies obtained from springs, from the old-fashioned, relatively shallow, dug wells, and from the generally deeper drilled wells that utilize artesian waters.

MOVEMENT OF UNDERGROUND WATERS

The underground waters rarely move in definite channels of appreciable size in any way resembling surface streams, and this occurs only in regions where thick limestone beds contain caverns. Such cavern streams of any size are practically unknown in this area, and for the world at large they may be said to constitute an extremely small proportion of the world's water supply.

Supplies which form the source from which practically all wells draw their water are to be regarded as coming from more or less saturated parts of porous beds, such as sand and sandstones, or beds with minute systems of joints, such as those common in crystalline rocks. In all such situations the underground water whether influenced by gravity or capillarity moves in the minute interstices of the rock or sand bed, and this movement may best be conceived as a slow seeping in which the rate of movement is a few feet per day, rather than a few miles as in surface streams.

WATER TABLE

The effect of the constant percolation of water into the ground is to saturate certain layers, and this saturated zone, or frequently its upper surface, is known as the water table. The position of the water table depends on the topography, the varying porosity of different rocks, and the amount of rainfall. Where precipitation is heavy the water table is near the surface, except on very steep slopes, while in arid regions or during dry seasons it retires to greater or less depths. The relation of the

water table to the topography is shown in the accompanying diagram

FIG. 76.-DIAGRAMMATIC SECTION SHOWING THE RELATION OF WATER TABLE TO SURFACE IRREGULARITIES.

VARIATIONS OF PRESSURE OR HEAD

Water pressures or head are very unequal even in the same bed or zone of saturation. They also vary greatly in different beds in the same region. Pressures depend upon a number of variable factors. These include: (1) The position of the water table, which varies with the factors already enumerated under that head; (2) the character of the water-containing beds, whether coarse- or fine-grained, and hence offering less or more resistance to the underground movement of the water; (3) changes in the adjacent beds as regards their coarseness of grain, and the effects of faults, joints, or other natural breaks. In artesian wells a fourth and most

Hydraulic

grade

FIG. 77.-DIAGRAM SHOWING THE LOSS IN HEAD BY FRICTION AND LEAKAGE.

important factor is the character and elevation of the source of the underground water for the particular bed penetrated. The principles governing the loss of head, or what is known as the hydraulic grade, are illustrated in the accompanying diagram (Fig. 77).

CAUSES PRODUCING ARTESIAN WELLS

The term artesian well has been employed in different senses in the past, and frequently in Maryland the term is used for only those wells which overflow at the surface. This is an incorrect conception. In this report, as well as in the publications of the U. S. Geological Survey, by an artesian well is meant any well which conforms to the hydrostatic principle by means of which confined underground waters tend to rise because of the pressure of the water column due to the ascending of the watercontaining beds to higher elevations. The presence or absence of a flow is dependent entirely on whether this pressure is sufficient to lift the water above the surface of the ground or not. It is customary in works on hydrology to compute these heads with reference to a general datum plane, usually that of mean sea level, but in the present report the more readily understandable datum is the surface, consequently throughout the

F

D

A B

E

FIG. 78.-DIAGRAM ILLUSTRATING ARTESIAN CONDITIONS IN THE COASTAL PLAIN.

text heads are given as + or with respect to the surface of the country where wells are located, although in the maps the contours are drawn with respect to depths below tide.

Artesian wells are possible where a porous stratum capable of saturation reaches the surface in a region where the rainfall is sufficient. This stratum should be confined between two impervious layers. The ideal conditions for obtaining an artesian well are indicated in the accompanying diagram (Fig. 78), which is practically an ideal vertical section from northwest to southeast across the Coastal Plain region of Maryland. The actual conditions are much more variable and complex.

Referring to the first diagram, there is a porous sand or sandstone with clay or shale both above and below it. Rain falling on the surface-marked catchment area sinks into the sand and gradually moves down the slope toward the southeast. If the position of a drilled well is enough lower than the catchment area so that the mouth of the well is below the

hydraulic grade, determined by height and friction in the bed and well as illustrated in Figure 78, there results a flowing well, the water rising to the hydraulic grade which is the "head" or static head of the well.

A variation of these conditions is furnished where the strata are gently folded, as in the western counties of Maryland, and the water-bearing bed rises in two directions as shown in Figure 79. In such situations there are two catchment areas and the underground waters are moving in two directions toward the intermediate valley.

Water-bearing or other geological beds are rarely continuous for scores of miles beneath the ground without some changes in thickness or character of materials, so that instead of the ideal conditions illustrated in the diagram the conditions approximate these ideal conditions either more or less closely. It may happen that the sand bed, like those of the Lower Cretaceous or of the Calvert formation, may not vary through very wide

BA

D

E

F

FIG. 79.-DIAGRAM ILLUSTRATING ARTESIAN CONDITIONS IN WESTERN MARYLAND.

limits, but the confining beds may vary, resulting in water-bearing sands now confined by clay lenses at one level and elsewhere at another level. In this case the head may be much greater beneath such a clay lens than it is where the clay cover is absent or of small extent.

SPRINGS

The mode of occurrence of springs is regulated by the geological structure, and hence there is a great variety in the conditions which produce them, depending on the relations of permeable and impermeable strata. Shallow springs in general are formed where the land surface is depressed in valleys, or on hill slopes so that it penetrates the water table. A normal hillside spring is formed where readily permeable beds rest on less permeable strata, as shown in Figure 80.

Variations from this normal type are dependent on the inclination of the strata, the flatness or irregularity in the surface of the underlying

impervious bed, and whether the water-bearing layer is covered by an impervious stratum. Other springs may be due to fairly defined channels in the surficial rocks and may be regular or intermittent in character, dependent on the geological structure. Shallow springs usually reflect the

FIG. 80.-DIAGRAM SHOWING AN ORDINARY HILLSIDE SPRING.

seasonal variation in the rainfall. On the other hand, springs that owe their existence to a saturated bed confined between impervious beds may be of large volume and regular flow and may often have a considerable hydrostatic head, particularly in mountainous regions.

Other springs are illustrative of ordinary artesian well conditions where the water-bearing bed under a considerable head is cut by a fault, as shown diagrammatically in Figure 81, and illustrates deep-seated springs, like those at Clear Spring or Big Spring in Washington County. The same results are accomplished in regions where the strata are folded and water in a confined, saturated bed, circulating at great depth under pressure,

Fissure spring

FIG. 81.-DIAGRAM SHOWING A DEEP-SEATED SPRING.

forces its way to the surface at the point of contact between the saturated bed and the overlying or underlying impervious beds. Springs of this sort are not uncommon along the Alleghany ridges in Washington County. The historic Berkeley Springs, near the Maryland border in West Virginia, is the best known instance of this last type of spring.