126 ARTESIAN WELLS ON THE ATLANTIC COAST. [bull. 138. 
CHESAPEAKE FORMATION. 
This is a conspicuous member of the Coastal Plain series in Maryland, 
although it is overlain in greater part by thin sheets of Columbia or 
Lafayette deposits. Its western edge extends to the crystalline rocks 
at a few points, and the formation thickens rapidly eastward to over 
1,000 feet at the coast line. It consists of a series of beds of sand and 
clay which dip gently eastward at a rate of about 15 feet per mile in 
Maryland and Delaware. These beds rise to the surface in succession 
on the dip, and outcrop diagonally across the Coastal Piain with a 
northeast-southwest strike. Clays predominate, and the sands are in 
•greater part of impalpably fine grain. The lower clay members con- 
tain a large proportion of diatom remains, and this diatomaceous series 
is a characteristic member of the formation over nearly its entire area. 
Interbedded with the clay and fine sands there are, at various hori- 
zons, beds of coarser water-bearing sands, some of which underlie wide 
areas and contain much water. At the base of the Chesapeake forma- 
tion there usually are gravelly sands, of greater or less thickness, under- 
lying the diatomaceous clays. These basal beds are generally water 
bearing. 
LAFAYETTE FORMATION. 
This is a thin sheet of gravels and loams which covers the plateau 
region of the "ridges" of the southern counties of the Western shore. 
It has been widely removed north of the latitude of Washington and 
from the area of the Eastern shore. It generally has water-bearing 
gravels at its base, and this water supplies all the shallow wells in the 
higher lands of St. Mary, Calvert, Charles, and southern Prince George 
counties. The water, however, is similar to that of the Columbia 
formation in being subject to surface contamination. 
COLUMBIA FORMATION. 
This occupies terraces along the tidal estuaries and the entire area 
of the region east of the Chesapeake Bay which is known as the 
"Eastern Shore." It is in greater part a thin sheet of loam merging 
downward into a gravel bed, and its total thickness rarely exceeds 25 
feet. The basal gravel contains water in moderate amount, and is the 
general source of supply for dug wells in the lower lands of the region. 
The water is usually of good quality, but in thickly settled districts 
and where surface drainage is defective it is subject to contamination. 
It is probable, also, that it is not sufficiently protected from surface 
water ever to be free from malarial germs. 
WELLS IN MARYLAND. 
Except in Baltimore, where there are many wells, the wells in Mary- 
land are relatively few in number and mainly in widely scattered 
groups. They have been generally successful in obtaining large water 
