Chapter I
Physical Geography and Geology of Delaware County
The surface of the county is hilly with very little exception in its western part, but somewhat level in its eastern portion. Its drainage is by several small streams, called creeks, that flow in a southerly direction and empty into the Delaware River. These and their tributary branches make Delaware a well-watered county. Almost every country house is supplied from a never-failing spring of pure, soft water, and nearly all the fields of every farm have running streams through them.
The flowing of these creeks down a surface inclined to the Delaware River, which is the southeastern boundary of the county, gives an abundance of water-power, which is used for various manufacturing purposes. The rapid flow of these streams and their numerous branches have cut deeply into the surface of the land, making it beautifully diversified by wood-crowned hills and fertile valleys and hill-sides. No one who has ever seen the charming scenery of this part of our State can exclude from the recollection of it the well-tilled farms, with their tastefully-planned homes, capacious barns, fields of waving grain, and the herds of cows that supply milk and butter of the very best quality to the Philadelphia market. Here grow luxuriantly all the fruits, grains, grasses, and vegetables of the temperate zone. The declension of the surface of the land toward the south brings it near to a right angle with the rays of the sun, which has an effect on its temperature that is equivalent to being a degree or more farther south. The lower altitude of lands touching tide-water also favors the mildness of the climate as compared with higher surfaces. Grass is ready for pasturing about ten days earlier in the spring than on the higher and more horizontal lands of similar quality a few miles farther north. The river has a considerable influence on the temperature of that part of the county bordering immediately on it. In winter the air may be for a long time at a freezing temperature before the river has ice on it, for the reason that the whole depth of water must be very near to the freezing-point before its surface can become ice, though the surface of the ground will be frozen by a single night of coldness.
Under such circumstances, and they occur every year, the two miles of width of water that is several degrees warmer than the general atmosphere has a very perceptible modifying influence. Fruits and flowers remain untouched by frost for several weeks after hard freezing has occurred in other parts of the county. In summer, evaporation keeps the river cooler than the surface of the land, which, becoming heated by the sun’s rays, radiates the heat into the air above it. The air expanding by the heat becomes lighter, and rises, and is replaced by the heavier air from the river, which flows with refreshing coolness and moisture over the parched land. These river breezes are of daily occurrence whenever the surface of the land is warm and dry, and their visits are delightfully acceptable.
The geology of the county is somewhat peculiar. Our rocks belong to the earliest formation known to geologists. They were formed by the first process of hardening, which occurred when the surface of the great red-hot drop of molten matter which now constitutes the earth had cooled to the hardening-point. Having been formed by cooling from a melted condition, they are crystalline in structure. It appears that they have not been submerged in the water of seas or lakes, where, if they had been, deposits of mud, sand, and gravel might have been washed upon them, to afterwards be hardened into rocks, but that since rocks have existed on the earth these have been a part of the dry land. They contain no traces of the remains of organic beings, such as are found in the stratified rocks that are formed under water.
In many parts of the county great fissures have opened, in the remote past, into which the liquid rock of the earth’s interior has been injected, forming what are known as dikes. Into these different kinds of rock have been forced, some being trap-rock and others serpentine. Coming from the earth’s interior, this liquid matter was intensely hot, and heated the rocks on both sides the dikes so much as to change their texture by semi-liquefying them, and thereby favoring a recrystallization into different forms.
Overlying the rocks of the country are deposits of gravel, sand, and clay. Some of these are results of the decomposition of the rocks themselves, but the greater part of them appear to have been brought from some other region, and the opinion is generally accepted that they were pushed from the country north of us by immense glaciers, that appear to have at one time covered all the northeastern parts of this country. The minerals of the county are very numerous. There are very few places in the whole country that offer such an extensive field for scientific research in this direction as the small county of Delaware.
Contributed by Ellwood Harvey, M.D., Chester
Source: Page(s) 1-2, History of Delaware County, Pennsylvania, by Henry Graham Ashmead, Philadelphia: L.H. Everts & Co. 1884
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