As the ice melted, the oceans rose. The rise of the oceans drowned the mouths of the rivers all along the northeast coast and created the fantastically irregular coastline and the wealth of harbors characteristic of Massachusetts and Maine today. And the new rivers redistributed the surface soil and the glacial debris all over the landscape. Long Island was built of such debris.
During the advance and retreat of each of these vast waves of ice, the plant and animal life retreated and advanced, driven southward by the cold along the ice front, then creeping back as the climate warmed up again. Actually, the retreats were not as far as used to be believed. The constant melting of the ice at the southern edge, even when the ice was advancing, created heavy fogs which somewhat confined the cold. Not far in advance of the ice there seems to have been a relatively mild, very damp climate. The animals, birds, and plants that had been displaced by the ice moved southward, but not far ahead of the ice itself; and each time the ice melted back, all forms of life crept northward again. Even plant life spread northward in those intervals, the seeds sometimes carried by the wind, sometimes in mud on the feet of birds and animals, sometimes in bird and animal droppings. Plant life’s return was somewhat more leisurely than the return of bird and animal life, but it came back each time.
At one time or another, all New England was swept clean by the ice. All the plant life that was here before the ice came was killed or swept away. The plant life we know today has crept back in the past 10,000 or 15,000 years. Tundra plants came first, of course, the Alpine and Arctic types of hardy dwarfs, shrubs, bushes, and low-growing plants that can endure severe cold. Then came the pines, the spruces, and the hemlocks. Last to return were the hardwoods. And of the hardwoods, the birch, the beech, and the maple were the first to get a foothold. After them came the oaks, the chestnuts, and the hickories.
With such a history, the Northeast provides a great variety of geographic and geological material for study, as well as a variety of biotopes or plant and animal environments.
The surface of the land is made up of soil and rocks. Soil consists of finely divided rock materials and decayed or decaying plant and animal material. The soil is a result of eons of the whole erosion process, together with the continual cycle of life, growth, maturity, death, and decay. But underlying the soil, and thrusting up through it in many places, is the basic rock of the earth’s solid crust.
The geologist classifies rocks according to their origin, igneous, sedimentary, or metamorphic.
Igneous rocks were formed by the cooling of molten material from within the earth’s crust. There are two basic kinds of igneous rocks, those that cooled inside the crust and formed such rocks as granite, and those that cooled outside in the form of lava, which produced such rocks as obsidian, basalt, and pumice.
Rocks erode and some of the minerals in them are dissolved in water. When surface rocks erode they become pebbles and sand and finely divided particles such as clay. These eroded materials are carried off by wind and water and deposited, usually in layers, and they become the materials for sedimentary rocks. Sandstone and shale and conglomerate, which is like sandstone with pebbles scattered through it, are typical sedimentary rocks. Natural cements such as lime, silica, and iron are filtered out of solution into such layers of sediment and bind the materials together. In the ocean, limestone is formed when the remains of shellfish and other animal life accumulate on the ocean floor; they have a high calcite or lime content, another natural cement.
When rocks already in existence are crushed together by the tremendous pressures of a convulsive earth and acted on by the earth’s internal heat and the heat resulting from those pressures, metamorphic rocks are formed. Such action changes sandstone into quartzite, limestone into marble, granite into gneiss (pronounced “nice”), shale into slate. The degree of change depends on the pressure and the amount of heat present.
Theoretically, there is a sequence in these processes. The magma, or internal igneous rocks, become lava. The lava is eroded into sand and pebbles and carried off and deposited as sandstone, sedimentary rock. Sedimentary rock is squeezed and heated by the earth’s convulsive pressures and metamorphosed. If the heat and pressures are sufficiently high, the metamorphic rock may be melted and changed to magma once more, which could spew out as lava and start the whole cycle over again. Actually, what we have is an overlap rather than a clear-cut differentiation, not only the three basic kinds of rocks but gradations of all three. And, of course, in almost every area there is a variety of rock forms, for the earth has jumbled them about and all the forces of erosion, from wind to glaciation, have scattered them. And because some rocks are more durable, more resistant to erosion than others, we have the geographic features of the land, the hills and valleys, the plains and mountains. These factors, as well as the variety of soils from one area to another, combine with weather and climate to create the various biotopes or particular areas of somewhat specialized forms of life.
There is the temptation to continue with geological speculation, which must be resisted. Anyone living in the Northeast is specially favored with a vast museum of geological material for study, with the whole story of rocks and the land lying there waiting to be read. I could spend years puzzling over the mysteries of the stones in the tumbled walls on my own land, for example, many of which were brought here from distant places by the glaciers and most of which still show the indelible marks of that long-gone age. On my desk right now is a small bowl of garnets I plucked from a weathered ledge of mica schist on the mountainside. In what ancient time and what warm seas were those magnesium silicates deposited in that rock, and what incredible forces metamorphosed the rock and created these dull red crystals that can be polished into semiprecious gems or crushed into abrasive covering for sandpaper or garnet-cloth? What convulsions and what vast erosions brought them into the open here on my ancient mountainside?
But the purpose of this chapter is to examine and speculate on the various kinds of environment for life. Let those whose deep interest is in the rocks follow out such questions.
In broad terms, that part of the continent which was covered by the glaciers of the Ice Age still shows traces of that time not only in its terrain but in its plant life. Perhaps we are still recovering from the effects of the ice. For example, there is an irregular line across New York, Pennsylvania, and the Midwest where certain kinds of plants seem to stop short—south of it you find such plants as sweet buckeye, Hercules’-club, and mistletoe, and north of it you do not find them. The soil, the climate, and even the terrain seem to be almost identical on both sides of that invisible line, and there is a wealth of plant life that pays no attention to the line. But there are certain trees and lesser plants that obey some invisible barrier. Actually, that line is the southern boundary of the ice sheet, and geological evidence proves it. The ice killed those plants, and they have never returned, for some reason. So we have two major biotopic areas still separated by a line that vanished perhaps 15,000 years ago.
There is another biotopic division of major proportions in the mid-South, and again it is a result, at least in good part, of the Ice Age. In the Cumberland Mountain area of Tennessee, Virginia, and western North Carolina is a region of remarkable variety of plant life. The hardwood forests there are both venerable and widely diversified. They include birch, black cherry, cucumber tree, ash, red maple, sour gum, several kinds of basswood, tulip tree, sugar maple, red and white oak, hemlock, beech, hickory, black walnut, and a score of other species. And the lesser plants are equally varied, the underbrush and the wildflowers, a vast wealth of them. The plant life of this area was in process of development long before the glacial ages. It is, in a sense, the mother greenhouse or nursery for most of the Northeast. Before the Ice Age, this wealth of plant life undoubtedly extended out from that area in all directions. It certainly reached well over the whole Appalachian area and probably into New England.
Then came the ice. This northern land of ours was denuded. That parent
nursery was not only untouched by the ice but apparently not affected greatly by the climatic changes. Its wealth of plant life continued to flourish. Then the ice finally melted. Here lay the land, vastly changed, its soil reconstituted where any soil was left, its climate relatively mild once more. And as the land began to emerge from the boggy, chill, foggy state in which the melting ice left it, hardy plants from the Cumberland began to move in. Oaks, chestnuts, and other nut trees that thrive in a mild climate were among the first, perhaps because returning animals such as squirrels helped plant their seeds. It seems now that the oak forests of the Northeast established themselves about 5,000 years ago, when climates all over the earth were warmer than they are today. Since that time the climate has turned cooler again, but the oak forests adapted themselves to the change.
Logically, in terms of the climate the white man has always known here, all of New England should have the North-woods type of forest cover, completely lacking in oaks and related hardwoods, with spruce, hemlock, white pine, birch, and maple predominating. Actually, this natural tendency creates botanical pockets here and there, special biotopes as the scientists would call them. In shady ravines, on north-facing slopes, on damp slopes above swamps even in southern Connecticut, typical North-woods trees are often found, hemlocks, black birch, beech, and sugar maples. And oaks and hickories are scarce. Such areas are always marked by damp soil, shelter from drying winds, and some natural protection from the intense heat of the Summer sun. Often you can go just over the ridge from such pockets and find a totally different environment, drier soil, more direct sun, and oak-hickory woods, an entirely different biotope though less than a mile from the little North-woods area.
The kind of trees has a profound effect on the soil and on the lesser plants. Maple, walnut, birch, basswood, and ash enrich the soil. Their leaves are relatively soft and sweet; earthworms eat them, bacteria work on them, and they soon become a layer of leafmold, a rich topsoil. Oak leaves, tough and full of tannin, decay slowly and sometimes form thick, fibrous mats where few bushes or lesser plants can find roothold and sustenance. If other trees take root there, they are the lesser ones—dogwood, hornbeam, sassafras—and few shrubs except blueberries, laurel, and hazel thrive. On the forest floor itself you will find Canada Mayflower, partridgeberry, wintergreen, sometimes trailing arbutus, but few others of the flowering tribe.
On the other hand, the type of terrain and the quality of the soil largely determine the kind of woodland that grows there. For example, rocky ledges and places with thin, poor soil are likely to have a growth of red cedar. Upland ground, dry and relatively poor in fertility, is typical oak and hickory country. Fertile but moderately dry soil invites sugar maple, walnut, white ash, white birch, and white pine. Damp, cool soil is preferred by spruce and hemlock. Riverbanks and swamp margins, especially those with full sunlight, invite swamp maple, black ash, willow, tamarack, cottonwood, and basswood. Almost invariably there is some overlapping of such areas. Red cedars, for instance, are constantly creeping down from the dry, sterile ledges on the mountainside and gaining a foothold in the margins of my moist, fertile pastures. There are a few oaks in the rich, damp soil along the riverbank. And there is an occasional swamp maple on the mountainside where I am sure it lacks the damp rootbed it usually prefers. But this merely proves that the boundaries of any biotic area are approximate, never clear-cut.
These overlaps make life interesting for the person who explores the outdoors, for there is always a surprise possible. Purple vervain, for instance, is a riverbank, damp-soil plant which I find only in the bogland or along the river. Usually. But one afternoon I was high on the mountainside when I saw a flash of color that certainly was the blue flower of that misnamed vervain. I made my way through the woodland toward it and came out in a tiny opening where a trickle of a seep spring oozed up among the rocks and made a little patch of bog, a wet spot not five feet across. A thread of spring water flowed from it perhaps twenty feet, then disappeared among the rocks. But that little damp spot had a patch of lush green grass, a couple of marsh violet plants, and one thriving stalk of vervain in full bloom. It had no business growing there, theoretically at least. Probably the seed from which it grew was carried to that unlikely spot by some bird with muddy feet. However it got there, it sprouted, grew, and was now in bloom.
In a sense, that tiny spot of boggy soil, so small I could jump across it, was a special biotope, a particular environment for such plants as purple vervain and marsh violets. Possibly there were a couple of green frogs there, too, though I didn’t see them.
Almost any area, even one of only a few square miles, will have its lesser specialized environments, just as my own place has. Even without a river or a bog, there may be meadow and woodland, or a low hill and a shallow valley. Even two or three trees can create a small environment of their own, with their shade and the effect of their decaying leaves on the soil beneath them. And any hill will have a slightly different type of life from that in the valley at its foot.
I can illustrate this even in our vegetable garden, a patch only about a hundred feet square, or in the lawn beside the house.
The soil here is typical valley soil, deposited first by the river, long ago, then added to by the wash from the mountainside, and finally topped by a layer of natural top-soil. Logically the soil in the garden should be uniform. But one half of the garden is dark, fairly heavy soil, and the other half is lighter, somewhat sandy soil. The division line runs right down the middle of the garden. The dark soil is ideal for leaf crops, the lighter soil is perfect for root crops. And even our ten years of cultivation, on top of perhaps forty or fifty years of cultivation before we came here, have not greatly altered the character of the soil in either side. There in that garden we have two different natural environments, side by side. If it were left to go back to natural growth there would be a slight difference in the wild plants that established themselves there.
Here on the side lawn the soil seems to be fairly uniform, but the presence of a big Norway spruce makes a noticeable difference. Under the spruce the grass needs constant care and pampering, but only a few feet away the grass grows lush and luxuriant. The tree makes all the difference, as any suburban householder learns. The tree casts shade, and its slow fall of needles alters the chemistry of the soil. Ground ivy, dandelions, and plantain, which thrive in shade and more acid soil, are constantly trying to crowd out the grass under the tree. Again, I have two minor biotopes, two separate plant environments, side by side.
Multiply such slight differences by a hundred or a thousand times and you have the difference in natural conditions between, say, a meadow and a woodland, or even between two areas in the same meadow.
But even these are essentially minor differences in the big picture, details to be noted after one has taken account of the major differences between the larger natural regions. Climate is the first of the governing factors. Then comes geography, the lay of the land. And after those first two come the matters of soil and moisture.
Climate is a matter of average temperatures, prevailing winds, and normal rain and snowfall. The United States has a variety of climates because of its size and varying terrain. Average annual temperatures, for instance, range from below 40° to above 70°. Average Summer temperatures range from below 60° to above 90°, and average Winter lows vary from minus 40° to plus 20°. Maine, upper Wisconsin, and North Dakota and the Rocky Mountains are the coldest areas; lower New Mexico and California, the hottest. Most of New England, upper New York State, and the upper Midwest are cool, and a broad area from the mid-Atlantic coast west to the Rocky Mountains is, on the averages, moderate in climate, averaging from zero to 20° below in Winter, 70° to 80° in the Summer. There are hotter and colder spots in all these areas, of course, caused by the lay of the land. But each of them is essentially one major biotope or environment.
Average annual precipitation, both rain and snow, is between 30 and 50 inches throughout New England, the Northeast, and the Midwe
st, with little patches of heavier precipitation here and there. The Deep South, except most of Georgia, has a wetter climate, between 50 and 70 inches of rainfall a year. But when it comes to the amount of sunshine and the number of clear days each year, the picture changes again. Lower New England and most of the Midwest have 140 to 180 clear days a year. This number drops to 100-140 in the Great Lakes area. And the pattern of snowfall is still different, snow-covered days averaging around 60 a year in the lower Midwest and lower New England, and 100 or more in the upper Midwest and upper New England.
And, finally, the growing season, the period of frost-free weather, follows still another pattern on the map. All New England and most of the Midwest as far as the Rockies averages from 80 to 120 frost-free days a year. But when you try to plot the dates of last Spring frost and first Fall frost you create a pattern even more complicated than the daily weather map. Frost follows the pattern of the land. My place here close beside the river, for instance, has almost two weeks more of frost-free weather than the hilltop farm of a friend only two miles away. Frost normally follows the valleys, so my place should be colder than his; but the river tempers my climate and reverses the normal difference from hilltop to valley. This one factor somewhat affects the character of all the natural growth on my land and that of my friend.
All these factors—soil, terrain, average temperature, frost dates, amount of sunshine, annual rain and snowfall—determine the natural life in any area. Even trees respond to slight changes in natural environment. To pick one example, the black walnut, though common in the Midwest and much of the Northeast, is rare in Connecticut and Massachusetts. But here in my own area, in a pocket of only a few square miles, the black walnut was thriving when the white man first arrived. Go fifteen miles in any direction and you won’t find a black walnut tree, but here it still grows vigorously. There must be something in the soil and climate of this small, particular area that satisfies the needs of the black walnut.