Outlines of the Earth's History: A Popular Study in Physiography

by Nathaniel Southgate Shaler

The group of marine marshes differs in many important regards from those which are formed in fresh water. Where the tide visits any coast line, and in sheltered positions along that shore, a number of plants, mostly belonging to the group of grasses, species which have become accustomed to having their roots bathed by salt water, begin the formation of a spongy mat, which resembles that composed of Sphagnum, only it is much more solid. This mat of the marine marshes soon attains a thickness of a foot or more, the upper or growing surface lying in a position where it is covered for two or three hours at each visit of the tide. Growing rapidly outward from the shore, and having a strength which enables it to resist in a tolerably effective manner waves not more than two or three feet high, this accumulation makes head against the sea. To a certain extent the waves undermine the front of the sheet and break up masses of it, which they distribute over the shallow bottom below the level at which these plants can grow. In this deeper water, also, other marine animals and plants are continually developing, and their remains are added to the accumulations which are ever shallowing the water, thus permitting a further extension of the level, higher-lying marsh. This process continues until the growth has gone as far as the scouring action of the tidal currents will permit. In the end the bay, originally of wide-open water, is only such at high tide. For the greater part of the time it appears as broad savannas, whose brilliant green gives them the aspect of rare fertility.

Owing to the conditions of their growth, the deposits formed in marine marshes contain no distinct peat, the nearest approach to that substance being the tangle of wirelike roots which covers the upper foot or so of the accumulation. The greater part of the mass is composed of fine silt, brought in by the streams of land water which discharge into the basin, and by the remains of animals which dwelt upon the bottom or between the stalks of the plants that occupy the surface of the marshes. These interspaces afford admirable shelter to a host of small marine forms. The result is, that the tidal marshes, as well as the lower-lying mud flats, which have been occupied by the mat of vegetation, afford admirable earth for tillage. Unfortunately, however, there are two disadvantages connected with the redemption of such lands. In the first place, it is necessary to exclude the sea from the area, which can only be accomplished by considerable engineering work; in the second place, the exclusion of the tide inevitably results in the silting up of the passage by which the water found its way to the sea. As these openings are often used for harbours, the effect arising from their destruction is often rather serious. Nevertheless, in some parts of the world very extensive and most fertile tracts of land have thus been won from the sea; a large part of Holland and shore-land districts in northern Europe are made up of fields which were originally covered by the tide. Near the mouth of the Rhine, indeed, the people have found these sea-bottom soils so profitable that they have gone beyond the zone of the marshes, and have drained considerable seas which of old were permanently covered, even at the lowest level of the waters.

On the coast of North America marine marshes have an extensive development, and vary much in character. In the Bay of Fundy, where the tides have an altitude of fifty feet or more, the energy of their currents is such that the marsh mat rarely forms. Its place, however, is taken by vast and ever-changing mud flats, the materials of which are swept to and fro by the moving waters. The people of this region have learned an art of a peculiar nature, by which they win broad fields of excellent land from the sea. Selecting an area of the flats, the surface of which has been brought to within a few feet of high tide, they inclose it with a stout barrier or dike, which has openings for the free admission of the tidal waters. Entering this basin, the tide, moving with considerable velocity, bears in quantities of sediment. In the basin, the motion being arrested, this sediment falls to the bottom, and serves to raise its level. In a few months the sheet of sediment is brought near the plane of the tidal movement, then the gates are closed at times when the tide has attained half of its height, so that the ground within the dike is not visited by the sea water, and can be cultivated.

Along the coast of New England the ordinary marine marshes attain an extensive development in the form of broad-grassed savannas. With this aspect, though with a considerable change in the plants which they bear, the fringe of savannas continues southward along the coast to northern Florida. In the region about the mouth of the Savannah River, so named from the vast extent of the tidal marshes, these fields attain their greatest development. In central and southern Florida, however, where the seacoast is admirably suited for their development, these coastal marshes of the grassy type disappear, their place being taken by the peculiar morasses formed by the growth of the mangrove tree.

In the mangrove marshes the tree which gives the areas their name covers all the field which is visited by the tide. This tree grows with its crown supported on stiltlike roots, at a level above high tide. From its horizontal branches there grow off roots, which reach downward into the water, and thence to the bottom. The seeds of the mangrove are admirably devised so as to enable the plant to obtain a foothold on the mud flats, even where they are covered at low tide with a depth of two or three feet of water. They are several inches in length, and arranged with booklets at their lower ends; floating near the bottom, they thus catch upon it, and in a few weeks' growth push the shoot to the level of the water, thus affording a foundation for a new plantation. In this manner, extending the old forests out into the shallow water of the bays, and forming new colonies wherever the water is not too deep, these plants rapidly occupy all the region which elsewhere would appear in the form of savannas.

The tidal marshes of North America, which may be in time converted to the uses of man, probably occupy an area exceeding twenty thousand square miles. If the work of reclaiming such lands from the sea ever attains the advance in this country that it has done in Holland, the area added to the dry land by engineering devices may amount to as much as fifty thousand square miles—a territory rather greater than the surface of Kentucky, and with a food-yielding power at least five times as great as is afforded by that fertile State. In fact, these conquests from the sea are hereafter to be among the great works which will attract the energies of mankind. In the arid region of the Cordilleras, as well as in many other countries, the soil, though destitute of those qualities which make it fit for the uses of man, because of the absence of water in sufficient amount, is, as regards its structure and depth, as well as its mineral contents, admirably suited to the needs of agriculture. The development of soils in desert regions is in almost all cases to be accounted for by the former existence in the realms they occupy of a much greater rainfall than now exists. Thus in the Rocky Mountain country, when the deep soils of the ample valleys were formed, the lakes, as we have before noted, were no longer dead seas, as is at present so generally the case, but poured forth great streams to the sea. Here, as elsewhere, we find evidence that certain portions of the earth which recently had an abundant rainfall have now become starved for the lack of that supply. All the soils of arid regions where the trial has been made have proved very fertile when subjected to irrigation, which can often be accomplished by storing the waters of the brief rainy season or by diverting those of rivers which enter the deserts from well-watered mountain fields. In fact, the soil of these arid realms yields peculiarly ample returns to the husbandman, because of certain conditions due to the exceeding dryness of the air. This leads to an absence of cloudy weather, so that from the time the seed is planted the growth is stimulated by uninterrupted and intense sunshine. The same dryness of the air leads, as we have seen, to a rapid evaporation from the surface, by which, in a manner before noted, the dissolved mineral matter is brought near the top of the soil, where it can best serve the greater part of our crop plants. On these accounts an acre of irrigated soil can be made to yield a far greater return than can be obtained from land of like chemical composition in humid regions.

Fig. 20.—Diagram showing mode of growth of mangroves.

In many parts of the world, particularly in the northern and western portions of the Mississippi Valley, there are widespread areas, which, though moderately well watered, were in their virgin state almost without forests. In the prairie region the early settlers found the country unwooded, except along the margins of the streams. On the borders of the true prairies, however, they found considerable areas of a prevailingly forested land, with here and there a tract of prairie. There were several of these open fields south of the Ohio, though the country there is in general forested; one of these prairie areas, in the Green River district of Kentucky, was several thousand square miles in extent. At first it was supposed that the absence of trees in the open country of the Mississippi Valley was due to some peculiarity of the soil, but experience shows that plantations luxuriantly develop, and that the timber will spread rapidly in the natural way. In fact, if the seeds of the trees which have been planted since the settlement of the country were allowed to develop as they seek to do, it would only be a few centuries before the region would be forest-clad as far west as the rainfall would permit the plants to develop. Probably the woods would attain to near the hundredth meridian.