On the Physical Features and Geology of Chignecto Isthmus.

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On the Physical Features and Geology of Chignecto Isthmus.
Alex Monro
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On the Physical Features and Geology of Chignecto Isthmus. BY ALEX MONRO, C.E. (From Bulletin Number V of the Natural History Society of New Brunswick, Read Feb. 3, 1885.) There are but few places in the Maritime Provinces where nature and history combine so many points of interest as exist on the Isthmus between Nova Scotia and New Brunswick. The great rise and fall of tides in Cumberland Basin, submerged forests and sea dykes, are subjects of much interest; and both nature and history point to this Isthmus as the chief highway occupied by the men of the stone age in their migrations from one side of the country to the other. Here, too, is the Missiguash River, tacitly acknowledged by England and France of old as the boundary between their possessions, and which is now the boundary between Nova Scotia and New Brunswick. On this Isthmus we can trace the outlines of forts, battlefields, dykes, roads, and other landmarks of the early history of the country. The hydrographical basin of the Bay of Fundy and the topography of the Isthmus are inseparable. This Bay and its two chief arms – one terminating in Cobequid Bay, at the head of the Basin of Minas; the other terminating in Cumberland Basin, are in general range of the great tide-wave which sweeps to the north-east along the Atlantic coast of the United States. In consequence of the contraction of the sides of the Bay of Fundy towards its head, there is a convergence of the tidal wave, which causes a very great increase in its height. The vertical height of the tide in Cumberland Basin at high spring tides is fifty feet; at Cobequid Bay it rises still higher; while at St. John it rises only about half this height. Around Cumberland Basin there are about forty thousand acres of marsh, enclosed by artificial dykes, the chief part of which is on the Isthmus. This great plain is divided into sections, separated from each other – except at Cumberland Basin – by long spurs of upland, known as the Jolicure, Point De Bute, and Fort Lawrence ridges. On the east of the marshes the town of Amherst, with its rich and picturesque scenery, spreads out before the eye; on the west, Sackville commands a view of one of the most extensive tracts of fertile land to be found in the Maritime Provinces; on the south lies Beau Bassin, the beautiful basin of the French Pioneers; and northerly, long ranges of settlements stretch along the ridges, across the Isthmus to Baie Verte; the distance across, in the narrowest part, being fifteen miles. These ridges and marshy plains are joined to a narrow belt of upland near the head of Baie Verte. Jolicure is four miles in length; Point de Bute, nine; and Fort Lawrence, seven miles. The average width of each ridge is about a mile, and their limits do not exceed one hundred feet above the marsh. The terminal points of the two latter ridges are within a mile of Cumberland Basin. The three great arms of the marsh vary in width from one mile to four, and in length from seven to nine miles. Each is traversed by a lacustrine stream, which discharges by a deep channel into the Cumberland Basin. Nowhere in North America does nature exhibit more remarkable tidal phenomena than at this Isthmus. The proximity of the counter-tides gives force to this view. In Cumberland Basin, the tide rises from thirty-five to forty-six feet over ordinary low water line, and at high spring tides over fifty feet. At times the water in Cumberland Basin is fully eighteen-and-a-half feet over that at Baie Verte; while at ebb tide the water in Baie Verte is nineteen-and-a-half feet higher than in Cumberland Basin. And at the head of the tideway in the rivers discharging into this Basin the tide rises to a still higher level. The time of high and low water is from two-and-a-half to three hours earlier at Baie Verte than in Cumberland Basin. But how was this physical condition in the relation of land and water caused? Sir William Dawson, in his “Acadian Geology,” says it was caused either by the “rupture of a barrier previously excluding the sea water, or an actual sinking or subsidence” of the land. He favors the subsidence hypothesis. However, since the publication of his “Acadian Geology,” a complete survey of the Isthmus has been made by order of the Dominion Government, with a view of uniting the waters of the Bay of Fundy with those of the Straits of Northumberland by means of a ship canal. The levels run across the Isthmus clearly show that Spring tides in Cumberland Basin are only twenty feet over the normal height of the tide in the Straits of Northumberland. That this elevation of the tide in Cumberland Basin over that of the Straits arises chiefly in consequence of the peninsular portion of Nova Scotia, I have no doubt. The original bed of Cumberland Basin may have been a shallow, fresh-water lake – a receptacle for the drainage waters of the Isthmus, which may have had their outlet over a rocky barrier to the westward. And when we consider the great depths of water in the Bay of Fundy compared to the depth farther seaward; and also in its arms, compared to the depth of water in the Straits of Northumberland, I think it might be inferred that the present great depth of water in the Bay of Fundy and its arms is largely due to the deepening and scooping-out action effected by the disintegrating force of the tidal-wave, and not to the subsidence of the land. There appears to be a gradual increase in the elevation of the surface tide-wave, as it ascends towards the head of the tide-ways in the bays and rivers, except where effected by local causes, while there is also a corresponding rise in the beds of these waters. In the Straits of Northumberland, between Cape Tormentine and Cape Traverse, where the Straits are narrowest, the ordinary depth of water at high water is only forty-two feet; in Cumberland Basin it is sixty-two feet at high water. The difference, twenty feet, being the height of the tides in the Basin above the tides in the Straits. Thus, it appears that the beds of these opposite waters have the same elevation above a common datum. There are several places in Cumberland Basin where the tides rise and fall to a depth of twenty feet over some of the once forest-clad uplands of the country. And it is highly probable that large areas of the ancient forests underlie the adjacent marshes. Indeed, where canals for drainage have been dug, trees are met with several feet below the surface. One of the ranges of submerged forests I surveyed in 1872 extends nearly half a mile along the northerly side of Cumberland Basin. At low water it had the appearance of being the southerly slope of an upland ridge, a part of which – Tongues Island – in the adjacent marsh, may be the summit. This submerged forest is from ten to twenty feet, vertically, below high water. I counted over two hundred stumps; in all about six hundred were counted. The appeared to be in the place of their growth, like an open forest. Some of the stumps were over eighteen inches in diameter. I observed several trees lying with their tops towards the head of the Basin. The trees were imbedded in clay, mud, and small stones. One tree measured fifty and another sixty feet in length. The [illegible] consisted chiefly of spruce and fir, with a mixture of other kinds of wood such as now grow on the Isthmus. Judging from their remarkable good state of preservation, and making allowance for the preservative effect of the salt water, it is obvious that these submerged forests belong to a modern period in geologic time. The lakes at the head of the marshes are shallow, and form chains of quiet expansions united by short discharging channels, which in some places are overgrown by shrubbery. Within the present century the Missiguash River and chain of lakes at its head were navigable for boats from Cumberland Basin. Within the last fifty years Indian canoes in large numbers followed this route to within three miles of the navigable waters of Baie Verte. This route has ceased to be navigable for canoes. Over the portages separating the counter-waters the Indians carried their canoes on their shoulders. At the head of the easterly arm of the Missiguash valley the water-shed between the counter tidal levels is two-and-a-half miles broad, and at the crowning part only thirteen feet above high water in Cumberland Basin, or about four feet above the Saxby tide of 1869; forming, as it were, an isthmus within an isthmus. Here two extremes, the low tides of the Straits and the high tides of Cumberland Basin, almost meet. This upland ridge is composed, to a depth of five feet, of soft alluvial matter, resting on clay. So slender, indeed, is this barrier, that a few rolls of tide in winter, but little higher than the Saxby tide, might force the waters of Cumberland Basin across the Isthmus to a level twenty feet below. And on the side of the Isthmus facing the Straits there are places along the terraces, or narrow marsh levels of the rivers, lines of “sea dykes.” These dykes are from eight to twelve feet broad, and three to six feet high. Probably they have been much broader and higher in prehistoric times. Except where streams are crooked the dykes are on both sides, but where very crooked they sweep from one side to the other. Their bases are generally on a level with the full spring tides of the present time. I observed one place, however, where the stream formed a sharp angle near the uprising upland, that the base of the dyke was from five to eight feet above the tide level. These dykes or mud walls are compactly formed of river mud and other superficial matter. In exposed places, where they are being worn down by the encroachments of the sea, stumps or trees and stones are exposed to sight. The stumps appear to be in the place of their growth, and in about the same state of preservation as those in Cumberland Basin. And, where the dykes are well sheltered from the sea, I have frequently observed large spruce, pine and birch trees growing on their tops.