History of Chittenden County, Vermont, with illustrations and biographical sketches of some of its prominent men and pioneers, Part 4

Author: Rann, W. S. (William S.)
Publication date: 1886
Publisher: Syracuse, N.Y. : D. Mason & Co.
Number of Pages: 1054

USA > Vermont > Chittenden County > History of Chittenden County, Vermont, with illustrations and biographical sketches of some of its prominent men and pioneers > Part 4

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GEOLOGICAL FORMATION OF THE COUNTY.

agencies of the globe was made, preparations were going forward which made this inevitable after a time. And here I come to debatable ground. I pre- sume that many geologists would strongly object to locating the upheaval of the Green Mountain ridge at the close of the Lower Silurian. The age of these mountains has been very warmly discussed, and the discussion has by no means brought the ideas of the disputants into harmony. Some would make them much older than the Silurian, even placing them in the Archaan ; but it seems to me that more than all the rest Professor Dana has reason and fact on his side in placing them where he does, and all subsequent investigations go to sustain him in his views. I have already quoted a paragraph which expressed these in general; but as the matter is of importance I would like to add one or two more extracts from the Manual of Geology. On page 196 we find the statement that-"In the Green Mountain region there are 2,000 or 3,000 feet or more of mica schist and slate, hydromica slate, gneiss, quartzite, and con- glomerate, which are probably of the Cincinnati series." Again, "Previous to the epoch of revolution the Green Mountain area had been a region of accu- mulating limestones through the Cambrian and Trenton periods, and of beds of quartzose sands and probably some limestone through the Cincinnati era. But here the rock-making over the region ended. Next came the upturning, in which the same rocks were lifted and folded and crystallized, and the Green Mountain region became dry land" (page 212). In the American Journal of Science and Arts for May, 1880, Professor Dana further says, as he sums up evidence given on previous pages : "It thus appears that on the mass of land which topographically belongs to the Green Mountain range that part which is already proved to be Lower Silurian in age and of one orological system, con- stitutes nearly one-half of the whole range." In this and subsequent articles Professor Dana shows that it is extremely probable that the rest of the Green Mountain range is of the same age. Professor Dana evidently does not sup- pose that the material out of which the Green Mountains were made was accu- mulated in a single period, but that "the limestones of the Green Mountain region include the limestones of successive periods from the Calciferous, and probably Primordial, to the Trenton." And again, Vol. XX, page 455, of the same journal-"The limestones and conformably associated rocks of the Green Mountain region from Vermont to New York Island are of Lower Silurian age." In a most valuable monograph on The Azoic System, by Professors J. D. Whitney and M. E. Wadsworth, these views of Professor Dana are quoted, and the authors remark: " Of the correctness of these statements in regard to the Lower Silurian age of the rocks in question, it seems to us that there can be no possible doubt" (page 462). It has seemed necessary to be thus full and explicit in this matter because of the wide diversity of opinion which has existed, and in some measure still exists, among geologists respecting the age of these rocks. The question also directly concerns the geology of this county,

HISTORY OF CHITTENDEN COUNTY.

for not only does a large part of the rocks of the county belong to the Green Mountain system, but one corner of Underhill reaches up along the west side of Mansfield and just takes in the Chin, which is the highest point of the whole range-4,430 feet above tide water, or 4,340 above the lake. Nearly the whole mass of Camel's Hump is included in Huntington and Bolton, and this mount- ain is 4,088 feet above sea level. There is a considerable degree of uniformity in the constitution of most of the Green Mountains, though there are some conspicuous differences. The two peaks with which in the geology of this county we have to do, are similar. Mansfield is made up of mica schist, hydromica schist, and chloritic hydromica schist. Camel's Hump is chiefly mica schist. It may seem hardly creditable to some that the stratified lime- stones and sandrocks of the Silurian can by any means have become the unstratified crystalline gneiss, schist, etc., of the Green Mountains. For a full discussion of this matter the reader must be referred to any recent treatise upon general geology ; and I may recommend those who care for more information concerning many matters necessarily passed by with very brief notice in this chapter, to read Dana's Manual of Geology or Le Conte's Elements of Geology.

The old theory of mountain making was, that through some great crack in the earth's crust, molten matter was thrown out, which hardened into rock, and now and then a hill or mountain has been so formed; but these are the excep- tions. Later it was believed that mountain chains were great upheavals, inter- nal forces thrusting up portions of the earth's crust. Now the common view is that mountain masses are due to lateral pressure, and that a mountain mass is not an arching up of the earth's crust, but a thickening. The pressure is due to subsidence, so that as one part of the earth's crust rises in mountain masses another part sinks in ocean abysses. What then was the process in the case of the Green Mountains ? They probably began in a subsidence, by which a gigantic trough several hundred miles long was formed. This trough began before the Cambrian, and in it were deposited the layers of sandstone, etc., of this period. Through the various epochs of the Silurian this trough sank deeper and deeper, sedimentary deposits at the same time filling it up, so that, although actually growing deeper, it really was all the time very shallow, or if the deposits were coextensive with the subsidence, it was not a trough at all on the surface. In time the accumulating deposits would become very deep, so deep that the lower part of the trough would be warm, if not hot, from the in- terior heat of the earth. By this heat, aided probably by moisture contained in the rocks, the lowest part of the trough would be softened and weakened. All this time the Atlantic Ocean bottom was sinking and thus a strong lateral pressure brought to bear upon the coast, and in time this pressure became so great that wherever in the neighborhood the crust of the earth was weakest it must yield to this push from the east. The Green Mountain trough is such a weak area and it breaks, rock masses slide over and upon each other, and by

GEOLOGICAL FORMATION OF THE COUNTY.

the friction great heat is produced, and the rocky masses not only rise in folds and ridges, but they are transformed into non-stratified, non-fossiliferous rocks, in short the limestone, sandstone and shale have become gneiss, mica schist, etc. Thus the elevation of the Green Mountains, which began in a depression and for ages continued such, has fairly taken place. The various kinds of rock found in metamorphic regions may be produced by a different amount of moisture, heat, etc., from the same original beds. Professor Dana says: "The differences between mica schist, mica slate, hydromica slate and clay slate appear to have arisen largely from differences of temperature attending meta- morphism." During metamorphic action soft rocks become hard, color and chemical character may be changed, and often the rocks are cracked and fis- sured, and these crevices may be filled with some mineral. All this is not done in a moment, but slowly, often extending through ages. In the case of the Green Mountains it occupied the interval between the Lower and Upper Silu- rian. At first a mere reef extending through the Silurian Ocean, the Green Mountain axis slowly rose above the water and finally was very much higher and larger than now, for the storms of millions of years cannot have raged against these rocky masses in vain, but must have worn them down greatly, and the great glacier must also have ground them and broken them, as we shall see presently. The metamorphism of the originally stratified rocks was not confined to the more elevated parts of this county. Scattered over the county and the State, also, are large masses of metamorphic rock which Mr. Wing has shown to be of this Cincinnati epoch. Under the headings "Talcoid Schists " and "Talcose Schists " the Vermont Report speaks of certain rocks which are abundant in some parts of the State. These rocks are now included under the name hydromica slates. These slates are quite abundant through the eastern part of this county and include a variety of rocks. The Vermont Report gives no less than fifty-four varieties, found in following this formation from Montpelier to Duxbury. In some parts of Vermont the formation in- cludes very extensive and important beds of steatite, or soapstone, and also of serpentine ; and a long list of minerals, few of them, however, of economical value, occur in these slates and schists. Although the mountains are not made of this rock, it occurs abundantly along their flanks. In this county it abounds in Underhill, about the lower slopes of Mansfield, and it extends through Jericho, Huntington and Bolton, varying in character more or less in each locality. There is another group of rocks which are largely developed in many parts of the State, though not found in large outcrops in this county. This is the Taconic system, once famous in geological discussions, but now abolished and placed under the Silurian. Under the rocks of this system the formerly called Taconic rocks have been treated, and it is mentioned here only because it occupies an important place in the Vermont Report as a distinct group, and some might wish to know why it was left out here. Another extensive forma-

HISTORY OF CHITTENDEN COUNTY.

tion, or rather so-called formation of the Vermont Report, is what that report calls the Eolian limestone, to which the limestone at High Bridge is referred, also other limestone in Burlington, Milton, Colchester and elsewhere. The ledge at the Lime Kilns extends from Colchester south to South Burlington, Shelburne and Hinesburg. Some of the outcrops of this have been quarried for white marble, and it is so far changed from ordinary limestone that it is in places a clear white marble, but so far it has been found full of flaws and seams. As to the age of this limestone the Vermont Report says: "We quite despair of satisfying ourselves on these points [the age, etc.] in respect to the Eolian limestone." And this is about where the matter rested until Mr. Wing worked out the problem and found, after much study in the field, that the formation was not one, but several, including rocks of the Calciferous, Trenton and vari- ous intermediate epochs.

At some time after the Lower Silurian rocks were completed the strata were cracked and fissured, possibly as a part of the commotion which occurred when the mountains were rising. Into these cracks molten matter, like the lava of a volcano, was forced from below, filling them, and to-day we find here and there the black limestone or shale cut sharply across by a band of very different material, much harder and usually of lighter color. Such intrusions from be- neath of melted matter are called dikes. Nowhere in the State is there such a display of these formations as in this county, and here they are mostly near the lake. They are most numerous in Shelburne, but they are found in Burling- ton, Charlotte, Colchester, Hinesburg, Williston, Essex, Richmond and Bolton. At Red Rocks is a dike twelve feet wide. There are two on Spear street about half a mile south of the Williston road. There is a trap-dike at Willard's Ledge. A dike crosses the north end of Juniper Island. There are about twenty-five dikes in Shelburne and not less than sixty in the county, and the hill south of the depot at Charlotte is of the same igneous rock. The material which fills the dikes is usually homogeneous and hard, compact and, of course, not stratified. In this region the dikes are trap and porphyry. Mr. Z. Thomp- son thus describes the system of dikes at Nash's Point, Shelburne : "The por- phyritic dikes at Nash's Point and vicinity are so numerous and irregular that I shall not attempt to particularize them. The whole surface of this point, embracing several acres, is strewn with fragments of porphyry, and it seems to be cut up by dikes traversing it in all directions. On the southeast side of Nash's Bay the bank is formed by a porphyry dike about twenty feet high for the distance of twelve rods. The slate has all been removed on the side next the bay down almost to the surface of the water, and the dike stands out like a huge wall about five feet thick built to support the bank." Sometimes, though, this phenomenon is rare everywhere. Dikes of different kinds of rock and different ages are found. Mr. Thompson describes a case of this sort on Shel- burne Point. He says that there is "a perpendicular face of porphyry, about

GEOLOGICAL FORMATION OF THE COUNTY.

eleven feet high and some rods in length, resting upon black slate and soil, cut- ting through the slate in an easterly direction. Beneath the porphyry are two parallel trap dikes, about eight feet apart and each about one foot wide; por- tions of these trap dikes are also found in the slate overlying the porphyry." From this statement we see that the porphyry dike was of later origin than the trap dikes across which it cut its way. The end of the Lower Silurian is, as one who will glance at the list of formations given in the early part of the chapter may see, only a little way from the beginning of the series, and yet by this time the solid foundations and the mountains and hills of Vermont were finished and the character of Vermont as an agricultural State was largely determined, for soil must come from the decomposition and the grinding of rocks, chiefly the former; hence the kind of rock, whether it be limestone, sandrock, schist, or some other sort, affects the fertility of the soil and also its moisture and drainage.

The scenery of the State, the form of its mountains, the number and char- acter of its valleys, gorges, cliffs, etc., all depend upon the kind or kinds of bed-rock. A very long interval, probably many millions of years, now elapsed, during which many minor changes may have taken place; land may have been upheaved and sunk, strata deposited and washed away, whole groups of living creatures have come into existence and been annihilated, but of all this we know nothing so far as nearly the whole State is concerned, and certainly in this county we have no record of any event between the beginning of the Upper Silurian and the end of the Tertiary. During this great interval the North American Continent grew southward beyond Pennsylvania. Thousands of feet, not less than forty thousand and probably more, of limestone, sandstone and shale, were formed, and great beds of it upheaved, crystallized and otherwise changed. The great coal beds east of the Mississippi grew, as generations and hundreds of generations of plants were deposited, as great beds of vegetable debris, and were slowly changed from this into hard coal. After this was the whole of the great middle period, the Mesozoic, and the first part of the last great era, the Cenozoic. In some parts of the State, notably at Brandon, there is a lignite formation, of great interest to geologists, which is of Tertiary age, and similar lignite has been found in small quantities in Colchester, and I be- lieve at one or two other places in this county. If any of this should be found to be of Tertiary age, then we should modify our statements somewhat. Im- agination might easily run wild through this vast unknown between the Cin- cinnati and the Quaternary, but its conclusions would have but little scientific value. There is much, which, reasoning from what we know took place in southern New York and the Middle and Western States, might have happened, but we know very little of what actually took place. There is one thing, how- ever, of which we may feel sure-that during all this time the elements were not idle. The powers of the air are far more potent than we are apt to think,

HISTORY OF CHITTENDEN COUNTY.

and when they have almost indefinite time in which to act, the results may be astonishing. As soon as any rock mass rises near the surface the waves at- tack it most relentlessly, and when it rises above the surface it is worn by both waves, and rain and frost. When the land was all elevated and mountains and plain were wholly above the sea, the work of erosion went on, rain drop and rill, rivulet and stream, all wore and furrowed the sides of the mountains and the surface of the plains. Slowly, but constantly, the wearing went on, and little by little the mountains and plains were borne to the sea, and the Ver- mont of to-day has been carved from the Vermont of Silurian days by these tireless agencies. But while vast changes, both in the physical character and also in the life of Chittenden county, occurred before the closing era of geolog- ical time, the Quaternary, there were very great changes still to come, and they were changes wholly unlike any that had heretofore taken place. The reign of fire and of water had for a time passed, and now came a reign of ice- The warm and equable climate which had prevailed for ages gave place to cold and, especially in Canada and the northern part of the continent, the whole country was raised, and upon it accumulated the snow and ice of a win- ter ages in duration. Just as modern glaciers come from snow masses, so from this continental snow mass a great glacier formed, and at last began to slowly creep south over the continent across the St. Lawrence, down over New England, year after year reaching farther south. Irresistible, relentless, it moved on, crushing, grinding, tearing all that opposed it. It was of such enormous thickness that it moved right on over valley and mountain, covering all except the highest peaks of the White Mountains. Hence, when this period was at its height all New England was one unbroken sheet of ice, with only a few mountain summits like islands along the White Mountain ridge. As this great ice sheet passed by and over ledges and hills, it must break off and rub off irregularities of all sorts, and grind the bits of rock so broken to sand or, finer still, to mud. The glacier has left its mark everywhere it went so plainly, that any one may trace its course and the direction of its motion. In many a place where a ledge has been uncovered by removing the sand or gravel that had been deposited over it, its surface is found strangely smooth, perhaps even polished, and scratched so evenly that it seems a work of design. The polish- ing and scratching were done by fragments of rock held in the bottom of the great ice mass. Great stones were carried a long distance by the same means, and whenever we find a more or less rounded mass of rock unlike any that is found about it, we may be sure that it came from some northern locality, and that it was brought thence by the glacier. Some of these boulders, as such stones are called, are of great size and weigh thousands of tons. They are found not only on low lands, but on mountains. Between the Nose and Chin on Mansfield there are several. On these places the same sort of scratches are found that we see below, and it is partly because of this that we know that the glacier went over

GEOLOGICAL FORMATION OF THE COUNTY.

the tops of our mountains. Scratches have been found as high as 5,500 feet on the White Mountains. Such an ice sheet, thousands of feet thick, and pressing upon the rocks below with enormous force (Professor Dana estimates the pressure of a glacier 4,500 feet thick at two thousand pounds on every square inch)-such a mass moving over the irregular mountain sides, already grooved and worn, would certainly wear them down very rapidly, and it is quite likely that the rounded form of many of our hills and mountains is due in part to this cause. The scratches, or stria, are a very certain and unmistakable proof of the great glacier ; modern glaciers make the same on a small scale. They do not occur anywhere and in any confusion, but exhibit a certain degree of order. They vary from fine lines, which are very common, to coarse lines or small grooves, and occasionally wide and deep grooves appear, such as one which I have in mind in the Connecticut river sandstone, which is about two feet wide and eight feet deep. The stria may be all in one direction or they may cross, and on Isle La Motte eight distinct sets were made out by Pro- fessor Adams. They are usually directed, according to Professor Dana, south- east.

The Quaternary age is divided into three periods: I. Glacial; 2. Cham- plain ; 3. Terrace; and all of these are represented over the whole county. It was in the early part of the Glacial period that the land was raised over the northern half of North America until it was several hundred feet higher than now in this region, and during this age the great ice sheet, hundreds of miles in length and breadth and hundreds, - yes, thousands of feet in thickness, moved over the surface of the country. At some time during the great interval, which, as we have seen, came between the Silurian and the Quaternary, plants began to grow over the land. Very likely here, as elsewhere, one great group of plants flourished for a time and then gave place to another, to be in turn replaced by still newer forms. However this may have been, we have no reason to doubt that over the whole State an abundant vegetation was growing before the cold of the Glacial period came upon it; and as this came gradually, the plants would be gradually driven southward to a more congenial climate, and animals as well as plants were driven out before the terrible icy foe that was to conquer everything that could not flee before its destructive march. After a time an- other great change came; the upraised continent began to subside, the cold grew moderate and the ice mass began to melt. As the great glacier melted away northward, local glaciers front Mansfield and Camel's Hump ran across the county towards the Adirondacks; but these small glaciers would not produce any very great effects. The sinking of this region (and the rest of New Eng- land sank with it) went on until the land was not only brought to its former level, but carried much below it. This is the Champlain period. From the melting of the ice mass great floods formed over the country, and in their course these would meet with vast masses of sand, gravel and other material accumu-

HISTORY OF CHITTENDEN COUNTY.

lated by the glacier, and would seize them and distribute them over the coun- try. The coarser material appears to have been moved during the first part of the period, and the finer later, when the flow of the stream was less turbulent. Some of the material deposited by these torrent streams was arranged in defi- nite layers ; some of it was simply piled up in a solid mass. That the material is of different sorts any one may see who will notice the variety of clays and sand thrown up in digging for sewers in most of our streets in Burlington, for all the soil upon which this city is built is of the Champlain age. In some of the beds the material was very nicely sorted by the stream which bore it on. Of course the larger and heavier stones and pebbles would sink soonest, and then that which was a little finer, and then the finest, and beds made up in just this way are found. It is believed that Lake Champlain was at this time 400 feet lower than now, and was not a body of fresh water, but a part of the St. Lawrence Gulf, which reached down the valley of the Hudson even to New York Bay, and was wider than now. As the climate grew milder plants, either such as had been here or new species, appeared. The climate was still much colder than now and the plants were such as now live in Labrador and Greenland, and we find their descendants on the top of the White Mountains. There are about a dozen of these now living on or near the top of Mansfield, never descending to less bleak regions. They are all small plants, easily cov- ered and protected by snow in winter, and growing most delicately and prettily in summer. On Mount Washington there are thirty-seven species of these cold- loving plants. In the latter part of the Champlain there were vast banks of clay deposited, and these Champlain clays are found very high above the pres- ent sea level; great banks of sand and gravel were also deposited along the streams, lakes and oceans. Sand and gravel are often found mixed in the same banks, as is the case with the ridge on which the university buildings stand. This ridge is 367 feet above the sea, Essex Center is 452 feet, Williston 402 feet, Colchester Center 225 feet, Charlotte, at the base of Sugar Loaf, 407 feet, Rutland 500 feet, Northfield 724 feet, and all of these towns stand upon de- posits of the Champlain period. That Lake Champlain was at this time an arm of the sea is shown by beds of marine shells found on what were its shores. In some places these shells, which are the same species as are now found in salt water, most often macoma fusca, saxicava rugosa, mya arenaria and mytilus edulus, though some other species have been found. These are found both in sand and clay. Just before reaching the Heineberg bridge the road goes through a cut in the sands of this period, and on the west side of this road in the sandy bluff the white shells may often be seen without leaving the carriage, and beyond the Mallet's Bay House are similar banks in which there are many of these fossils. Other localities might be given, for they are quite common. From the height of the Champlain clays, which are certainly of aquatic, and, as shown by the fossils, of marine origin, we infer the depth of the ocean at this

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History of Chittenden County, Vermont, with illustrations and biographical sketches of some of its prominent men and pioneers, Part 4

Author: Rann, W. S. (William S.) Publication date: 1886 Publisher: Syracuse, N.Y. : D. Mason & Co. Number of Pages: 1054

Author: Rann, W. S. (William S.)
Publication date: 1886
Publisher: Syracuse, N.Y. : D. Mason & Co.
Number of Pages: 1054