Wilbraham is situated in such a manner that its area lies within two broad physiographic provinces that cross Massachusetts from north to south. The Wilbraham Mountains which so dominate the geography of the town are part of the Central Upland of Massachusetts, while the portion of the town west of the mountains lies within the Connecticut Valley Lowland.
The rocks of the Central Upland are ancient rocks of sedimentary and igneous origin. Nearly all have been altered by metamorphism until they bear little resemblance to their original form. A very large volume of rock has been removed from this upland, including much of the original extent of the rock formations which remain today. The metamorphism that altered the rocks that are now exposed over the upland took place before the overlying rock was removed by erosion. Most of the mass of the Wilbraham Mountains is made up of one rock formation; the Amherst schist. This rock, where it crops out in ledges and roadcuts, is seen to be a dark grey, coarse grained, quartz-muscovite schist. It contains some vein quartz, feldspar clots, biotite, hornblende, and occasionally some graphite. In places it contains dikes and lenses of granite pegmatite that were forced into it during metamorphism. For the most part, the rock weathers to a grey to grey-brown color, although some beds with higher iron content weather to a rusty brown color.
Another schist formation, the Irving hornblende schist, occurs near the base of the Wilbraham Mountains on the east side. Along most of Twelve-Mile Brook, this rock unit may be found making up the steep east side of the valley. Where the rock crops out, it is weathered to a dark grey color. The fresh rock is a schist formed of hornblende or actinolite and angular quartz grains. The hornblende and actinolite needles lie in the foliation plane of the schist with random orientations. The rock tends to break along the foliation planes, and is rather resistant to weathering and erosion.
According to Professor Emerson of Amherst College, these two rocks originated as sedimentary material deposited in a shallow sea at least 250,000,000 years ago; at about the same time as the great coal deposits of the southern Appalachians were being formed. Recent research has indicated that they may, in fact, be even older. The deeper portions of the sea in which the sediments that formed these rocks were deposited lay west of Wilbraham, and the sediments themselves were derived from sources in the eastern part of the state. The older hornblende schist appears to have been a limy sandstone, while the younger Amherst schist seems to have been a shale with occasional beds of limestone.
Field study of these rocks as they are exposed across the central upland shows that at some time after their formation they were folded, uplifted, and intruded in numerous places by large masses of igneous rock. The intruding igneous rock was granitic in character, and migrating fluids from the intrusions soaked through the older sandstones and shales and helped to alter them to nearly their present condition. In the Wilbraham area, the intruded mass of igneous rock is known as the Monson granodiorite. Extreme reaction about the margin of this granitic intrusion seems to have produced a shell of dark colored rock known as the Dana diorite. The first range of low hills east of the Wilbraham Mountains is made up of the narrow band of Dana diorite on the west side and the Monson granodiorite on the east side.
Although both of these rocks are dark grey to black, the Dana diorite is schistose, coarse grained, and more nearly black, due to its abundance in hornblende. The smaller white, sometimes pink, grains in the rock are feldspar grains. Biotite, magnetite and several other dark minerals occur in varying amounts in this rock. The Monson granodiorite is gneissic, finer grained, and a uniformly lighter grey in color. The dark mineral present is biotite mica, and the lighter constituents are quartz and feldspar. This rock has been extensively quarried in Monson. The schistose and gneissic (layered) structure of these rocks indicates that the processes of alteration by metamorphism continued after these rocks were intruded.
The ancient rocks that are exposed today in the eastern half of Wilbraham and throughout the Central Upland of Massachusetts were once continuous to the west. The creation of the Central Upland, and the separation of these rocks from their western counterparts began during the Triassic Period about 190,000,000 years ago. A great system of fractures developed in the rocks of this region and movements of these rocks began to take place. Before these movements were completed the rocks in the area that we now call the Central Upland were uplifted in a lofty mountain range, and those in the Connecticut Valley Lowland were depressed repeatedly to form a long troughlike valley running north to south across Massachusetts and Connecticut. The fracture that marked the east side of this troughlike valley and the beginning of the mountains is believed to lie buried near the base of the Wilbraham Mountains and to run just east of, and parallel to, Main Street in Wilbraham.
Westward drainage from those mountains during the Triassic Period carried sediments derived from weathering and erosion in the mountains into the troughlike valley and spread them over the valley Boor. Today these sediments are found as the red Sugarloaf arkose and Longmeadow sandstone, and the red and grey Chicopee shale. Study of these rocks where they are exposed in the Connecticut Valley Lowland yields an interesting history of the time of their formation.
Two of these formations underlie the lowland portion of Wilbraham. The coarse, conglomeratic Sugarloaf arkose lies immediately adjacent to the Wilbraham Mountains on the west, and the younger, finer grained, well sorted Longmeadow sandstone lies mostly buried beneath the western portion of the town. Deep wells drilled for household use penetrate these red sedimentary rocks, and reveal their presence beneath the overlying alluvium. On the campus of Wilbraham Academy, just east of the wooden track, and on bath sides of Monson Road below Bolles Road, there are outcroppings of the Sugarloaf arkose that exactly mark the eastern boundary of the Triassic valley, These outcrops contain abundant plant fossils that reveal something of the nature of the area at the time. The plants therein preserved are marsh plants characteristic of a warmer and drier climate than that which we experience today. The red color of the sedimentary rocks, and the manner in which the beds spread out over the lowland area also indicate a warmer and drier climate than at present.
The crustal movements that produced the Central Upland and Connecticut Valley Lowland were attended by considerable volcanic activity, Extensive lava flows separate the Sugarloaf arkose and Longmeadow sandstone elsewhere in the valley, but do not appear to do so in the Wilbraham area where the Longmeadow sandstone lies directly on top of the Sugarloaf arkose.
Triassic life was abundant, and numerous tracks, trails, and impressions have been found on the bedding planes of the Triassic sedimentary rocks, particularly in the Longmeadow sandstone and the Chicopee shale. Two species of molluscs have been identified from outcrops in Wilbraham: Anoplophora (Unio) wilbrahamensis, and Anoplophara (Unio) emersoni. Otherwise there are abundant plant fossils of identifiable quality and occasional Dinosaur tracks, and impressions of insects and crustaceons are found.
After the close of the Triassic period, there seems to have been little geologic activity save long continued erosion for at least 160,000,000 years, until the advent of glaciation.
About a million years ago the climate of the earth grew much colder than it had been for millions of years, and the first of a series of glaciers began to grow and spread over large portions of the earth's surface. Somehow New England seems to have escaped all of the stages of glaciation except the last one; About 70,000 years ago, the Wisconsin stage of glaciation began and snow and ice began to accumulate over New England and the northeastern North American Continent faster than it could melt way in the summers. Gradually the topography of the land -- hills, valleys, and mountain ranges -- disappeared beneath the deepening ice. Eventually the ice thickened unbelievably, reaching thicknesses of over a mile in this area, and nearly two miles in Labrador and eastern Canada. As the ice thickened it also began to move over the land's surface, outward from it! thickest portions. The ice moved across Massachusetts toward the south-southeast.
Soil and rock caught up in the moving ice rasped the rocks of the earth's surface. Ir. Wilbraham, the softer Longmeadow sandstone and Sugarloaf arkose were cut away more rapidly than the rocks in the Wilbraham Mountains, so that the mountains were carved into sharper relief. The effects of the rasping of the rocks in the Wilbraham Mountains can be found on the surface of exposed rock along the crest of the mountain. Where the rock has been exposed for many years, only the scratches and grooves made by boulders being dragged over the surface of the rock remain as evidence of the passage of the glacier. But, where the rock has recently been exposed, its surface frequently shows a polish produced by the Bner particles of rock and soil that were scraped over it by the moving ice.
The glacier reached its maximum thickness and extent about 35,000 years ago. Thereafter it continued to advance across this area, but it gradually shrank in size and thickness. The edge of the shrinking glacier gradually retreated inland from Long Island Sound, reaching this area perhaps 12,000 to 15,000 years ago, and the area was finally free of ice about 9,000 years ago. The waters from the melting glacial ice flowed over the land in many directions, and at many elevations, and at one time or another most valleys and depressed areas were temporarily Hooded. The entire Connecticut Valley Lowland was Sooded for hundreds of years. The ponded waters in this lowland reached from Main Street in Wilbraham westward to the foothills of the Berkshires in Granville and Westfield, and from the Vermont line to south of Meriden, Connecticut. From the Holyoke Range southward this giant lake is referred to as glacial Lake Springfield.
The Chicopee River system carried a very large portion of the waters draining into the lake in this area. The present Chicopee River is only a shadow of its ancestral self. Millions of cubic yards of clay, silt, sand and gravel that had been dropped over the land's surface by the melting glacial ice were Bushed into the former Chicopee River and its tributaries. These rivers carried this material down to the edge of the glacial lake at Wilbraham and Ludlow. Much of the coarse sand and gravel (from which present day supplies are drawn) was quickly deposited in a delta that gradually grew westward into the lake for several miles. The finer sand, silt and clay was carried farther out into the lake before it was deposited. The broad extensive swampy area west of the Wilbraham Mountains, including Cedar Swamp, was a low area in this great delta, and is poorly drained today because of the long distance over which the top of the delta extends at a nearly uniform elevation. The broad flat areas that begin in the western part of Wilbraham and extend westward through the Sixteen Acres section of Springfield are the top of this great delta. The higher, rounded hills that project above the general level of western Wilbraham are deposits of glacial till, a sediment deposited beneath the advancing glacial ice. These hills rose above the waters of glacial Lake Springfield, and it is possible to find the beach line of the glacial lake cut into their slopes. The occasional fiat-topped patches of sand and gravel found elsewhere in Wilbraham, particularly in the valleys east of the Wilbraham Mountains, are related to tributary drainage when the glacier was melting off the land, and are not directly related to the deposits in glacial Lake Springfield.
Eventually, and gradually, glacial Lake Springfield drained away as the earth's crust slowly sprang back into position when relieved of the great weight of ice, the volume of ponded melt-waters from the glacial ice disappeared, and the region began to assume climatic and drainage characteristics similar to those of the present time. The modern streams began to establish themselves and to adjust their valleys to present conditions. The melting of blocks of ice embedded in the delta and remaining after the main glacial mass had melted away left kettle-holes scattered over the fiat lands. Crystal Lake near the Academy's art building and Bruuer's Pond on Main Street opposite Monson Road are ponds that occupy two of these kettle-hole depressions. Vegetation gradually reclaimed the surface of the land. Except for the valleys east of the Wilbraham Mountains, and the steep slopes of the mountains themselves, Wilbraham has been changed very little since the draining of glacial Lake Springfield. The bouldery soils in the mountain area and on the higher elevations elsewhere in town have developed from the glacial till left when the glacier melted, and the sandy or muck soils of the lowland areas in the town have developed from the old lake-bottom sediments. Until enough time has elapsed for the streams draining to the Chicopee and Connecticut Rivers to deepen their valleys, little geologic change can be expected in Wilbraham.
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