Glacial Geology

by James S. Wishart


I. Observations and Interpretations:

In Monroe County, or indeed, in any of the adjacent areas, one sees many evidences of past glacial activity. These evidences are the landscape itself: in places, rolling and marked by rounded hills; or peculiar elongated hills that have been described as "like the inverted bowl of a teaspoon," with the steeper slope toward the north; in other places depressions that have no outlets; or rounded boulders varying in size from one to more than ten feet in diameter.

When the soil is removed from the bedrock in this area, one observes a sharp boundary between the soil and the rock, and not a gradational change. This indicates that the soil has not developed from the weathering in situ of the rock, but rather that the soil has been transported to its present location by some transporting agency. The surface of the bedrock appears smoothed as if ground by some great rasp. In some places the surface of the rock may be marked by many scratches or striae. Boulders that are found in the soil are rounded rather than angular and approximately half of them are foreign to this area.

Study of these accumulations of the rock waste reveal that some of the material is a heterogeneous mixture; in other instances the rock waste has been sorted out into layers of varying coarseness. It is believed that these layers were formed by streams washing sand and gravel from the melting glacier, in which they were frozen, into bodies of standing water. Here the larger, heavier particles were dropped first, and the finer material was carried further into the lake.

As geologists have studied the area it has become apparent that great volumes of water were produced by the melting of the glaciers that once covered the area. Sometimes the water was impounded long enough to leave well defined beaches. As the water level dropped, streams, draining the area, cut down quickly through the material left behind by the ice. As soon as the streams had cut down as far as the level of the lake into which they drained, the streams began to widen their valleys. This process was repeated a number of times. The evidence of this is found along some of the present day streams in the form of terraces. Terraces are particularly noticeable in the valleys of some of the smaller streams draining into Lake Ontario today, and are frequently noticed north of Ridge Road. Terraces are found in many stream valleys around Rochester.

Agassiz proposed the idea of continental glaciation more than a century ago. Many people have examined the Monroe County area and have written about the local evidences of continental glaciation. One of the more prolific writers was H. L. Fairchild. Many of his published articles first appeared in the Proceedings of the Rochester Academy of Science.

In other parts of the United States, particularly in the upper Mississippi Valley, we find evidences of at least four advances and retreats of great ice masses which developed in Labrador and Keewatin, and spread outward as more snow accumulated than melted. The evidence in New York state is rather limited compared to what has been found elsewhere. In Rochester and the surrounding area the evidence indicates one invasion of the ice although in scattered instances we see what may be evidence of more than one glacial advance. From study of existing ice sheets and the debris left behind by the ice as it melted away, it is postulated that the ice mass that covered this area was between 5,000 and 7,000 feet thick.

Undoubtedly the great weight of the ice sheet depressed the surface of the earth in this area. Studies by Fairchild, Daly and others indicate that the surface of the earth in the Rochester area has risen about 250 feet since the ice left. To the north, where the ice was thicker and heavier, the rise has been greater and to the south where the glacial sheet was thinner, the rise has been less. Evidence to support this idea of the land rising is found when shorelines of former lakes have been identified and their present elevations measured. Obviously at the time the lake was in existence the shoreline was level. Today, changes in elevation of these former shorelines aid in determining how much the land surface has changed since the ice age.

When all of the evidences of the work of ice are studied, it is found that some are associated with the advancing ice sheet, some developed as the ice melted away or receded.

II. Glacial History

Apparently the ice advance was gradual. Snow falling, crystallized, melted a little and gradually changed to ice. As more fell than melted away the ice sheet grew in size and it began to move. Melting occurred on the surface and along the edges of the ice. Water under the ice congealed and froze into it fragments of underlying rock and soil.

Some of this rock material probably accumulated along the edge of the ice sheet. Because of the clay-like nature of much of it, plus an abundance of meltwater, at times it was probably a sticky, viscous mass. Ice pushing against it probably rode up and over it shaping it into the many elongate, oval hills found in this area. One of the best known of these hills is Methodist Hill, found on Route 15, a short distance north of the Thruway. It is made up of unstratified clay and boulders, with a steeper slope to the north. It is called a drumlin. Similar hills are found along Clover Street near the Lehigh Station Road and at many other places in Western New York. (Fig.6.) These drumlins are believed to be a phenomenon developed as the ice sheet pushed southward. In many places we find a veneer of other glacial wastes partially covering the drumlin, indicating that the drumlin was formed first, and later, other material was deposited on it.

One of the classic areas for the study of drumlins is that shown on the Palmyra quadrangle, where more than 900 drumlins are mapped on a single topographic sheet. Hill Cumorah, scene of a Mormon pageant every summer, is a well-known drumlin.

As the ice continued its advance it extended toward the southern edge of New York State. There as it melted, great piles of waste material accumulated in what is known as a terminal moraine. Large morainal deposits are found south of the Finger Lakes area, and extend in places to the Pennsylvania line. The belt of hills running east and west across the southern end of the Finger Lakes area can be followed westward. They are plainly visible near Dansville and between Nunda and Portageville. They are the accumulation of materials frozen into the ice and carried far from their source by the great transporting action of the glacier. Then, as it reached its southernmost point of advance, the melting ice furnished great quantities of rock material, which streams, emanating from the ice, dropped along the front edge of the ice sheet.

The southern edge of the ice sheet gradually melted and the ice front retreated northward. Little by little, as the ice disappeared other features appeared. Lakes formed from the melting ice developed along the edge of the ice sheet. Chunks of ice, detached from the edge of the glacier often foundered in the shallow water and were partially buried by rock debris. Later when the ice was gone ponds appeared in the hollows once occupied by the ice. Some of these ponds continue to exist today--as at Mendon Ponds Park. These bodies of water are called kettle ponds and the depressions in which they are found are referred to as kettles or kettle holes. Many of these are found in the masses of morainal material. This uneven topography is called hummocky by some; others refer to it as "swell and swale." Characteristically moraines are recent enough phenomena that good drainage has not yet been achieved, and swamps on or near hilltops are quite common.

When streams flowing from the ice entered lakes, the sediment was dropped and the combinations of change of current, gravity and grain size resulted in a sorting action. Layers of sediment were produced, with the coarser, heavier particles being dropped near the ice edge and finer particles being carried farther from the ice before sinking to the bottom. This type of hill made of stratified layers and roughly conical in shape, is called a kame. Because of their stratification these hills are a good source of sand and gravel. Many sand and gravel pits in the Rochester area are located in kames.

Streams flowing through, in and under the ice carried much rock waste. Frequently these bits of sand and gravel mixed with boulders settled in the stream channel. When the ice melted there was nothing to hold them in place and they slumped to form long, sinuous, roughly stratified hills which are called eskers. Examples of hills of this type can be seen at Mendon Ponds Park. One is located east of One Hundred Acre Pond; the other is located west of the pond and has its southern end in the parking area at the Devil's Bathtub. Its northern end is partially covered by a kame in which there is a large gravel pit on the east side of Clover street a few hundred yards north of the north entrance of the park.

Lakes formed by the melting ice had the ice front as one shoreline. The water was impounded by higher land to the south. Thus when the ice front was in the vicinity of the Mendon Ponds Park area, a lake extended southward to the Bristol Hills. Lake sand and beaches on these hills are the evidence. Fairchild found several such beaches at different elevations, indicating several different lake levels.

Apparently the earliest lakes formed at the ice edge drained to the west. Lake Warren is the name applied to the lake in which the kames in the Mendon Pond area were deposited. This lake is believed to have been the last one that drained westward from this area.

As the ice continued to melt back to the north, other lakes at lower elevations developed.

At the time the Pinnacle Hills were being formed a lake called Lake Dana extended from the ice front just north of the Pinnacle Hills, to the Bristol Hills. Some of the higher kames in the Mendon area were probably islands in this lake. Drainage channels in the Macedon area support the idea that water in this lake flowed eastward, draining the lake.

The land north of the Pinnacle range was gradually uncovered. This is the plain on which the city of Rochester is located. Here there were no great accumulations of sand and gravel, but rather a sheet of sand, clay, gravel and boulders all mixed together was left covering the rocks. This mixture is referred to as glacial drift.

Eventually the ice mass was someplace in the basin of the present Lake Ontario. Streams such as the Genesee River, draining higher land south of Rochester, flowed into another glacial lake that was located between the ice front and the Ridge Road. Glacial geologists refer to this body of water as Lake Iroquois.

Ridge Road lies on top of a bar developed by wave action in this lake. Along the ridge, and south of it, there are many areas of muck land. At the time Lake Iroquois was in existence, the muck areas were lagoons between the lake edge and the bar which is now Ridge Road.

The Genesee River, flowing into this lake carrying large amounts of silt from the area to the south, formed a delta. Today the town of Irondequoit covers much of this delta. The western part of it extends west of Lake Avenue, and Charlotte High School stands near the edge of the delta. The ponds and valleys of Durand Eastman Park are also located in this delta.

At the south edge of Lake Iroquois, where Irondequoit Bay and valley are now located, there was a long narrow bay that extended southward at least as far as East Rochester. Into this bay, streams carried material which was sorted by wave action, gradually filling the bay. The bay itself occupied a valley cut long before the ice age by a northward flowing stream which drained the same general area as the present day Genesee River. For this reason, the bay is said to occupy part of the valley of the pre-glacial Genesee. Irondequoit Creek cut a channel through this filling of lake sand.

Eventually, as the ice continued to melt, the Thousand Islands area was opened and the waters of Lake Iroquois drained to the sea. Since the land surface had been depressed by the great weight of the ice, it is believed that sea water actually invaded the Ontario basin. As streams in the area adjusted to the changes they cut down into the rocks and glacial wastes overlying them.

Thus the Genesee Gorge was extended northward. A part of the earlier valley floor through which it cut remains today as a ledge under the west end of Driving Park Avenue Bridge. In the area of Ellison Park and northward along Irondequoit Creek and Bay, streams cut down into the sediments dropped into the lake during Lake Iroquois time. The sand cliffs in a number of places show the lake sands, resting on top of the earlier glacial till.

As streams draining into the basin adjusted themselves they gradually began to widen their valleys. Then as the land rose, the streams had to cut down through their previous beds and so a series of terraces were developed. Twenty or more of these terraces have been mapped in the Corbett's Glen area, some of them being much more pronounced than others. In this way, streams flowing north across the county have carved terraces along their paths.

With the continued passing of time, and the removal of the great weight of ice, the whole land area began to rise. Since it rose more in the area to the north and east of Rochester than it did to the south and west, the basin of Lake Ontario has been tilted, flooding the southern shore. The ponds along the shore of Lake Ontario (northwest of Rochester) and Irondequoit Bay are really drowned stream valleys, caused by flooding the valleys as the Ontario basin has tilted.

While all of this happened in the past and many have contributed to studies and explanations, the individual who can go see for himself will enjoy noting places where these things can be seen.

For this reason, there are a few suggestions of things to look for:

Ground moraine--
rolling topography, easily noted along Clover Street, especially just south of Jefferson Road.

Drumlins--
along Highway 15, north of Thruway; on Lehigh Station Road, along Clover St.; Dumpling Hill on Scottsville Road.

Kames--
Mendon Ponds Park; Gillette Road; Victor; Pinnacle Hills; Scottsville.

Eskers--
East and West Esker in Mendon Ponds Park; Cartersville Esker near Pittsford.

Lake Sands--
Cliffs in Ellison Park; along Atlantic Avenue Dugway; along Highway 104 on both sides of Irondequoit Bay.

Delta Deposits--
Kings Highway in Irondequoit; Seneca Park Ave.; Durand Eastman Park.

Stream Terraces--
Genesee Gorge at Driving Park Ave.; Allens Creek in Corbetts Glen.


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