Glacial History
The landscape itself speaks out in North America, describing glacial
events to anyone willing to listen. Rounded boulders up to 10 ft in
diameter, sharp non-erosional transition from soil to bedrock, smooth
bedrock surface marked by scratches and striations, heterogeneous mixture of
rocks foreign to this area - all of these describe a very recent ice age in
geologic past of our planet.
The Ice Age in North America started approximately two million years
ago, and ended about six thousand years ago. This was a rather long
glaciation episode that was marked by several glacial advances and retreats
(which were caused by climate fluctuations in the Pleistocene). However, in
New York the geological evidence preserved tells us only about the latest
glacial advance - Wisconsin glaciation. The evidence of earlier advances,
although preserved in other Northern states, is absent in New York - it has
been destroyed or obscured by the last (Wisconsin) surge of ice. Thus,
Wisconsin glaciation is the only major glacial stage represented here.
The climax of Wisconsin glaciation occurred approximately twenty
thousand years ago. At that time ice sheets covered all of the up-state New
York and extended Southward as far down as New York City. In some places
the ice sheets of the glacier were more than a mile thick (calculated from
isostatic rebound). The glacier overrode Adirondacks! (Even Mt. Marcy, the
highest point in New York State, was covered by ice.)
Except for the terminal moraine, all other morainal deposits in New
York State are recessional, indicating periods of stagnation as the ice
retreated across the state toward the North.
Morainal topography everywhere in the state seems to be very pitted.
This is due to the presence of kettle holes. Kettles occur when blocks of
ice are isolated from the receding glacier terminus. These blocks of ice
are then partially or totally covered by moraine, lake or outwash plain
sediments. When these blocks of ice melt, the sediment overlying them
collapses and the melted ice leaves in its place surface depressions, or
kettle holes. Some of the kettle holes later fill with water and become
lakes (this happens if these depressions intersect the water table). These
lakes are usually small, have a rounded outline, and have poorly integrated
feeder and outlet streams. Both kettle ponds and kettle holes can be
observed in Mendon Ponds Park.
Morainal areas in the region often contain kames. Kames terraces
are formed by sediment deposited within meltwater streams which flow on the
top of the glacier. (Kame terraces are formed after glacial recession. Kame
fields differ from moraine in the cleaner, sandy, water-washed character of
their material.) Thus, the meltwaters flowing from the ice front into Lake
Dana built the kame deposits of the Pinnacle Range in the present Rochester.
The fine sediment was deposited further out into the lake, and developed the
present large clay plain which is to the South of the Pinnacle Range. (The
Pinnacle Hills lie at the eastern portion of the Rochester-Albion moraine.
This range consists of linear but irregular kame deposits that extend
approximately four miles from Brighton, adjoining Rochester on the
southeast, westward of the Genesee River.) The great thickness of silts and
clays was revealed by excavations for building the University of Rochester
River Campus and the Strong Memorial Hospital.
Lake Dana was followed by Lake Scottsville. Lake Scottsvilee was
located to the South of the Pinnacle moraine and to the West of the drift
ridge (along South Avenue and East Henrietta Road). The basin furnished by
this lake is now the place of accumulation of sediments that are brought
down by the Genesse river.
Numerous other lakes were formed between the continental ice sheet
and high ground as the Pleistocene epoch drew to a close. These lakes were
the receiving basins for the meltwater. Each time a new spillover outlet
was uncovered at a level lower than the existing one, the water level
changed accordingly. Some of these lakes are lake Ontario and Lake
Iroquois. The present Lake Iroquois was formed at the extinction of Lake
Dawson, when the ice receded and allowed the waters of the earlier eastern
Lake Iroquois and Lake Dawson to merge. During the time of Lake Dawson,
Niagara falls and Lake Erie also came into existence.
When the ice mass was in the basin of the present day Lake Ontario,
streams such as Genesee drained into Lake Iruquois. However, the sediment
carried by these streams gradually filled the valley cut by pre-glacial
Genesee, and the Genesee river had to cut a new path.
Another feature frequently found in regional morainal topography is
the esker. Eskers were deposited by meltwater stream channels like kames
were, but unlike kames, eskers were deposited beneath the ice sheets. Eskers
are long and narrow ridges - sinuous landforms winding irregularly across
the landscape, such as the Mendon Ponds Park esker.
Cirques - depressions representing original areas of snow
accumulation - are bowl-shaped amphitheaters that are very characteristic of
Adirondacks. (Retreat of ice did not mark the end of glaciation there - ice
stayed longer on the high peaks.)
The Finger Lakes hold another great testimony of glaciation. The
lakes occupy troughs. Troughs were first carved out by streams, then gauged
by moving ice, and dammed in the South by the Valley Heads moraine. The
region between Finger Lakes and Lake Ontario is a great example of drumlin
fields. Drumlins occur when glaciers locally readvance and override a
previously deposited ground moraine. Drumlins are hills of glacial debris.
They are elongated in the direction of ice movement. The alignment of
drumlins shows that the Lake Ontario basin served as a spreading center for
ice moving outward in the region of Buffalo and Watertown.
(From Robyn Hannigan, Laboratory 5, Geology 101, University of Rochester,
fall 1994)