TIMELINE: GEOLOGY: PRE-HISTORIC

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~ ROCHESTER'S HISTORY ~
AN ILLUSTRATED TIMELINE

GEOLOGY

MILLIONS OF YEARS AGO

245 to 2 MILLIONS OF YEARS AGO

100,000 YEARS AGO

12,000 YEARS AGO

MILLIONS OF YEARS AGO

Many millions of years ago this entire area was covered by a warm, shallow inland sea.
The chain of Taconic mountains grow and erode away leaving red shale and sandstone. Traces of these rocks can be seen at the bottom of the gorge below the Lower Falls. Other mountains develop and erode away leaving more shales and limestone, also visible in the river gorge.
About a million years ago the earth cooled dramatically and the first of the great Ice Ages takes over. Four times the polar ice has advanced over this area, covering Rochester with glacial ice up to two miles thick. The glaciers in this area did not gouge the surface away as mountain glaciers do, they worked more like sandpaper rounding and smoothing the contours. The primary change was actually in the material they left behind. Vast areas in Canada were scraped down to bedrock and this dirt was pushed south giving some areas of the Genesee Valley a covering of glacial till 300 feet deep.

Today's geography would be very different had the glaciers not made their appearance. There would be several deep valleys with ridges separating them. The many ridges and rounded hills would not be present. The lakes would never have formed by the normal action of erosion. The soil would be poor.

245 to 2 MILLIONS OF YEARS AGO

(Mesozoic and Cenozoic Eras)

Area was rich in plant life, first with ferns, mosses, and conifers, then evolving to flowering plants, forest trees and palms.
Animal life would include massive amphibians, and then the giant reptiles of the land and air.

This region may have been home to early horses, camels, rhinoceros and other animals that were driven off by the advancing glaciers.

100,000 YEARS AGO

In the last million years there have been four glacial ages that have covered the area. Each age scraped and gouged, and sometimes buried evidence of the previous glaciers. The earliest glacier to leave evidence in the area arrived here 100,000 years ago. It was about 2,000 miles in diameter, and 1,000 to 10,000 feet high. This ice would have put about a ton of pressure on each square inch of the ground below. At the end of its 40,000 year stay it had compressed the earth below by as much as 2,500 feet! Before the ice would finally melt away, it would advance and recede across Rochester five times. Each time scraping away old soil and depositing new. The glacial till (soil left by the glaciers) in the Genesee Valley is between 200 to 300 feet deep.
Genesee River gradually evolves through erosion of the region.

The Genesee River starts as a spring (today, on the farm of Dave Slaybaugh, in Gold, PA). About five miles north of Avon the river turns east at Rush taking the route of Honeoye Creek, until just before Honeoye Falls, where it turned northward again following the Irondequoit Valley where it joined the Ontario River at a location near the center of today's Lake Ontario.
The Genesee and the Nile, are the world's only major rivers that flow North.

12,000 YEARS AGO

Mammoth and Mastodon live in western New York
Retreating glacial ice heads northward, finds new strength and readvances to the south, then stalls. Melting ice at this static point slopes to the north at an angle of 20 to 30 degrees. The meltwater along this front, deposits mounds of debris forming the Pinnacle Range, a range of hills extending four miles, forming Cobb's Hill, Pinnacle Hill (the tallest at 230 feet), the hills of Highland Park, Mt. Hope Cemetery and the University of Rochester River Campus. They are described as a recessional kame moraine. This means that the hills were not formed by the furthest advance of the glacier, but by the melt water carrying sand and/or gravel off the ice and depositing it in well sorted deposits. There are also deposits of glacial till spread over this area. This means that after the range was formed the glacier advanced again, covering the area with ice. This last glacial appearance must have been a weak effort, because it did not scrape the hills away, mearly flowed over top of them. The exit of the ice sheet wasn't a retreat of the ice mass as in the past, but just the melting away of the ice, which droped the well mixed debris trapped in the ice (glacial till) to the ground.
The river channel between Rush and Honeoye Falls is filled by glacial debris (the Portageville Moraine). Lake Scottsville (elev. 540 feet) is formed along the present river route, with the Pinnacle Range damming the northern exit. Water flow continued northward through a depression in the hills at the University of Rochester, where it joined Lake Dawson (elev. 480 feet) about a mile north of the rapids that were created when the river crossed the Niagara Escarpment.

Evidence of how important the glacial action has been to this area as we know it, is all around us. It is easily seen in the fertile soil, The Great Lakes, and the Finger Lakes. Consider this more subtle, yet profound example: Had the Pinnacle Range at the University of Rochester been 20 feet higher, Lake Scottsville would not have broken through there. The debris that had blocked the channel near Avon was at a lower elevation, and would have become the new outlet. The running water would easily wash away the blockage, re-opening the river's original, gradual course to Irondequoit Bay. The Genesee River would not have found its new, more direct course northward. There would be no falls for early settlers to use for power, and here would be no Rochester.

Retreating ice exposes a new outlet, and Lake Dawson's level rapidly drops 45 feet, creating Lake Iroquois (elev. 435 feet). This in turn drains Lake Scottsville, and moves the mouth of the Genesee a mile further north, to a point just north of the Veteran's Memorial Bridge. Rapids at the Niagara Escarpment have eroded to form a 40 foot falls, just north of today's High Falls. Two new sets of rapids are formed between the escarpment and the river mouth

Lake Iroquois remained long enough, so that it's shoreline can still be seen. Extending 150 miles from Sodus to Lewiston it forms a ridge that the Indians would follow as a primary east-west trail. Stagecoach lines follow it from 1818 to 1848. Today's NY Route 104, or Ridge Road follows this same geologic formation.

Ice clears the Thousand Island Outlet, and the water level drops again forming Admiralty Lake (elev. 50 feet). The ground in this area has been so compressed by the absolutely massive weight of the two mile thick ice sheet, that the lake level is 200 feet lower than today's Lake Ontario. The Genesee River has to extend it's length by seven miles to reach the new shoreline.
The two lower sets of rapids become the Middle and Lower Falls, moving southward in the process. The lower gorge south of Stutson Street is carved by erosion.

The Genesee River drops about 220', over three falls within the Rochester City limits.

Land slowly rebounds from glacial weight. (Imagine squeezing a rubber ball, when you release the pressure it returns to it's original shape.)Because more weight was removed from the Canadian side of the lake, it is raising faster. Even though the land has risen 250 feet in Rochester, northern portions of the lake have rebounded nearly 500 feet. The faster rebound on the Canadian side is causing Lake Ontario is literaly spill onto New York, forming the many ponds and the wetlands along the lake shore. The original outlet of the river has also flooded, forming Irondequoit Bay. The the lower gorge along the river's new course is flooded to the Lower Falls.
When the glaciers finally left the area they left behind many signature deposits and features. Debris includes random pockets of material ranging from the size of sand, to an Irondequoit field that originally was covered with 30 to 80 ton boulders. These 'erratic' were taken from northern Canada by the advancing ice. Melt water from the glaciers formed rivers, on and in the ice. Debris and gravel would collect along the beds of these rivers and when the glacier finally melts, this debris is dropped to the earth forming long snake-like piles called eskers. There is a very impressive esker in Mt. Hope Cemetery. Mendon Ponds Park also has many notable glacial features including kettles which are large circular depressions in the ground formed when huge chunks of ice, buried in the ground melt away.

Pinnacle Hill is the highest point in Rochester at 749'.
The Erie Canal crosses the city at 508' above sea level.