The Honeoye Lake Book: The Geology of Honeoye Lake

Bedrock Geology

To comprehend the bedrock of the Honeoye watershed, the story begins hundreds of millions of years ago. It is a tale of drifting continents. In our modern understanding of earth's history, the global surface is viewed as a region of dynamic change. The surface consists of a series of crustal plates that "drift" or "float" on a semi-plastic lower layer. The plates are constantly moving, colliding, rifting apart or simply grinding by each other.

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A basement of Precambrian rocks (over 1100-1300 million years old), ancient igneous and metamorphic rocks that anchor the North American crustal plate, underlies most of New York State. These ancient rocks were buried by eroded sediment swept into the region when early oceans covered it, a time when crustal plates had drifted apart. As sand, silt and mud built up, pressures transformed the sediment into sandstone, shale and limestone. Creatures of the early ocean environment were trapped and fossilized in these newly formed rocks. Crustal plate collisions produce continental uplift that exposed these layered rocks to weathering, and the slow cycle of eroding the landscape began anew. The lands of western New York rose almost horizontally with just a slight regional dip to the south. More resistant sedimentary rock layers produced escarpments, while the weaker rocks were weathered away. Stream and river patterns were cut into the landscape, with major east-west drainage channels at the base of the escarpments. The slow process of continental weathering was abruptly altered at the beginning of the Pleistocene, the Great Ice age, approximately 2 million years ago. A new gent, moving ice, would be responsible for rapid change and a complete reworking of the local landscape. 

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Today this region in part of the hilly, glaciated Appalachian Plateau. The sedimentary bedrocks are Upper Devonian (360-375 million years old) and consist largely of shales, limestones, siltstones and sandstones belonging to the Genesee, Sonyea and West Falls Groups. The northern portion of the watershed rests on a layer of black petroliferous shales, gray silty shales and thin beds of fossiliferous limestone known as the Genesee Group. The harder limestones often form the cap rock of small waterfalls within the ravines that flow to the lake. Underlying the central watershed region is a foundation of light to dark gray shales interbedded with siltstone. This layer is known as the Sonyea Group. The watershed to the south rests on the West Falls Group, a series of gray shales, siltstones and sandstones. Resistant sandstone, source of a popular "flagstone" used by the local residents, caps the highest elevations to the south.

 

Surface Geology and Topology

Throughout most of the Honeoye Lake watershed, a mantle of glacial till covers the sedimentary bedrock. These deposits vary in thickness, composition and surface topography. They give grim testimony to how different the Finger Lakes region was during the Pleistocene Ice Age. It is generally accepted that the Ice Age began about two million years ago. Over this long interval, at least four major advances of continental ice sheets were triggered by fluctuations in the Pleistocene climate. Erosional and depositional evidence for each major advance may be found in the northeastern states. However, well-preserved features of the most recent Ice Age advance (called the Wisconsin stage) are found in New York. Although this final surge of glacial ice undoubtedly fit the landscape pattern left by earlier advances, it obliterated any evidence that they may have left behind. 

Honeoye Lake occupies a "trough", originally a southward flowing stream valley that was scoured by the slowly advancing ice. Believed to have been nearly two miles thick, this ice must have exerted a tremendous pressure on the landscape. Debris trapped beneath it served as an effective cutting edge on the landscape. What had been a narrow V-shaped stream valley was now transformed to a broad U-shaped trough. Later, as the ice margin waned away, the Honeoye valley flooded with glacial meltwater and overflowed to the southeast into the Canandaigua trough, at that time occupied by a periglacial water body known as Glacial Lake Naples. Further retreat of the ice margin uncovered lower drainage channels to the north, and eventually the modern stage of Honeoye Lake came into existence. Today, the lake drains northward through Honeoye Creek about 14 miles to Honeoye Falls, then turns westward for 8 miles eventually to join the Genesee River at Golah. Flow continues northward t Lake Ontario at Rochester, New York. 

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During the immediate post-glacial years, significant siltation occurred in the lake basin. Soil formation was just underway, vegetative cover was sparse and glacial meltwater was everywhere. Stream erosion began cutting gullies into the landscape, carrying materials to the lake and initiating the formation of points. Prominent bedrock exposures were present were erosion of the thin overlying glacial till was complete. Large glacial erratic boulders, impossible to move by water action, were left perched on higher ground throughout the watershed. 

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Soils to the north are derived from the Genesee shales and are fair agricultural lands. The landscape consists of rolling topography. To the south, agricultural value of the soil declines due, in part, to steeper terrain, shallower depths and more acidic pH. The forested inlet wetlands are dominated by a thick accumulation of muck that is subject to repeated inundation during the growing season. Moranic fill of sand and gravel are found farther south in the Honeoye valley. Excessive stoniness limits their agricultural potential. 

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