Jerusalem Geology Sign Text Released
As the June 20 sign dedication at the Branchport overlook approaches, the Bluff Point Association has released the texts of geology and history signs. The geology text is below, and the history text appears in a separate article.
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BLUFF POINT & KEUKA LAKE GEOLOGY
THE SEABED BECAME A PLATEAU
The Keuka Lake region belongs to a topographic province called the Allegheny Plateau. The shale and sandstone formations found here represent mud and sand deposits that settled on a shallow sea floor over millions of years. These sediments gradually became rock under the pressure of successive layers in a process called lithification. Many area gorges and glens contain fossils such as trilobites, brachiopods, and corals. Geologically speaking, the surface rocks belong to the West River Group and are sandstones, siltstones, and shales originating from the erosion of the Acadian Mountains to the east. These mountains were a product of the collision of the North American and Avalonian Plates. The region first became dry land more than 300 million years ago during late Devonian times. A long period of uplift and erosion began. During this time, about one vertical mile of rock material was eroded from the original two-mile-thick stack by a system of rivers that developed in the area. One such river occupied the Keuka valley.
GLACIERS COVERED THE AREA
The Ice Age or Pleistocene Period began about two million years ago, bringing a succession of glacial ice sheets from the north. Evidence has been found indicating there were at least 10 and possibly 20 distinct cold periods that could have resulted in an ice sheet advance and subsequent melt-off. Each glacier scoured the landscape under its enormous weight and removed rock material, erasing evidence of the preceding glacier. The continental ice sheets slowly flowed into existing river valleys as valley glaciers, massively reshaping the topography. The valleys running north-south were greatly deepened and changed from a V-shaped cross-section to a U-shaped one by the south- flowing ice. As the advancing ice sheet thickened, ridges between the glacially deepened valleys were sculpted by the flowing ice into streamlined forms as seen in the ridges between the east and west branches of Keuka Lake. The last continental glacier was at least one mile thick.
THE BLUFF WAS AN ISLAND
As the last glacier slowly melted and retreated northward, it stalled about 17,000 years ago for a period of time and dumped what has been called the Valley Heads Moraine. This formed a barrier preventing southward drainage of glacial meltwaters. A part of this barrier remains today as the hilly topography between Hammondsport and Bath.
Around 15,000 years ago the water level in the Keuka valley rose to about 1,000 feet above sea level, causing an overflow of the west branch into the east branch. As water flowed between the present day Keuka Lake State Park and Route 54A, the Bluff became an island for about 300 years. Meanwhile, meltwater continued to flow over the ridge from Keuka Lake to Seneca Lake, eventually forming the present outlet between them. Keuka’s level gradually lowered to its present elevation of approximately 714 feet above sea level.
THE FINGER LAKE THAT IS A Y
When underlying shale layers were eroded, harder sandstone on the Bluff’s crest resisted. As ancient rivers flowed around the Bluff, it remained creating today’s Y shape. Major inlets to Keuka Lake are located at Hammondsport and Branchport. The outlet is in Penn Yan, where the lake level is regulated by a set of gates. From this outlet, water drains to Seneca Lake and then northward through the Seneca-Oswego-Oneida Rivers Drainage Basin, eventually reaching the Atlantic Ocean via Lake Ontario and the St. Lawrence River.
The highest points in the Town of Jerusalem are on the Bluff and the Pinnacle (the hill behind you), each about 1,400 feet above sea level. There are spectacular views from Esperanza Road and along Skyline Drive.
References:
Isachsen,Y.W., E. Landing, J.M. Lauber, L.V. Rickard, and W. B. Rogers, Geology of New York, New York State Education Department, 2000
New York State Geological Association 58th Annual Meeting Field G, New York State Geological Association, 1986
USGS Quad Sheet, Lake Level
Van Diver, Bradford B., Upstate New York: K/H Geology Field Guide Series, Kendall/Hunt Publishing, 1980
Von Engeln, O.D., The Finger Lakes Region, Cornell University Press 1961
Image Attributions & Captions: (clockwise from upper left)
Bluff Point & Keuka Lake Geology photo, Ryan Toaspern
“Glacial Ice Sculptured our Fingers Lakes Region,” topographical image, Bryan Isacks, Ph.D., Geomorphology, Professor Emeritus, Cornell University, for the Paleontological Research Institution Museum of the Earth, Ithaca, NY
Glaciers followed existing streambeds leaving Keuka as a “Y”-shaped lake as they receded. (Black lines show where the southern edge of the last receding glacier was 17,000 years ago.) About 15,000 years ago, the water rose so high that the Bluff became an island for about 300 years.
Bluff with Both Branches photo, Bill Hecht
The east, west, and south branches of Keuka Lake embrace the Bluff.
Gullies and Glens photos, Annette Toaspern and Nancy Peek
The force of water and glaciers contributed to the formations of gullies and glens found around Keuka Lake.
Finger Lakes Watershed and Keuka Lake Watershed maps, Keuka Lake Association
Sign courtesy of Bluff Point Association