A Brief Geological History of Ithaca and Tompkins County:

From Cambrian to Cornellian

Tompkins County owes its stunning topography to its intricate and variegated Phanerozoic geologic history. Of principal influence over the region’s surficial geology are Devonian Period sedimentary processes as well as glaciation events which took place during the recent Pleistocene Ice Age — a period which still strongly influences global climatic conditions. In short, the collision of proto-continents during the Ordovician Period generated a massive mountain range (Taconic Orogeny), the remnants of which constitute the modern Appalachian chain. Erosion of said mountain range, deposition of sediment in tropical deltas, and subsequent lithification of sediment in conjunction with calcariferous marine organism skeletal detritus generated the sedimentary bedrock that comprises the vast majority of Tompkins County’s visible strata. Erosion and glaciation have largely removed successively deposited strata. During the Mesozoic and Cenozoic eras, further plate tectonic movement gave rise to the contemporary continental configuration. Pleistocene glaciation sculpted Tompkins County’s ubiquitous hills, gorges, and lakes. Human habitation is responsible for the region’s latest surficial alteration, and continues to transform the region’s appearance.

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Introduction

Tompkins County — situated in upstate New York and home to the city of Ithaca — displays some of the most dramatic and geologically informative escarpments of sedimentary rock in the eastern United States. Tompkins County’s sheer topography is not only aesthetically stunning; it provides the careful observer a means to decipher a wealth of the region’s geological history. The purpose of this narrative, then, is to orient the reader with regard to the geology of Tompkins County and to provide sufficient historical context for the informed observation of the region’s spectacular landscape.

Fig 1. — Taughannock Falls, Tompkins County, New York (Browne 2010). Taughannock Falls are 66 meters high and are the tallest single drop waterfall in New York State (Taughannock State Park 2012).

Tompkins County’s surficial geology is comprised heavily of sedimentary rock deposited and lithified during the Devonian Period (the period of time ranging from about 416 to 359 million years ago). Indeed, the oft-fossiliferous stratigraphic units exposed in the Tompkins County region are among the World’s most important with regard to the study of Devonian happenings and conditions (Williams 1906; Titus 1993; Allmon and Ross 2008).

Also of paramount importance to the contemporary topography of Tompkins County is its recent glacial history. The expansion and subsidence of massive ice sheets gave rise, in large part, to the region’s craggy terrain. Beginning approximately 75,000 years ago and concluding approximately 11,000 years ago, a series of glacial and interglacial periods defined Tompkins County’s modern landscape (Fullerton 1986; Pair 1996; Cadwell and Muller 2004; Allmon and Ross 2008; Merguerian 2010).

Although the scope of this report will encompass the local geologic history of the entire Phanerozoic Eon (the time period ranging from 542 million years ago to the present) and give brief mention to certain contextualizing Precambrian events, this narrative will give particular focus to the time periods corresponding to the visible stratigraphy of the region.

Location — Geographical and Geological Settings

Tompkins County comprises 492 square miles of upstate New York. It is home to more than 100,000 people, or approximately 206 people/mi2 (United States Census Bureau 2009; Wolfram|Alpha 2012). The city of Ithaca is both the county seat and the County’s largest city, with approximately 30% of the County’s population (Hijmans 2012; Wolfram|Alpha 2012). The Ithaca city center is located at ~ 42°26'38.26"N, 76°30'6.77"W with the boundaries of the roughly square Tompkins County lying approximately 20km in each direction (Google Earth 2012; New York State GIS 2012).

Fig 2. — Location and scale map of Tompkins County and county seat Ithaca (Google Earth 2012; New York State GIS 2012).

The southern portion to Tompkins County is dominated by rugged hills and an irregular, rugged, rolling landscape, with elevations reaching more than 2,000 feet above sea level (Tompkins County Planning Department 2008). The northern portion, meanwhile, has gentler terrain and, due to its fertile soils, is home to a viable agricultural industry. In fact, approximately 25% of Tompkins County is covered in arable land (Lewis 1930; Tompkins County Planning Department 2008; New York State Department of Agriculture and Markets 2012).

Tompkins County’s most conspicuous feature is Cayuga Lake, which extends approximately 18km into Tompkins County from the northern border (Mullins 1989; Tompkins County Planning Department 2008; Google Earth 2012; New York State GIS 2012). Cayuga Lake is the second largest of the Finger Lakes (Tompkins County Planning Department 2008), after Seneca Lake (Mullins 1989) and is the longest, widest, and one of the deepest of the Finger Lakes — at its deepest, it reaches 53 feet below sea level (Figiel 1995; Allmon and Ross 2008). The elevation of the surface of Cayuga Lake is 382 feet above sea level (Google Earth 2012; Google Earth 2012); the highest point in the county is the crest of Connecticut Hill, at 2,099 feet above sea level (Tompkins County Chamber of Commerce). Tompkins County has approximately 39 km of shoreline along Cayuga Lake (Tompkins County Planning Department 2008; Google Earth 2012; New York State GIS 2012). Cayuga Lake is situated in a glacial valley; slopes along the lakeshore are steep, as are the numerous gorges and tributaries that make up the Cayuga Lake watershed (Levine 2003). Within the Cayuga Lake watershed lie hundreds of waterfalls, cascades, and cataracts (Kurtz 1883). Elevations of the gorge walls can reach as high as 300 feet, one reason why the county is home to such an abundance of waterfalls. Approximately 80% of the county’s land area drains North into the Finger Lakes, which eventually drain into Lake Ontario (the southernmost 20% of the county eventually drains into the Upper Susquehanna River) (Tompkins County Planning Department 2008).

The modern topography of the Cayuga watershed originated in large part when continental uplifting began to take place approximately 200ma (million years ago) in the early Triassic Period (Sanders 1963; Tompkins County Planning Department 2008). The topography was largely altered during the last ice age, specifically by two distinct glaciation events. This glacial action formed the flatlands that dominate the northern part of the county as well as reversed the direction of drainage in the basin from south to north (Tompkins County Planning Department 2008). It is the visible component of this topography and the corresponding geological history thereof that will be the focus of this paper.

Discussion— A Brief Geological History of Tompkins County

Approximately 830 million years ago, all the world’s landmasses were joined together in an ancient supercontinent called Rodinia. Soon thereafter, this ancient supercontinent began to split apart when a massive mantle plume below modern day China initiated continental rifting (Li 1999). There are multiple estimates with regard to when the rifting of Rodinia began; these estimates range from approximately 828±7ma (Li 1999) to 600ma (Allmon and Ross 2008). Furthermore, Baltica and Laurentia may have shared a common drift history until ~600ma (Torsvik 1996).

An ocean, formally known as the Iapetus Ocean, began forming between the rifting landmasses — the birth of this ocean is estimated to have taken place in 615±2ma (Kamo 1989). The Iapetus Ocean continued to widen until approximately 500 million years ago (Mac Niocall 1997; Allmon and Ross 2008). The Phanerozoic Eon began at 542ma with the first appearance of trilobites and archaeocyatha, although there is some disagreement among paleontologists as to the methodology of defining the dividing point between the Proterozoic and the Phanerozoic — this debate is formally known as the Cambrian lower boundary problem) (Rozanov 1967).

The two largest of these ancient continents were roughly equivalent to modern day North America and Europe (Li 1999; Allmon and Ross 2008). The Proto-North American continent (the North American craton) is known as Laurentia and the Proto-European continent is known as Baltica. Rifting of these landmasses ceased approximately 500ma, following which Laurentia and Baltica began drifting towards one another (Torsvik 1999; Allmon and Ross 2008). As these two continents approached one another, smaller islands and landmasses in between rammed into the eastern coast of ancient North America and the western coast of ancient Europe and Africa. As these smaller islands and landmasses collided with Laurentia and Baltica, they generated massive mountain ranges along the margins of each proto-continent (Dana 1880; Blackwelder 1914). The North American remnants of these massive American mountains are collectively referred to today as the Appalachian Mountain Range — a mountain chain that extends from modern day Alabama to southeastern Canada. The Appalachian Mountain range was largely generated by three mountain-building events: the Taconic orogeny, the Acadian Orogeny, and the Alleghenian Orogeny, respectively (Encyclopedia Britannica 2012).

Fig 3. — Paleogeography of the Ordivician Period (Wikimedia Commons, based upon Laurie and Webby 1992; Torsvik and Rehnström 2003; Blakey 2011).

With regard to the various mountain building events of this period, of particular significance to Tompkins County is the Taconic Orogeny (Dana 1880). The Taconic orogeny of eastern North America was “not, as traditionally defined, a single orogenic event that occurred at the end of the Ordovician period, but rather a complex series of orogenic episodes or climaxes spread over the larger part of that period” (Rodgers 1971). The Taconic orogeny is largely recognized to have occurred during the late Ordovician Period and a portion of the early Silurian Period — from approximately 450ma to 440ma (Zen 1968; Berry 1970; Bird and Dewey 1970; Rodgers 1971). This complex of orogenic events resulted in mountain formation over a large area — potentially as far north as Quebec and as far south as Virginia (Dana 1880; Blackwelder 1914; Rodgers 1971). The Taconic Orogeny is largely responsible (either directly or indirectly) for the formation of the hills and peaks found within Tompkins County.

From the beginning of the Phanerozoic until the end of the Missippian Period, sea levels were, on average, substantially higher than they are today. These elevated seas were high enough to cover a considerable amount of modern-day North America. There were, however, intermittent periods of sea level decline (Haq 1987). In fact, eustatic sea level change occurred during short (less than 1–3my) intervals throughout the Ordivician and Silurian (McKerrow 1979). This may be due to the fact that, at the time, the icecap of the supercontinent Gondwana (which comprised the landmass of most of today’s southern hemisphere continents) underwent relatively rapid changes in size (thus affecting eustatic sea level) (McKerrow 1979). One such interval of importance, especially with regard to the history of Tompkins County, occurred in the mid Ordovician Period (Haq 1987; Allmon and Ross 2008; Wolfram|Alpha 2012).

Fig 4. — Phanerozoic sea level with Silurian Period highlighted (Wolfram|Alpha 2012).

As sea level declined, the rate of evaporite accretion increased — especially in contemporary New York State (and Tompkins County). As a result of the abundant evaporation of seawater during the Silurian Period, massive salt deposits formed in modern-day New York State (Alling and Briggs 1961). This stratigraphic unit is known as the Salina group (Alling and Briggs 1961; Swezey 2002). These salt deposits are mined for commercial purposes; one of the largest and deepest salt mining operations is located in Tompkins County, northeast of Ithaca on the shore of Cayuga Lake (Allmon and Ross 2008).

Sea levels rose again in the early Devonian Period (around 415ma). At that time, the area that constitutes the contemporary northeastern United States was located near the equator. As such, the prevalent climate was tropical (subject to occasional and intense tropical storms) and benthic communities thrived (Craft 1987).

Fig 5. — Paleogeography of mid to late Devonian (~385ma) showing the location of Euramerica (modern North America and western Eurasia) (Wikimedia Commons). Tompkins County is most famous for its middle and late Devonian strata.

As the skeletons of marine organisms that create calcium carbonate (CaCO3) shells accumulated on the seafloor, layers of lime mud began to accumulate and eventually pile on top of one another (Ginsberg and Lowenstam 1958). The lithification of this lime mud, along with the accumulation of the massive quantities of sediment that were being eroded from the proto-Appalachian mountain range and deposited in these Devonian deltas, formed the thick layer of Devonian sedimentary rocks that can be found throughout much of New York State (Leo 1967; Craft 1987; Allmon and Ross 2008; Rogers 2011). The fossiliferous Devonian sedimentary rocks of Tompkins County contain abundant Brachiopods, Bryozoa, Clams, Trilobites, Crinoids, and Cephalopods — all of which help geologists and paleontologists to understand the Devonian climate, environment, and biota (Craft 1987; Allmon and Ross 2008). Specifically, we can interpret that these sedimentary layers must have originated in a warm, high nutrient (WHN), shallow marine environment.

As a brief aside, this is one instance where the geological concept of uniformitarianism does not provide an adequate means to understand local geologic history. Because there are very few modern WHN environments (James 1997; Allmon 2007), it is difficult to utilize presently operating processes to characterize the Devonian depositional environment largely responsible for Tompkins County’s sedimentary composition. In fact, most modern platform carbonates accumulate in cool waters, generally colder than 20ºC (James 1997). Thus, a more complex application of present conditions and processes becomes requisite. In fact, the contemporary climate (as well as the surficial geology of Tompkins County) is heavily influenced by the Pleistocene Ice Age — such glacial intervals represent less than 10% of Earth history (Kauffman 1987).

With regard to interpreting the paleo-environment of Devonian Tompkins County, we can not only learn from the fossilized biota contained within the sedimentary strata, we can learn from the composition of the strata itself and changes thereto. When looking broadly at the continuum of Tompkins County’s sedimentary groups and formations, it becomes apparent that in the middle Devonian, shale and limestone-rich formations prevail. However, this prevalence shifts toward sandstone-dominated formations and groups in strata deposited during the upper (late) Devonian (Allmon and Ross 2008). This is very convincing evidence that the ancient delta wherein these sediments were deposited prograded westward. A basic study of the dynamics of sediment transport and deposition will yield the understanding that more massive, denser sediment will be deposited more readily than smaller, less dense particles. Thus, as a sediment-depositing delta progrades, for any given stratigraphic column, sedimentologists expect to see a progression from finer to coarser sediment material (Stanley 2009).

As the Devonian Period came to a close, the shallow seas that had covered the expanding delta comprising modern day New York State began to recede. Throughout the Carboniferous Period (359–299ma) and the Permian Period (299–251ma), rocks were indeed deposited over modern day New York State. However, erosion has removed all Carboniferous and Permian rocks from the state (Miller 1989; The Paleontology Portal 2006).

Although there are no Carboniferous or Permian rocks in Tompkins County, important paleographic changes took place during these time periods. During the Early Carboniferous, the oceans separating Euramerica and Gondwana began to close. By the late Carboniferous, Euramerica and Gondwana had collided, forming the western half of the soon-to-be-supercontinent Pangaea. During the Permian, the supercontinent Pangaea developed further — plate tectonic movements brought the cratons that today make up North China, Siberia, and Australia, closer together. The western portion of Pangaea was covered in a vast desert and reptiles spread across the face of the supercontinent.

A massive extinction event, the Permian–Triassic extinction, brought the Paleozoic Era (and the Permian period) to a close. The Permian–Triassic extinction event was the most severe extinction event in Earth’s history, albeit an often over dramatized one (Rhodes 1967), in which up to 96% of marine species (Benton 2005) and 70% of terrestrial vertebrate species becoming extinct (Sahney and Benton 2008). Land plants, however, fared surprisingly well (Rhodes 1967). There are a variety of proposed mechanisms for the extinction event, which may have occurred in up to three distinct pulses (Sweet 1992; Jin 2000; Yin 2001; Sahney and Benton 2008). Proposed mechanisms for the extinction include a shift in ocean circulation driven by plate tectonics and climate change (Hotinski 2001; Tanner 2004; Kheil 2005; Winguth 2005) increased volcanism and coal/gas fires (especially in relation to the Siberian Traps) (Chung 1995; Renne 1995; Jin 2000; Ogden and Sleep 2012), anoxia (Wignal 1992), extraterrestrial impact (Becker 2001), the sudden release of methane clathrate from the sea floor (Krull 2000), and various combinations thereof.

During the Triassic Period, the supercontinent Pangaea became fully formed; that is, all of the continents were joined and sutured into one large landmass (Torsvik 2003; Allmon and Ross 2008). Throughout the Mesozoic Era (the time period from 251–65.5ma which encompasses the Triassic, Jurassic, and Cretaceous Periods) dinosaurs and mammals occupied the vast majority of the habitable land on Earth, including Tompkins County. However, sedimentary rocks from the Mesozoic area are not present in Tompkins County — largely due to erosion — and thus there are no dinosaur fossils to be found in the area (Allmon and Ross 2008).

The Mesozoic Era came to an end approximately 65.5ma with the Cretaceous–Paleogene extinction event. This large-scale mass extinction took place in a geologically short period of time. The prevalent theory describing the mechanism for extinction involves one or more extraterrestrial impacts. Evidence for this theory takes the form of an iridium rich layer of sedimentary material at the time of the extinction as well as several impact craters dated approximately to the time of the Cretaceous–Paeogene extinction (Horowitz 1979; Alvarez 1980).

With regard to the Cenozoic Era, the oldest episode of geologic history recorded in the stratigraphy of Tompkins County is the most recent Ice Age, which began approximately 2ma (Allmon and Ross 2008). Recent glaciation shaped much of the striking topography of Tompkins County. The evidence for Pleistocene glaciation abounds; this evidence provides geologists with a wealth of information pertaining to the nature and dynamics of the glaciers that traversed this region during the Pleistocene epoch. Glacial erratics, which are visible throughout Tompkins County, provide information regarding the existence, source, and trajectory of glaciers. Glacial striations, which can also be found throughout Tompkins County, provide information relating to the direction, size, and height of glaciers (Stanley 2009; Merguerian 2010). Terminal moraines also provide evidence of glacier source, size, and trajectory. No discussion of glaciation in Tompkins County would be complete without at least some cursory information pertaining to the formation of the Finger Lakes (and Cayuga Lake in particular).

The 11 Finger Lakes formed in roughly the same manner. Advances of glaciers during the Pleistocene carved deep trenches into pre existing river valleys. These glaciers left evidence of their progression (especially from the most recent glaciation). Terminal moraines at the southern end of the Finger Lakes provide direct evidence of the bedrock carving process responsible for their formation (Ridky 1990; Allmon and Ross 2008). The latest glacial episode reached its zenith approximately 21ka. At that time, glaciers covered the majority of New York State. In addition to terminal moraines, drumlins — which form beneath flowing glaciers — provide evidence of Pleistocene glaciation in Tompkins County.

Fig 6. — Evidence of glaciers in the Finger Lakes region. Drumlins, highlighted in this image, are elongated hills comprised of glacial sediment (Paleontological Research Institute).

Ithaca’s famous gorges formed as a result of the same glacial episodes that are responsible for the genesis of the Finger Lakes, although these gorges formed during the interglacial periods and during the transition out of glacial periods. As east–west flowing streams interacted with north-south moving glaciers, the seeds of Tompkins County’s contemporary topography were planted. Glaciers repeatedly filled the river valleys that would eventually become the Finger Lakes. As they did so, east–west running tributary streams often became dammed by glacial ice. Lakes formed behind these dams and cascades of water flowed over them as they made their way to the proto–Finger Lakes. As the dam-forming ice receded, the lake levels declined, thus the gradient from highland streams to these intermediate lakes increased. This higher gradient leads to more rapid currents and higher rates of erosion. These streams eroded downward, forming the gorges characteristic of modern-day Tompkins County (Allmon and Ross 2008; Jordan, personal communication, 2012).

Pleistocene fossils in Tompkins County also contribute to geologists’ understanding of the region’s geologic history. The remains of mastodons are of particular interest in the region, as more than 130 mastodon skeletons have been found throughout New York State, including Tompkins County. One such mastodon, discovered in Duchess County, can be seen on display in Tompkins County at the Museum of the Earth (Allmon and Ross, 2008).

The geology and topography of Tompkins County has been central to its human history. From the Native Americans who were drawn to the region for its evaporite salt deposits to the thriving contemporary viticulture industry to the founding of Cornell University, Tompkins County — and its contemporary topographic configuration and pattern of human habitation — is a product of its 542-million-year Phanerozoic history.

Conclusion

The visible strata in Tompkins County were deposited primarily during the Devonian Period, and sculpted during glaciation that took place during the Pleistocene Ice Age. Although pre-Devonian sediment does exist in Tompkins County, it is, for the most part, buried beneath the Devonian strata (and thus not visible on the surface). Furthermore, strata were also deposited in Tompkins County after the Devonian period, but these have largely been removed by erosion — the history of Tompkins County from the Devonian to the Quaternary has been inferred from the geologic information contained in neighboring regions.

Appendix — Brief Summary for High School Students and Teachers

Tompkins County’s geologic history is both interesting and multifaceted; the region’s spectacular landscape is a product of this history. This brief appendix serves as an introduction to and summary of Tompkins County’s Phanerozoic (542ma–present) history, and is suitable for high school-level instructors and students.

Processes which took place hundreds of millions of years ago (largely during the Devonian Period, 316–259ma) created much of Tompkins County’s visible bedrock, which is visible in its gorges, cliffs, and valleys. Relatively recent processes, specifically glaciation events (the advance and retreat of glaciers) during the last Ice Age (Pleistocene, 1.64ma–11ka) sculpted the dramatic scenery seen in Tompkins County today. This summary will give particular focus to these important periods, but will briefly introduce Tompkins County’s entire Phanerozoic history.

At the beginning of the Phanerozoic Eon, two prehistoric continents — Laurentia (the precursor continent to North America) and Baltica (the precursor continent to Europe) — were rifting away from one another, an ocean called the Iapetus between them. By about 500ma, however, they had stopped rifting apart and began to drift towards one another. As these two continents collided with one another (as well as all of the islands in between the two), a mountain chain, now called the Appalachians, was born. The genesis of a mountain range is called an orogeny, and the orogeny of consequence to the Tompkins County region is called the Taconic Orogeny.

As wind, ice, snow, and rain came into contact with these mountains, the once massive peaks began to erode. Rivers and streams carried sediment from these mountains into deltas. During the Devonian Period, massive amounts of sediment accumulated in deltas and, along with the skeletal remains of ocean-dwelling creatures, turned into sedimentary rock in a process called lithification. Deposition and lithification of sediment continued to occur in the Tompkins County region after the Devonian Period; however, following a concept called superposition, these layers were deposited on top of the Devonian strata. These layers have since been eroded.

During the Mesozoic era there were almost certainly dinosaurs present in Tompkins County. Unfortunately, there is no fossil evidence of their existence in the region. This is because the fossil-bearing sedimentary rocks that would have recorded evidence of dinosaur presence in Tompkins County have since been eroded.

Throughout the Mesozoic and Cenozoic eras (from 265–65.5ma and from 65.5ma–present, respectively) plate tectonic movement gave rise to the contemporary continental configuration. The region’s topography has been largely shaped by its recent history. That is, during the last glacial episodes (namely, during the Pleistocene — the epoch lasting from 1.64ma–11ka) glaciation sculpted Tompkins County’s ubiquitous hills, gorges, and lakes. As glaciers moved across the region, they carved deep trenches into the bedrock (some of which became the Finger Lakes). As the climate warmed and the glaciers melted and receded, melt-water gave rise to extensive erosion, which contributed to the formation of Tompkins County’s numerous gorges, waterfalls, and glens.

Most recently, human habitation has dramatically affected Tomkins County’s landscape. From the Finger Lakes quarry to Cornell University, human habitation certainly has left a mark on the County’s topography — and will continue to do so in the future.

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