This is the first of several posts, reporting the roadside geology of western New York and central Vermont. Today, we will visit Binghamton, New York. This small city (urban population less than 50 thousand) sits at the confluence of two perennial, gravel-bedded rivers (Fig. 1).
Figure 1. View from Confluence Park in Binghamton, NY, showing the Susquehanna River (left side) and Chenango River (right). Note the weir on the Susquehanna, which maintained sufficient depth for a lock (Fig. 2), which permitted access further upriver.Figure 2. Photo of submerged lock that operated in the 1800s along the Susquehanna River. Similar structures are present along the Potomac in VA and MD. The outline seen in this image may be only the foundation. The stonework was probably removed to create a clear channel for the modern bridge (Fig. 3).Figure 3. Photo from the confluence of the Susquehanna and Chenango rivers, showing the bridge that has replaced the lock structure shown in Fig. 2. Note the gravel bar to the center right of the image, where a small tributary has been channelized but still is depositing large gravel and small boulders in the Susquehanna channel.
Enough of Holocene and Anthropocene geology. The fascinating thing about this region is that it preserves a huge volume of sediment eroded from mountains that were growing during the Devonian Period, about 350 million-years ago (Fig. 4).
Figure 4. Geologic map from Rock D, with Binghamton near the center. Note the tan-shaded area that expands from Albany westward. These rocks were originally sediments, deposited from a high mountain range located somewhere east of Albany, carried by ancient rivers as far as 500 miles to the west . (The scale is in the lower left of the image: 100 km is about 63 miles.) This humongous basin, collecting boulders, sand, silt, and mud about 350 my ago, was preserved because it was a precursor of what was to come 100 million years later, when Africa and N. America collided, burying these rocks deep enough to save them from erosion, but not deep enough for them to lose their sedimentary character. This perfectly preserved basin–frozen in time as if in a museum–is called the Catskill Delta.
We didn’t have the time or resources to go on a quest for rocks that would reveal what was happening during the Devonian Period, so we took some photos of charismatic blocks that had been removed from their original location and “deposited” along the path that followed the Chenango River through downtown Binghamton (Figs. 5 and 6).
Figure 5. Photo of slab of mudstone (not in original orientation), showing irregular ripples accentuated by silt against a matrix of mud. This is a very common environment in river flood plains during high-flow events, when gentle currents separate silt from mud. Flow during these intervals is insufficient to form unidirectional ripples, and the result is seen in this image. The sample is about two feet across.Figure 6. Photo of silt surface in shallow water from unreferenced rocks from the Catskill Delta. Note that this bedding plane has a more criss-crossing pattern of “ripples” and contained more silt (light-colored). This sediment was probably deposited in a similar environment to Fig. 5. Both were quickly buried during a flood that occurred not long after these fragile sedimentary structures were created. If you were to step on these sediments in a modern stream, they would be what we term “mud” and avoided if possible.
The title of this post refers to the outer limits of a broad plain that was receiving gravel, sand, silt, and mud from a rapidly rising mountain belt–probably like western North America today (e.g. the Sierra Nevada mountains). It wasn’t a continental collision, but it was pretty massive, with elongate swaths of sediment subsequently buried by what came later.
Timothy R. Keen 
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