September 14, 2020
By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
First, some background. Thomas Cole’s 1836 painting popularly called “The Oxbow” isn’t really called that. The actual title of the work is “View from Mt. Holyoke, Northampton, Massachusetts, after a Thunderstorm.” I have always known the painting by its shorter adopted title, and I always assumed it was not based on an actual location and was something of a synthesis of Cole’s observations of various rivers. I sourced the image of the painting below from this link, which offers some interesting background.
Earlier this year, I became aware of the longer, geographically-specific title and learned that the painting does portray a real location with a particularly interesting geologic context. The Oxbow is located at 42.290590N 72.635695W just south of Northampton; Cole’s vantage point on Mt. Holyoke is east-northeast. The second image below shows an inclined view from a prominent outcrop on the ridge; it seems to match well with the perspective in the painting.
The Oxbow loop itself is just under 1 mile (1.6 km) across at its widest, and today is abandoned by the Connecticut River; a small channel keeps the two waterways connected. I don’t know if The Oxbow was abandoned in 1836. Even if it was, Cole would have certainly removed such an unfortunate detail to produce this iconic piece, which might not have looked so good with a cutoff meander. I doubt Cole would be amused by my re-touched version below, made possible by Microsoft Paint.
The rendering of the curve of The Oxbow is excellent and should please any geomorphologist, but the geologic setting of the painting is itself a particularly interesting detail. I was surprised to find out that Mt. Holyoke is developed on tilted basalt flows in the Mesozoic Hartford Basin, which accumulated sedimentary and volcanic rocks during the breakup of Pangaea.
These rocks are much younger than the surrounding Appalachian rocks and formed in a subsiding tectonic basin, but today they are just another part of the regional landscape in which rock types and their resistance to long-term erosion are the major control over topography. In today’s eastern North America, significant exposures of hard rocks, like the Holyoke basalt, always make prominent (not necessarily high) ridges or mountains, regardless of the original tectonic origin of the hard rocks. A present-day mountain in eastern North America therefore does not need to be related to the assembly of the Appalachian Mountains; it only needs to be supported by enough hard rock adjacent to enough weaker rock to create topographic prominence (see the end of the post). This general topographic regime contrasts with tectonically (more) active settings, where fault movement or volcanic accumulation are major controls on topographic highs and lows.
Overall, the Hartford Basin is still a minor topographic basin because most of the rock filling it is sedimentary layering that is much more erodible than the surrounding Appalachian bedrock. Again, this reflects rock erodibility, not tectonic movement. The resulting low, flat landscape is good for development and agriculture. In the image below, the Hartford Basin does not need to be outlined; its higher population density and extensive land development make it stand out as a light-colored zone.
Paleozoic metamorphic and igneous rocks that frame the basin make rougher topography. These areas remain forested, as they are less attractive for habitation and farming when so much easily worked land is nearby.
Mt. Holyoke rises 400 ft or so (~120 m) above the surrounding flatlands, which have developed on the more erodible sedimentary layers deposited in the basin before and after the basalt flows. The erodibility contrast is fairly extreme, creating the very abrupt topography that offered Cole such a dramatic view despite fairly modest topographic relief. The basalt flows and surrounding sedimentary layers were tilted and faulted during basin growth, causing the basin fill to dip (tilt) towards the eastern edge of the basin. Erosion after the end of the basin development has etched out the once-buried portions of the basin fill, and exposures of hard layers at the surface now create topographic high points that are completely unrelated to high (or low) points during basin formation. The model shown below gives a general idea of how basin structure corresponds with topography in the modern-day Hartford Basin setting.
The pink layer represents a resistant basalt layer. Where it meets the surface, ridges develop. A river meanders along the foot of one of the ridges atop a much weaker exposure of sedimentary rock. Note that topography is tiny compared to the thickness of the basin fill. There are thousands of feet of remaining basin fill beneath the land surface (up to 10,000-13,000 feet (3-4 km) ) of today’s Hartford basin, but Mt. Holyoke only rises 400 ft (120 m) above the adjacent sedimentary landscape. Generalized cross sections of the Hartford Basin abound, but the overwhelming majority are drawn looking north, opposite of the first few images in this post. I am not sure about the use of Connecticut River Basin for to describe the geologic Hartford Basin as shown in the image below sourced here; either way, the conceptual layout of the sketch is good.
In addition to being the setting for The Oxbow, the Hartford Basin’s sedimentary fill is the source of Portland Brownstone, which was a much sought-after construction material in the second half of the 19th century. In New York and many other major American cities, a large number of historic buildings utilized the brownstone, which is very recognizable. There is plenty to read about quarrying of the brownstone and basalt (or “traprock”); this link, this link, and this link provide some information and very basic conceptual sketches of the basin setting, which can be compared to the sand model above.
Mt. Holyoke may be the most prominent ridge within a Mesozoic basin in eastern North America today; its competition would be features in the Newark Basin in New Jersey and Pennsylvania. These exhumed Mesozoic Basins produce interesting outcrop patterns; this link shows some model examples compared to the real thing. The development of erosional topography in the exposed basin rocks is a good reminder that the appearance of modern-day eastern North America largely reflects regional uplift and erosion occurring well after the breakup of Pangaea.
For comparison, another small ridge developed on resistant Mesozoic rock can be found close to my present location in Greenville County, South Carolina. Pax Mountain is a long, narrow ridge that rises 350 ft (just over 100 m) above the surrounding landscape (35.066216N 82.322472W).
The mountain is supported by silicified fault gouge (crushed rock) within a Mesozoic fault zone. Normally, ground-up rock in faults is more erodible than surrounding rocks. In this case, silica-rich fluids moved through the ground rock while it was still at depth and impregnated it with quartz, making it exceedingly hard. As erosion un-buried the silicified rock, it developed notable prominence above the intact mica- and feldspar-rich metamorphic rock surrounding it. The silicified zone is quite narrow, but hard enough to make a prominent ridge that offers nice views and pricey real estate.
Like Mt. Holyoke, this mountain has nothing to do with Appalachian assembly and everything to do with rock strength and the prolonged erosional history of eastern North America.
This post was originally published on The Geo Models blog and reproduced here with permission.