Half a Hundred = A Good Time
November 8, 2011 by Chuck Bailey
William & Mary’s Geology Department turned 50 years old in 2011. We celebrated the half a hundred mark with a weekend wingding on campus and in the field. Nearly one hundred alums were in attendance and by my reckoning a good time was had by all.
Founded in 1961, the Geology department has graduated nearly 800 majors. The feté brought together many of these alums from across the decades, current students, departmental friends, emeriti faculty, and present-day faculty. The crowd arrived in the department on Friday night for a voluble reception.
Assembled at the outcrop along the Falls of the James River, Richmond.
On Saturday morning we rolled westward to the Falls of the James in Richmond for a field trip dedicated to former Professor Bruce Goodwin, who introduced legions of W&M students to this exceptional exposure of granite and its erosive fluvial features. We remembered Bruce and then cast our eyes towards the rock and riverbed to understand the geologic history of the Fall Zone. The banter on the outcrop was rich.
Hope Huntington, Christine Turner, and Trudy Phelps (class of 1971) on the field trip.
Watch video snippets of the field trip
Back on campus that evening, participants exchanged their fleeces and boots for more upscale fashion at the banquet. As department chair the master of ceremony duties fell to me. I recounted a departmental history (although, I must admit, strict historical accuracy was tossed under the bus). Other speakers recollected past adventures and made a strong case for why the department has been a vibrant community for 50 years.
Recent alums enjoying the banquet.
For me the weekend will be among the most memorable and satisfying I’ve had in my fifteen years at the College. To see so many students, former faculty, and friends was special. We won’t wait another 50 years for another celebration!
It All Averages Out
October 14, 2011 by Chuck Bailey
This morning I handed back the graded mid-term exam to the Geology 312- Weather, Climate, and Change class. The average (or mean) score was 78% with the high grade topping out at 92%. The grade distribution is skewed to the left (that is there is a long tail of lower grades) with a noticeable absence of high-end grades- this is a typical pattern in the larger classes that I teach. My exams are difficult and given the time constraint set by an hour-long exam, scores in the high 90’s are quite rare. Students earning scores of 85% did great. I don’t assign letter grades per se, but from the illustration below you get a sense of my thinking on that matter.
Grade distribution for GEO 312 mid-term exam
Students are creatures of habit and typically sit in the same seat throughout the semester, thus I was able to determine the average grade per row. The classroom (Tyler Hall 102) has six rows- between 9 and 12 people sit in each row. There are some discernible trends in exam scores between the rows. Row 1 (the front row) had the highest average and the row average dropped back to Row 3, there is a curious ‘grade inversion’ between Row 3 and 4, and then the grades fall off towards the back row. Now these are just averages for the rows, not everybody in the back row did poorly. A measure of grade variation per row is given by the standard deviation (shown as the gray horizontal lines), note the wider spread in grades in the back two rows. So just what is the best row to sit in? The use of averages and standard deviations will be important as the class works on their regional climate projects, discerning the variability of temperature and precipitation over time.
Speaking of averages, the cumulative precipitation for 2011, measured at William & Mary’s Keck Environmental Field Lab, is spot on for an average year. Typically, by the second week of October, Williamsburg has received just over 1,000 mm (1 meter or 39 inches) of precipitation- this is based on time series data collected between 1949 and 2010.
An average year might not seem so interesting, but how the average was reached is more informative. The plot above illustrates the weekly precipitation (red columns), the cumulative precipitation for 2011 (the thick blue line), and the cumulative yearly average (dashed purple line), which steadily climbs throughout the year. Precipitation events, such as the passage of weather fronts and thunderstorms, do not occur at an even or measured pace. Some weeks Williamsburg receives no precipitation, whereas other weeks we get drenched. By mid-August (week 33) campus had received 600 mm of precipitation for the year and was over 25% below average. Hurricane Irene (in late August) changed all that by adding over 200 mm in just under a day. As summer turned to fall, a consistent flow of southerly air has repeatedly brought rain to Williamsburg and in the process topped up W&M’s rain gauges to their long-term average. La Niña conditions are set in the equatorial Pacific Ocean, this typically brings warmer and potentially wetter conditions to the mid-Atlantic. There are still some 10 weeks left in 2011 and it remains to be seen whether precipitation for the entire year will all average out.
Mountain Mayhem
September 27, 2011 by Chuck Bailey
As I’ve noted in these posts before, Geology Departmental field trips are unique as they bring together the W&M geologic community in a way that staying on campus never could. The Fall Field trip took an enthusiastic crew of students and faculty to the Blue Ridge Mountains for a weekend getaway. Our timing was just right as an on-shore flow of moist air brought rain and a dreary Saturday to Williamsburg. The mountain mayhem began at Big Meadows in Shenandoah National Park. We savored the irony that Big Meadows, one of Virginia’s wettest locales with a yearly precipitation average of 130 cm (50”) per year, was dry while Williamsburg was wet.
Sorting out the wind direction at Big Meadows (photo by Linda Morse).
Mellowing the marshmallows!
Professors Greg Hancock and Jim Kaste got the discussion started as we pondered the flattish landscape of Big Meadows, and hiked into Hogcamp Branch to consider stream dynamics and the role that bedrock plays in water chemistry. The ascent of Bearfence Mountain took us from the basement complex (always my favorite), up through outcrops of sandstone in the Swift Run Formation, to a rocky spine of greenstone exposed along the crest of the ridge. At Rockytop Overlook we basked in late afternoon sunbeams. The scene was so sublime that some faculty broke into song, songs extolling the virtues of steep slopes and rocky tops. We made camp in the twilight, devoured bowls of chili, and reveled by the fire long into the evening.
To get a better sense of the mayhem- Check out video snippets from the field trip
On Sunday morning we shook off the dew and began with a jaunt along the Appalachian Trail to exposures of sandstones, conglomerates, and siltstones in the Weverton Formation—its depositional environment, way back in the early Cambrian period (~540 million years ago), was vigorously debated. Another hike took us to Calvary Rocks, where well-cemented quartz sandstones of the Antietam Formation revealed their secrets. Our last debate focused on the landscape—are the Blue Ridge Mountains growing, shrinking, or in some long-term steady-state? Any thoughts? (comments welcomed)
Our weekend excursion to the Blue Ridge was uplifting (pun intended!). For an old mountain range the Blue Ridge is more dynamic than you might think.
The crew at Bearfence Rocks.
Hark Upon the Gale! The Hurricane Irene Report
September 7, 2011 by Chuck Bailey
Hurricane Irene raked North America’s East Coast and put a kibosh on the start of William & Mary’s Fall semester. Students were sent packing to safer locales while most faculty and staff hunkered down in Williamsburg. Irene delayed opening convocation by a week, but last Friday the choir belted out a spirited version of William & Mary’s school song- James Southall Wilson’s “Our Alma Mater”- and the semester was truly under way.
Enhanced infrared satellite image of Hurricane Irene at 5 p.m. (EDT) August 27th, 2011. (From- http://www.ssec.wisc.edu/~scottb/110825-29_g15_ir_irene_anim.gif)
For my money, the best line in the song is “Hark upon the gale!” There are varied interpretations as to what “hark upon the gale” really means- I view hark as an active rather than a passive verb. During Irene the Geology faculty did more than just listen to the gale- they were out in the gale collecting data before, during, and after the event.
On Friday afternoon the W&M campus was quiet, but the atmospheric pressure had started its telltale decline. By the wee hours of Saturday morning, August 27th the wind was blowing steadily from the east and the rain began. Professor Greg Hancock measured the cumulative precipitation at his house- in the early afternoon the rainfall rate exceeded 1” per hour, deluge-like rates! By noon the wind was coming from the northeast, and as the Hurricane’s center moved to the northeast off Virginia’s coast, the wind steadily shifted from the north to the northwest and then to the west. Sustained winds at Yorktown ranged from 15 to 23 m/sec (~30 to 50 mph) with the peak gusts, at nearly 30 m/sec (66 mph), occurring near midnight. These winds blew down trees and shut off the power to most Williamsburgers.
Data plots for Hurricane Irene in the greater Williamsburg area.
At his house near York River State Park Professor Jim Kaste was monitoring a small perennial stream and collecting water samples for chemical analysis. The normal base flow for the stream is quite small (<0.01 cubic feet/sec), during the height of Irene stream flow exceeded 6.5 cubic feet/sec- more than two orders of magnitude (>100x) greater than normal flow! Even a week after the storm, the stream has yet to fall to its base flow condition. His environmental geochemistry class is now running the water samples to measure the dissolved load carried by the stream and assess what fraction is derived from Irene versus groundwater.
Discharge of a small stream near York River State Park. Note logarithmic scale.
On Sunday morning the damage from downed trees was widespread. I lost two trees to the storm, but none fell on the house- my next-door neighbor was not so fortunate. In between chain sawing and cleaning up I measured the orientation of the blown over trees in my neighborhood and then later on campus and around Williamsburg. The orientation of the downed trees tells a story about the kinematics of the wind during the storm. There is a clear bimodal pattern- the majority of the trees fell towards the southeast and southwest. Almost no trees fell toward the west, northwest, north, and northeast.
Rose diagram illustrating the orientation and % of downed trees. Note- this is a map view.
Prior to Irene’s arrival I’d hypothesized that most trees would fall towards the west and southwest as winds from the east and northeast (on the leading edge of the storm) would do the damage. As rose diagram illustrates my hypothesis was generally incorrect.
Incorrect hypotheses warrant further inquire so I decided to investigate the dynamics of the wind, leading me to calculate the cumulative wind energy for winds from various directions during the storm. Utilizing the wind data from the Yorktown Coast Guard station, I sorted the wind data into directional bins (in 10˚ increments) then for each observation determined the wind power acting over 1 square meter via this relationship:
Power (in Watts) = 1/2 x (air density) x (area) x (wind velocity^3).
I used a standard density for air of 1.225 kg/m3 and an area of 1 m2. Here is an example calculation-
1/2 x (1.22 kg/m^3) x (1 m^2) x (10 m/sec)^3 = 612 kg m^2 /sec^3 = 612 Watts
Wind measurements were made every 6 minutes, so to convert to energy I multiplied the power by 360 seconds (the number of seconds in 6 minutes). Power is energy per unit time, so to get energy from power- multiply by time. I then summed each 6-minute interval for a given bin to yield the cumulative wind energy from each direction.
Well over 60% of the cumulative wind energy came from the northeast (that is wind blowing towards the southwest) while the cumulative wind energy from the northwest is markedly lower. This is likely because the wind blew consistently from the northeast for many hours (see the wind direction plot above). The correlation between the orientation of downed trees and directional wind energy is less than stunning. Perhaps there is a better metric for understanding the downed trees.
The next step was to calculate the power (in kWatts per square meter) from the peak gusts from a particular direction. This yielded a bimodal pattern with peaks in both the southwest and southeast quadrants- much closer to the observed tree fall data. It seems reasonable that the peak winds toppled trees.
Another significant observation is that most trees fell during the last half of the storm. Although the energy and even peak gusts coming from the northeast (first half of the storm) were high, more trees fell due to winds from the northwest (last half of the storm). It had been dry for some weeks in Williamsburg and it took time for the rainfall to saturate the soil, and saturated soils are invariably less cohesive and weaker than dry soils.
Hurricane Irene was an exciting event for the William & Mary earth science community. Collectively, the storm damage to campus and the community was modest- that is a good thing. Although many were disappointed with the media’s hype of the storm, Irene tracked out just as meteorologists had predicted. In Williamsburg Irene was an asymmetric storm with copious rain in its long opening act and a tumultuous windy finish. The storm set the tone for the early days in my Weather, Climate, and Change class. As the Fall semester unfurls, Geology faculty and students will continue to work out the intricacies of Irene’s environmental impact .
So as the song says… “Hark upon the gale! Hear the thunder of our chorus”
It’s Not My Fault
August 23, 2011 by Chuck Bailey
This afternoon, our Geology faculty meeting was adjourned by a motion from the floor. A 20-second motion from the floor, but more to the point, a 20-second motion from the Earth. Virginia and the East Coast experienced a moderate, but widely felt earthquake at 1:51 p.m. (local time). It was quite a jolt.
The earthquake’s epicenter was about 60 km (~40 miles) northwest of Richmond, Virginia and occurred in the central Virginia seismic zone- an area of modest (or so we thought), but persistent seismic activity in the Piedmont. This region is laced with ancient faults that formed 200 to 300 million years ago when Virginia was at the frontline in an ugly collision between tectonic plates. I study these fault zones. Today’s temblor makes it clear that these faults are 1) not inactive and 2) have the potential to produce significant and damaging earthquakes. We have much to learn about the stresses that cause faults to slip this far from modern tectonic plate boundaries (in this case at the Mid-Atlantic Ridge some 3,000 km from central Virginia) and the hazards that these old, but restless, faults pose. It’s why we do research at William & Mary.
An earthquake presaging a hurricane- this could be quite a semester to study the Earth!
Williamsburg’s Triple H’s
August 19, 2011 by Chuck Bailey
Welcome to William & Mary, it’s the middle of August and the weather in Williamsburg is…
Word cloud of terms describing Williamsburg's summer weather from a survey of Introductory Geology students.
Williamsburg’s Triple H’s- HAZY, HOT, and HUMID.
For me August is not Williamsburg’s finest month and I’m not the only one with that point of view. Faculty colleagues who, in August, moved to Williamsburg from New England, northern California, and Colorado have all thought “just what have I gotten myself into?” If you are new to William & Mary, fear not- Williamsburg’s weather gets better as autumn arrives. Next week the semester begins and I’ll be teaching Geology 312- Weather, Climate & Change, a second-level course pitched to a diverse audience. It is summer so there’s no surprise about the hot, but why is William & Mary wrapped in a humid blanket of air at this time of year and what about all that haze out there?
Map from the National Weather Service (http://www.weather.gov/forecasts/graphical/sectors/).
Before tackling that question it is important to consider just what is humidity and how best to define humidity? The Earth’s atmosphere is a mixture of various gases; H2O in the vapor phase forms a small, but important, fraction of the atmosphere. Humidity, in a broad sense, is the amount of water vapor in the atmosphere. The trouble is that humidity can be defined in a number of ways- including relative, specific, and absolute humidity, as well as by mixing ratios and with saturation vapor pressures. The amount of water vapor that air can contain changes with temperature; warm air holds more water vapor than cold air. Relative humidity, a commonly used meteorological metric, is
(water vapor content of the air/ water vapor capacity at that temperature) x 100
When air is saturated with water vapor the relative humidity is 100%. Typically, relative humidity changes over the course of a day- as the temperature rises, the air’s capacity to hold water vapor increases and consequentially the relative humidity drops. Another measure of humidity is dewpoint, the temperature at which the air is saturated. Moist, humid air has dewpoint temperatures of greater than 15˚ C (~60˚ F).
The plot below illustrates data collected at W&M’s Keck lab from 2004 through 2010. Notice that the average relative humidity tops 80% in August, September, and October and stays above 77% for half the year (from June thru November). But the dewpoint temperature reaches its maximum, over 20˚ C (67˚ F), in July and August and that is the key. The air harboring the most moisture envelopes Williamsburg in mid to late summer. Although the relative humidity in October and November remains high, the dewpoint temperature is much lower during the summer and these autumn months are among the most pleasant in Williamsburg.
Data from W&M Keck Lab (2004-2010). Blue shaded area and bars are standard deviation of data.
But why such moist air during the summer? As Bob Dylan croons- “the answer, my friend, is blowin’ in the wind”. Consider the wind roses compiled from a weather station at Yorktown (15 km east of Williamsburg): during the summer months the wind blows from a southerly direction (southeast, south, and southwest) nearly 50% of the time. During the winter months the wind comes from the south just over 30% of the time and the dominant wind direction is from the northwest.
Seasonal wind rose for Yorktown, Virginia. Arrows point in direction the wind blows towards (i.e. a SW wind blows towards the NE). Southerly winds illustrated in blue.
That southerly wind brings with it moist, marine air from the Gulf of Mexico and tropical Atlantic Ocean- stoking up humidity in the southeastern United States. Why the seasonal shift in wind direction? Questions like that are a significant part of the Geology 312 course. Here is the quick answer- during the northern hemisphere summer a stable high pressure (anticyclone) tends to develop over the Atlantic (known as the Bermuda High or Azores High). The clockwise flow of air around this high-pressure system conveys warm and moist air to the southeastern United States. Weather fronts and tropical storm development complicate the pattern, but the regional climate is dictated by this stable summer air mass in the Atlantic. During the winter months the high is weakened and excursions of a southerly jet stream can bring cooler and drier continental air masses to southeastern Virginia.
In addition to moist air, the southerly wind is currently bringing lots of haze with an acrid smoky smell and even ash from a wildfire that’s burning in the Dismal Swamp. What a welcome to Williamsburg! Carbon is being liberated from both modern and ancient vegetation (in the form of peat buried in the swamp)- carbon cycling plays a huge role in climate. Just how much carbon is being liberated, where will it go and what are the consequences? More questions that the Weather, Climate, & Change class will tackle starting next week. The semester begins on Wednesday, I can’t wait.
Orange arrows illustrate wind direction in southeastern Virginia during the afternoon, August 18th, 2011. Wind data from National Weather Service.
Making Hay with the Alberene Dream Team
August 5, 2011 by Chuck Bailey
August is here and a new semester looms just around the corner. As the old saying goes it is best “to make hay while the sun shines” and Alberene Dream Team did just that, they baled a bunch of “research hay” during their summer field campaign in the eastern Blue Ridge Mountains. Let’s review.
The Dream Team examined nearly 500 outcrops, measured geologic structures until fingers tingled, and brought a pile of rocks to Williamsburg in order to learn their secrets. We climbed to the top of many a green mountain chasing geologic contacts, we prospected many a quarry, and we floated the James River searching for faults exposed in the river bottom.
Molly Hahn strikes a pose amidst fractured granitic basement rock.
Molly Hahn teased out the geometric distribution of fractures in the bedrock, there are two recognizable fracture sets throughout much of the study area and most developed as extension fractures under a northwest-to-southeast oriented maximum principal stress. Alex Johnson pinned the basement/cover sequence contact at a few locations, and much to my chagrin the contact geometry is not consistent with a major fault in the Alberene quadrangle.
The always dapper Alex Johnson measures geologic structures.
Kevin Quinlan’s research framed the Scottsville rift basin from both a structural and sedimentological perspective, his detailed analysis of en-echelon diabase dikes in the James River suggest a component of sinistral shear affected these rocks.
Kevin Quinlan checkouts Triassic conglomerate exposed along the James River.
Andrea Jensen and her advisor Professor Brent Owens mapped the distribution of mafic and ultramafic bodies that intrude both the basement and cover sequence. During the semester they will analyze the rock’s chemistry and model the emplacement conditions of these bodies.
Andrea Jensen framed by a hulking outcrop of ultramafic rock.
Geologic field work is difficult work: the heat, the humidity, the vegetation, and insect fauna conspire to make geologic mapping in the summer as much a physical challenge as an intellectual endeavor. There is much still to learn in the field and our laboratory analyses await, but the Dream Team’s research progress has been splendid and their geologic map of the Alberene quadrangle is fetching.
- The Dream Team’s simplified geologic map of the Alberene 7.5′ quadrangle, Virginia. The various colors illustrate the distribution of the different bedrock, gr- granite.
One of our major research goals was to determine whether a tectonic suture (an ancient fault zone separating rocks that originated on different tectonic plates) etches the eastern Blue Ridge foothills. The geologic map reveals a distinctive parallel pattern between the basement complex in the northwest and the units in the overlying cover sequence to the southeast (excepting, of course, the intrusive metagabbro and diabase dikes). The overall concordance of rock units and geologic structures makes it clear that no terrane-bounding fault are present here. Kudos to the Alberene Dream Team.
Mid-Summer Dream Team Report
July 13, 2011 by Chuck Bailey
The Alberene Dream Team has left the building and is now safely ensconced back in the eastern Blue Ridge Mountains. The Dream Team took a well-earned respite from fieldwork last week to compile field data in the Geology Department.
Alberene Dream Team in action somewhere among the green hills of central Virginia. From left to right- Andrea Jensen, Molly Hahn, Alex Johnson, and Kevin Quinlan.
In my original post I noted that the Alberene Dream Team is conducting research in the eastern foothills of the Blue Ridge Mountains and producing a geologic map of the Alberene 7.5’ quadrangle. Since that post I have been asked- just what is a quadrangle?
- Part of the Dream Team’s digital field map. Red boxes are 1 km by 1 km. Colored dots are different rock types, black numbered symbols are geologic structures, and solid lines are contacts.
A quadrangle, in geographic parlance, is a rectangular area of land illustrated on maps produced by the U.S. Geological Survey. A 7.5’ quadrangle encompasses 7.5’ (minutes) of latitude by 7.5’ of longitude. In Virginia a 7.5’ quadrangle is ~11 km by 14 km. The Alberene 7.5’ quadrangle takes its name for the small village of Alberene (37.886˚ N, 78.617˚ W) that was once a company town where soapstone was quarried.
The Dream Team spent most of the last month conducting fieldwork in the Alberene quadrangle. During the summer, the riot of vegetation that cloaks the landscape and obscures bedrock outcrops makes fieldwork in Virginia challenging. Nevertheless, the Dream Team has canvassed the Alberene quadrangle, examined over 250 outcrops, collected many kilograms of samples, and measured a multitude of fractures and faults.
In addition to producing a detailed geologic map and collecting the primary data for their senior theses, the Alberene Dream Team is filming clips to make a short documentary on the process of doing geologic field research.
Video Snippet- Mapmakers Discussion
Professor Brent Owens and I are off to join the research team tomorrow. Over the next few weeks we’ll be canoeing the James River to investigate Triassic fault zones, we’ll be searching for exposures of a curious igneous rock rumored to crop out at Thomas Jefferson’s Monticello, and we’ll be hiking up and over the green hills to pinpoint the location and geometry of geologic contacts. And as my research students say, this promises to be a smashing good time.
The Alberene Dream Team
June 10, 2011 by Chuck Bailey
The summer of 2011 will not be a quiet one in the Geology Department. In addition to a full complement of Geology majors working on research, the department is abuzz with construction as we convert the geology library into a new and commodious classroom. More than a dozen Geology majors are currently on campus collecting data and readying themselves for field campaigns.
I am collaborating on a project with Professor Brent Owens and four undergraduates to better understand the geology of the eastern Blue Ridge region in central Virginia. Our study area is located about two hours northwest of Williamsburg and the research is supported by a grant from the U.S. Geological Survey. We are producing a geologic map of the Alberene 7.5’ quadrangle, a patch of ground about 155 km square located just south of Charlottesville, in order to learn the geologic secrets of the Blue Ridge.
The Alberene Dream Team. From left to right- Andrea Jensen, Kevin Quinlan, an astonished Professor Brent Owens, Alex Johnson, and Molly Hahn discuss geologic research in the eastern Blue Ridge.
So why study this region? The rocks in the Alberene quadrangle are old and record a long history that includes two episodes of mountain building and two episodes of crustal rifting. Some Appalachian geologists place a tectonic suture (the boundary between ancient tectonic plates) in the eastern Blue Ridge whereas others are more circumspect. We are going to resolve that issue.
Shaded relief map of the Alberene 7.5’ quadrangle and surrounding area. Note the southwest-to-northeast ‘grain’ of the topography, a reflection of the underlying bedrock layers and their structure.
Molly Hahn’s research is focused on understanding the geometry and timing of brittle deformation (faulting and fracturing) in the rocks of this region. We’d like to know when the rocks broke, but deciphering the fracture geometry also aids in predicting the distribution and abundance of groundwater. The Alberene quadrangle is endowed with numerous mafic rocks (dark and dense rocks loaded with iron and magnesium) whose age and tectonic origin have long been debated; Andrea Jensen’s research will determine when and how these rocks formed. Alex Johnson is mapping the contact between 1 billion year old granitic rocks (exposed in the northwest part of the area) and a sequence of younger metamorphosed sedimentary rocks- is this boundary a fault or an unconformity (if you are uncomfortable with unconformities check out-Who’s unconformable? ). Rocks in the southern part of the area are sedimentary rocks of Mesozoic age (~200 million years old) and formed in a rift basin that presaged the opening of the Atlantic Ocean- Kevin Quinlan is working to understand the structural architecture of this basin and its later reactivation.
We leave for the green hills of the eastern Blue Ridge early on Monday morning. As the summer rolls on I’ll provide updates on the adventures, misadventures, and research discoveries of the Alberene Dream Team.
Who’s unconformable (Part 2)?
December 23, 2010 by Chuck Bailey
It took well over a week to crawl out from under the pile of 170 final exams, a ‘gift’ delivered by my Geology 110 course, but the grading is now done and the holidays are here. This quick post draws to a close the research that my structural geology seminar completed. Recall that the seminar conducted field research in the Blue Ridge Mountains of Shenandoah National Park to determine whether the contact between the underlying greenstones of the Catoctin Formation and sedimentary rocks of the Weverton Formation is conformable or unconformable. Here is a selection from our geologic maps and cross sections.
Part of the geologic map from the structural geology seminar’s field area. Zc- Catoctin Formation, Cw- Weverton Formation, Ch- Harpers Formation. The orange line is the contact of interest for our study.
Geologic cross section illustrating the nature of the folded strata. Zc- Catoctin Formation, Cw- Weverton Formation, Ch- Harpers Formation.
Geologic map draped over the topography in Google Earth. Oblique view of the terrain, view to the south. Yellow diamonds are outcrops examined this semester.
Close up of Weverton conglomerate with clast of Catoctin Formation.
Our research indicates that the contact is unconformable and that an interval of erosion took place after the Catoctin lava flows cooled, but before the sand and gravel of the Weverton Formation was deposited. A number of different rock types are exposed at the top of the Catoctin Formation, an observation consistent with an erosional upper contact. Clasts in Weverton sandstones and conglomerates are mostly quartz and feldspar, minerals derived from the erosion of a granite, but rare clasts of Catoctin basalt do occur and indicate that erosion of the Catoctin preceded the Weverton Formation. The unconformable boundary between these geologic units may be the product of regional uplift followed by thermal subsidence as rifting waned and the ancient North American continent broke apart some 550 million years ago. I am delighted with the research conducted by this semester’s structural geology seminar; their final work was of high quality and they’ve added another piece of understanding to the geologic puzzle that is the Appalachian Mountains.