November 14, 2013 by Chuck Bailey
This post begins what I plan to be a recurring series on drainage basins and watersheds. For earth scientists interested in landscapes and surface hydrology: drainage basins are a fundamental component of these natural systems.
A drainage basin consists of all the terrain that contributes water to a particular stream or river. For instance, rain that falls on the Geology building at William & Mary runs off into College Creek, College Creek flows south into the James River, and the James River debouches into the Chesapeake Bay at Hampton Roads. Drainage basins are diverse, ranging in size from near continental portions (i.e. Mississippi/Missouri and Amazon) to modest creeks that drain a few square kilometers, and small drainage basins are nested inside progressively larger basins. In what drainage basin are you reading this post?
I grew up on a patch of rolling upland terrain near Ivy, Virginia about 10 km west of Charlottesville in Albemarle County. I spent much of my youth in the local creek that meandered across the rural landscape. This was Ivy Creek. It is a small stream, typically a meter or two or three wide, with a sandy to gravelly bottom and steep muddy banks (1 to 2 meters high).
My friends and I had many adventures in Ivy Creek and its tributaries. We traipsed through the creek on hot summer days searching for cool water and deep shade. After snowfalls we’d build a snow ramp on the edge of Ivy Creek Branch, take to our sleds, glide down the nearby hill, hit the ramp, and often land askew in the partly frozen creek – good times!
Fond memories aside, Ivy Creek and its drainage basin are quite ordinary. Because the Ivy Creek drainage basin is so typical (or perhaps classic) it makes a nice embarkation point for an ongoing discussion about drainage basins.
The Ivy Creek watershed encompasses nearly 60 square kilometers (23 sq. miles). The stream begins along the northwestern slope of the Ragged Mountains and flows in a northeasterly direction for over 20 kilometers (~13 miles) where it joins the South Fork of the Rivanna River (a tributary to the James River).
The Ragged Mountains are a group of subdued and rounded hills underlain by granitic bedrock that solidified about a billion years ago. For a not so subdued and phantasmagoric story of this range read Edgar Allan Poe’s “A Tale of the Ragged Mountains” which ostensibly takes place here. All things considered, the old granitic bedrock in the Ivy Creek watershed is relatively uniform and homogeneous. As such, Ivy Creek flows across bedrock with a similar strength and erodibility throughout its drainage basin.
Consider Ivy Creek’s gradient: at its headwaters near Taylors Gap the stream has a steep gradient (~40 meters per km) that gradually lessens (~2-3 meters per km) near its confluence with the Rivanna. The longitudinal profile of Ivy Creek is steep near its headwaters and becomes progressively less steep further downstream. This concave up profile is very typical of many stream systems. Why do streams typically have such a profile?
Since the bedrock in the Ivy Creek basin is similar throughout the basin it is unlikely that the bedrock is exerting much of a control on the stream gradient. In a future post we’ll examine a stream that crosses bedrock of varying hardness and erodibility, but let’s get back to Ivy Creek.
Near Taylor’s Gap, Ivy Creek is perhaps 1 meter wide (~3’) and quite shallow; near its confluence with the Rivanna, Ivy Creek is upward of 5 meters wide (~16’) and plenty deep. As Ivy Creek flows downstream, traversing its basin, tributary streams join the main stem adding their flow to Ivy Creek, enlarging the channel and increasing the stream discharge.
These relations hold true for many stream systems:
- Stream channel slope (S) decreases downstream
- Mean stream discharge (Qm) increases downstream
Essentially, the gradient of a stream channel is proportional to an inverse function of its mean annual discharge. Perhaps a stream’s discharge plays a major role in controlling a stream’s gradient. I’ll leave it at that for the moment, but I am curious as to your thoughts.
November 4, 2013 by Chuck Bailey
My family has a tradition of going camping about once per semester. Back in the spring of 2011, as the Appalachians were beginning to green up, we headed west to Rockfish, Virginia for a weekend camping trip to my Uncle Joe’s farm. Joe’s farm is located in the foothills of the Blue Ridge Mountains and I’d call the scenery sublime.
Bedlam reigned as we unpacked and set up camp, so much bedlam that I needed quiet time alone. Off I set on a hike through the woods. A half-mile of hiking brought me to the verge of this old abandoned quarry.
It was a commanding view with spectacular rocks exposed in the walls of the quarry. The bedrock here is a metamorphosed conglomerate, a coarse-grained conglomerate chock-a-block full of clasts, many larger than basketballs. The clasts are fragments of older rocks that were eroded, transported, and deposited long ago.
The conglomerate is stratified, but the layers have been tilted and are lying on end. These layers were tilted past vertical and are overturned – that is, the older layers are structurally above the younger layers (an inverted stratigraphic sequence).
I was giddy with geologic questions. In what depositional environment was this conglomerate deposited? Why are these layers tilted and overturned?
The quarry I’d stumbled upon exposes the Rockfish Conglomerate, a geologic unit in the eastern Blue Ridge first defined by Wilbur Nelson in 1932 for outcrops along the Rockfish River about a kilometer to the southwest (geologists commonly name stratigraphic units for a particular location where the rocks are first described and typically are well exposed, it’s known as the type location). This curious conglomerate has been studied by a number of geologists, but nowhere could I find any reference to the exposure at the quarry.
Since the “discovery” I’ve taken classes to see these rocks and invited other geologists to these outcrops. Callan Bentley at Northern Virginia Community College wrote a blog post and captured a lovely Gigapan image from an early visit to the quarry. Just last year Zach Foster-Baril decided to tackle the Rockfish Conglomerate as his senior research topic. Zach just presented the results of this work as a talk last week at the Geological Society of America’s annual meeting in Denver.
We have learned much from our field and lab work over the past year. Our geologic mapping reveals that the Rockfish Conglomerate was deposited in a trough that was eroded into the underlying and older granitic basement rocks. The trough was later filled with the coarse sediment that became the Rockfish Conglomerate. Years earlier I’d posited that the contact between the basement rocks and the overlying metasedimentary cover rocks was a fault, but Zach’s detailed field data make it clear that the boundary is an unconformity.
As the rocks in the Rockfish area are tilted into an almost vertical orientation we can turn the geologic map on its side to approximate the original geometry of this sequence.
Instead of north, south, east, and west we’ve got stratigraphic up (top) and down (bottom) with the oldest rocks at the bottom. From this perspective it is clear that the granitic basement rocks (green) originally were beneath the Rockfish conglomerate (orange), which occupies an irregular trough a few hundred meters deep. An intrusive gabbro body (lavender), as sills and dikes, cuts both the granitic rocks and overlying metasedimentary rocks.
In what environment were these sediments (now thoroughly lithified rocks) deposited?
Cobbles and boulders are not easily transported: high-energy processes are needed to move bowling ball-sized rocks. Suitable conditions occur in mountain streams where fast-moving turbulent waters rage during floods. As the torrent reaches the mountain front, the current slackens and the stream’s load of cobbles and boulders is deposited at the mountain front creating a distinctive wedge of alluvial deposits out into the valley.
But fast moving water effectively sorts sediment based on its grain size. Particles of similar size are deposited along with each other; so as pebbles are deposited, the sand and mud (the smaller particles) continue to be transported by the flowing water. The Rockfish Conglomerate is a very poorly sorted rock and lacks many of the sedimentary structures we’d expect in alluvial fan deposits.
In the 1980’s Frederick Wehr, at the time a graduate student at Virginia Tech, studied the exposures along the Rockfish River and concluded that the sequence was not a terrestrial alluvial fan deposit, but rather a subaqueous marine deposit formed from glacial outwash and mass flow turbidites. Our observations at the quarry are consistent with Wehr’s interpretations. A glaciomarine origin for these rocks is compatible with 1) the wide range of grain sizes (boulder to silt/sand), 2) the prominent parallel stratification present throughout the Rockfish Conglomerate, and 3) the conformable nature of the overlying strata in the Lynchburg Group.
After the talk my colleague Michelle Markley from Mt. Holyoke College plaintively asked Zach to show the audience some dropstones.
Dropstones are literally stones dropped from above into sediment layers at the bottom of a lake or ocean. They are inferred to form when rock laden lake or sea ice rafted off glaciers melts, thereby “dropping” the sediment, from boulders to mud, that was entrained in the ice. Dropstones commonly deform or deflect the underlying strata and are draped by younger layers of sediment. These sedimentary features are not only visually stunning, but typically taken as key evidence for glaciogenic sedimentation into a basin.
Michelle’s question was right on point, we’d not offered up any dropstones for viewing.
Individual dropstones (called by some geologists – lonestones) are not common in the Rockfish Conglomerate. The picture below highlights a candidate for a Rockfish dropstone, but it won’t win any dropstone beauty contest.
A few plausible reasons for the lack of readily identifiable dropstones in the Rockfish Conglomerate include: 1) the coarse conglomerate was deposited in a proximal position relative to the glacial ice and as such the flux of coarse-grained sediment was huge, 2) abundant ice-rafted debris falling into a coarse mixture of sand is less likely to stand out as a dropstone when there are lots of clasts (no lonestones here), and 3) these rocks experienced later deformation and metamorphism such that the matrix is a recrystallized mixture of quartz and mica which does not preserve fine-scale details of the depositional environment.
Zach’s research provides new data on the Rockfish Conglomerate, but many questions remain. A few include:
- When was the Rockfish Conglomerate deposited? Stratigraphic relations indicate that the Rockfish Conglomerate was deposited during the Neoproterozoic Era between 570 and 1,000 million years ago. The possible age range for when these rocks were deposited is >400 million years, that’s lousy age control.
- What created the trough in the basement complex into which the Rockfish Conglomerate was deposited? Possibilities include either glacial or fluvial erosion during a low stand of sea level.
- Is the Rockfish Conglomerate associated with widespread glacial episodes that occurred during the Neoproterozoic? The Neoproterozoic was a time of tremendous climate oscillation, some researchers argue for a Snowball Earth in which glacial ice covered much of the planet.
- How much strain/deformation has the Rockfish Conglomerate experienced? Outcrop-scale deformation structures are common. To better understand original sedimentary geology we need to quantify the amount of shortening, stretching, and rotation these rocks enjoyed.
- When was the Rockfish Conglomerate deformed, metamorphosed, and tilted? Regional data suggest these events occurred during the Paleozoic Era, but as with the Neoproterozoic: the Paleozoic encompasses an expansive amount of Earth history.
That is the nature of research – some questions get answered and other new ones arise. The good news is that Gussie Maguire, a Geology/English double major and mystery tweeter, has taken up the charge. Her thesis research is aimed at answering some of the outstanding questions regarding the Rockfish Conglomerate. On we go.
August 14, 2013 by Chuck Bailey
In April I delivered a talk on “Finding Faults in Old Virginia” as part of William & Mary’s Tack Faculty Lecture Series. Our study of Virginia’s faults is ongoing and one current project is focused on the boundary between the Blue Ridge and Piedmont regions.
Virginia’s Blue Ridge Mountains are underlain by a sequence of antiquated rocks (some older than a billion years) that have been ensconced in North America for a considerable time. During the tectonic collision that created the Appalachians (~250 to 300 mya) bits and pieces of other continental blocks were accreted onto North America—today these exotic terranes are exposed in the Piedmont.
The boundary between the Blue Ridge and Piedmont geologic provinces is a fault: in northern Virginia, it’s the Mountain Run fault and in southern Virginia, it’s the Bowens Creek fault. In central Virginia, the structure that separates the Blue Ridge from the Piedmont is less clear.
Some geologists posit the fault at the Blue Ridge/Piedmont boundary is a fundamental crustal boundary in the Appalachians, whereas others view the structure as minor and insignificant. Geologists also disagree about the kinematics (movement history) along this fault: some argue for reverse or thrust faulting, in which Piedmont rocks were shoved up and over Blue Ridge rocks, while others suggest faulting involved a wrenching movement – in which Piedmont rocks slid laterally past Blue Ridge rocks.
Enter Parker Campbell, a William & Mary Geology major, whose senior research is focused on understanding the Blue Ridge/Piedmont boundary in central Virginia. Off to the field we went. But in the summertime, central Virginia is cloaked in thick foliage concealing the bedrock. Rather than don full field gear to cope with the riotous vegetation, we took to the waters of the James River for a float trip to the outcrops. The James River does an effective job of eroding its bottom and margins such that bedrock outcrops are plentiful.
Our flotilla of three canoes launched onto the Tye River on a pleasant morning. The float would take us 32 kilometers (20 miles) down the river to the little village of Howardsville on the James. We chose this stretch of river because, according to the 1993 Geologic Map of Virginia, we’d cross the trace of the purported Bowens Creek fault four times.
Two kilometers (1.5 miles) into the float the Tye joins the James River and here we crossed to the cliffs on the southeastern bank. An exceptional set of outcrops rise steeply from the water and expose strongly deformed rocks—this is what we’d come for.
This sequence of marble and schist is replete with asymmetric folds, boudinage, mineral veins, and mullions—all of which provide clues as to the rocks’ deformation history.
We examined many outcrops on the two-day trip and collected a cornucopia of samples at riverside outcrops including marble, schist, quartzite, metagraywacke, quartzose phyllite and greenstone. We camped on a low sand bank in mid-channel and passed a warm evening with the waters of the James sluicing close by. Our stomachs enjoyed an eclectic summer menu of fresh and pickled produce.
Good times, all in the name of research!
Back in Williamsburg, Parker’s next tasks are to prepare a suite of petrographic thin sections for microscopic examination and sift through the structural observations to work out the rocks’ deformation history. All William & Mary geology majors complete an independent senior research project—their efforts provide new and valuable data that help to answer an array of outstanding to longstanding geological questions and conundrums. I’m excited about what we observed in the field and am looking forward to collaborating with Parker as we work to understand the geologic history of this enigmatic fault zone.
July 29, 2013 by Chuck Bailey
As I noted in my last post our summer geologic field research took us to the Beehive State. Our work is primarily focused on Wayne County in the south-central part of Utah. Created in 1892, Wayne County forms an expansive rectangular block of nearly 2,500 square miles. The county is sparsely populated with about 2,700 human inhabitants: that’s an average population density of just over one person per square mile (plenty of elbowroom). The Fremont River neatly traverses the county from the high plateaus in the west, through spectacular red rock terrain, and into the low desert in the east.
By any measure Wayne County is a scenic place, and in an effort to publicize the rugged beauty of the county a local civic group coined the term Wayne Wonderland in the 1920s. Eventually, that wonderland became Capitol Reef National Monument (now a National Park). Today, nearly ¾ of a million people visit the park per year.
Typically, my research students working in a particular region create a research team name. Some past groups include the Grenvillian UberJocks (1999), the Browns Cove BearBait (2006), and the Alberene Dream Team (2011). This year’s research team has embraced the sobriquet – the Wayne WonderMonkeys.
Indeed, we are studying the geology of Wayne County and the county’s landscape is surely a wonderland. But, why the monkey moniker? In the modern era, monkeys are not part of the Wayne County fauna. But as my 11-year old stepdaughter adroitly points out, monkeys are both smart and energetic creatures—an apt description for a group of W&M undergraduates wandering about Wayne County, Utah.
Once again our research is supported by the U.S. Geological Survey’s EDMAP program and we are producing a geologic map of the Lyman 7.5’ quadrangle in cooperation with the Utah Geological Survey. The Lyman 7.5’ quadrangle lies in the southeastern part of the Fish Lake Plateau at the boundary with the Colorado Plateau and Thousand Lake Mountain (elev. 3400 m) dominates the landscape to the east. We focused on the Lyman quadrangle because it’s the place to go to:
- Understand the kinematics and timing of the Thousand Lake Fault System, a major fault that forms the boundary of the Colorado Plateau. Geomorphic evidence suggests that this fault system experienced Quaternary slip, but the magnitude and timing of that movement is poorly understood. Hanna Bartram is evaluating the history and potential seismic hazard associated with this fault.
- Understand the origin of the Rabbit Valley salient, a distinctive topographic feature that extends westward from the flank of Thousand Lake Mountain. The topography is distinctly hummocky and salient could be an old and very large mass movement deposit. Zachariah Fleming’s research is focused on the origin of this enigmatic terrain.
For a more animated view of the Wayne WonderMonkeys’ research and field experiences witness this video they posted.
July 15, 2013 by Chuck Bailey
I’ve just returned to Williamsburg after a month of field research in Utah at Fish Lake and the High Plateaus. I journeyed to Utah with a team of four W&M undergraduates, nicknamed the Wayne WonderMonkeys (more on their name later). June brought copious rain to Williamsburg (more than 25 cm (10”) fell on campus), all the while we enjoyed a long run of sunny and dry weather in south-central Utah.
Fish Lake is the largest alpine lake in Utah. It covers an area of ~10 square kilometers (~4 square miles) at an elevation of 2,688 m (8,843’). This lake formed in a graben, a structural trough created between two normal faults. Grabens develop in regions where the Earth’s crust is extended and stretched. Our past research has documented the geological structure of the Fish Lake region and we’ve mapped the bathymetry and sediment distribution of the lake. Fish Lake’s bottom is covered by copious amounts of smelly organic-rich mud, a bit of sand, and even boulder deposits in places. But how much sediment lies at the bottom of Fish Lake: a few meters (~10’) or a few hundred meters (~1000’)?
Erika Wenrich’s ’14 senior thesis project focuses on determining just how much sediment lies at the bottom of Fish Lake. One way to determine the sediment thickness at the bottom of the lake would be to drill a core out in the middle of the lake. Drilling a full core from a lake’s bottom is an expensive proposition (say, a million dollars or so). A far cheaper method uses gravity to estimate the sediment thickness. Gravity, or more precisely the acceleration due to gravity, is not the same at all locations on Earth.
The acceleration due to the Earth’s gravity (denoted as g, spoken in casual conversation as ‘little g’) varies according to one’s latitude (greater at the poles, less at the equator), elevation (greater at sea level, less at altitude), the surrounding terrain, and due to the density variations in materials below the Earth’s surface (the denser the material, the greater the acceleration due to the Earth’s gravity). We use an instrument, intuitively named a gravimeter, to measure g at a point on the Earth’s surface.
We measured gravity at 55 locations in the Fish Lake basin. At each location we calculated what the theoretical gravity should be at that point (by taking into account the latitude, elevation, and terrain). The difference between the observed or measured gravity and the theoretical gravity at a point is the gravity anomaly. These anomalies provide insight about the subsurface materials and their geometry.
Erika’s research plan involves 1) constructing a gravity anomaly map of the Fish Lake basin, and 2) making a 3D model of those gravity anomalies to estimate the thickness of sediment in the basin. Our task this summer was to lay down a grid of gravity observations around Fish Lake. This winter, when Fish Lake is frozen over, we’ll go back and measure the gravity at sites on the lake itself.
A common question we are asked is “Why do you want to know the sediment thickness in Fish Lake?” or put another way “Who cares how much sediment is in the lake?”. We are interested in the sediment at Fish Lake because it contains a record of tectonic activity as well as climate and environmental change over at least the last two ice ages (past 150,000 years), and perhaps much further back (~1 or 2 million years). Knowing the amount and geometry of the sediment package is a key step towards procuring funding to actually drill a lake core and learn the secrets buried beneath Fish Lake.
May 21, 2013 by Chuck Bailey
Remember the Alberene Dream Team from the summer of 2011? This talented group of undergraduates poured themselves into research projects aimed at understanding the geology of the eastern Blue Ridge Mountains that summer and continued their work as part of their senior research during the academic year. Alex Johnson, the youngest member of the Alberene Dream Team, graduated from the College last Sunday (Andrea, Kevin, and Molly were in the class of 2012). The week before landing his diploma Alex helped lead a raucous field review across the Alberene quadrangle.
The U.S. Geological Survey funded our research and as such we have a responsibility to get this geologic map and the attendant data published so the results are accessible to the wider world. A necessary step towards publishing a geologic map is the field review. We invited geologists from the U.S. Geological Survey, the Virginia Division of Geology and Mineral Resources, academics, and other interested individuals to join us for a day in the field and asked for their critical comments on the map and our geologic interpretations.
We visited 10 outcrops: from roadside exposures of the basement complex, to an old soapstone quarry, to a magnificent outcrop of metabasalt with deformed pillows along the Hardware River. Alex Johnson and John Hollis framed most of the discussion with Professor Brent Owens and I chiming in on occasion. We argued our case and learned from other geologists.
This summer we’ll do a bit of targeted fieldwork to better resolve some problem spots on the map and make revisions based on the comments we received during the field review. Once that is complete we’ll submit it for yet another review. It is a long road to publication, but an important road to travel nonetheless.
April 12, 2013 by Chuck Bailey
Last week the William & Mary Geology department played host to a group of international geoscientists that descended upon Williamsburg from Japan and Oman. They were at William & Mary to attend the 3rd Critchfield Conference which focused on the Indian Ocean Basin: Navigating the 21st Century Marine Silk Road. Prior to their conference duties, we had the good fortune to rope them into delivering seminars in the Geology department and meeting with geology students.
Professor Toshio Mizuta, the former director of the International Center for Research and Education on Mineral and Energy Resources (ICREMER) at Akita University, Japan discussed his research on Kuroko-type massive sulfide deposits. Professor Takashi Uchida, Professor of Earth Science and Technology at Akita University, presented an overview talk on non-conventional energy resources such as gas hydrates. Collectively, their talks highlighted some new frontiers of mineral and energy exploration. As a mineral resource-limited island nation, Japan has focused much effort on seafloor mapping in a quest for discovering new resources.
Professor Abdullah Al-Ghafri of the University of Niwza, Oman delivered a seminar to a packed house that focused on his research on Aflaj, an ancient water management system used in arid regions through the world. Later this year I will be starting a geologic research project in Oman and Dr. Al-Ghafri will play a key role in helping me build connections with other Omani scientists. There were also representatives from the Oman embassy in Washington, D.C. and the Sultan Qaboos Cultural Center in attendance.
Geology is a science in which both time and place are important, and as such the Geology Department is well positioned to forge ahead into the realm of international education and research. In the not-so-distant future, we aim to run a geology and environmental field study program in Oman. A joint field trip with Japanese faculty and students to Alaska to explore base-metal deposits is also a possibility. Exciting times ahead.
November 26, 2012 by Chuck Bailey
Here is the opening question from the last problem set in my Earth’s Environmental Systems course (GEOL 110).
I thought my clues were amply generous. The photograph is of Palace Square in St. Petersburg, Russia. Google Earth is a great tool for checking out this scene and determining the latitude and longitude (59.9˚ N, 30.3˚ E). Inspecting the scene in Google Earth makes it clear that the photographer was looking to the South across Palace Square from a window in the Hermitage Museum.
That was the prelude. The 2nd part of the question asks.
Mike Blum, W&M Academic Technology Specialist Extraordinaire, took the photograph. His wife, Professor of Modern Languages Bella Ginzbursky-Blum, often directs William & Mary’s summer study program in St. Petersburg. I first saw the photo framed on the wall at their house and was captivated by the scene. When I asked to use the photo I told them NOT to tell me the date and time at which the picture was taken—if I’m going to ask W&M students to figure that out I’d better be able to do the same.
So just how do we figure out the date and time when the photograph was taken? The key lies in the shadow cast by the massive Alexander’s Column at the center of the square. Alexander’s Column is a large monument erected in the early 1830’s to celebrate Russia’s victory over Napoleon’s armies. It stands 47.5 meters tall and is reputably the world’s tallest column made from a monolithic block of rock (granite, in this case).
By making a triangle between the column and the shadow we can determine the solar elevation angle (Ψ) by either measuring it directly on the photo with a protractor (my students say that’s old school because they last used protractors in elementary school) or by using a dash of trigonometry. The solar elevation angle at the moment the photo was taken is 47˚.
The solar declination (δs) follows an annual path that crosses the equator at the equinoxes and reaches 23.45˚ N (Tropic of Cancer) on June 20th/21st and 23.45˚ S (Tropic of Capricorn) on December 21st/22nd. This path is graphically illustrated by the analemma, that strange figure-8 shape lurking in the Pacific Ocean on many globes.
Recall that St. Petersburg is located at 59.9˚ N latitude and we measured a solar elevation angle (Ψ) of 47˚ at Alexander’s Column. For the sun to reach a solar elevation angle of 47˚ in St. Petersburg the solar declination must be at 17˚ N or further north (Ψ = 90˚ – [Latitude of St. Petersburg – δs]), reading the analemma reveals that the solar declination is ≥17˚ N from May 9th until August 4th. At any other time of the year the sun never gets high enough in the sky to reach a solar elevation angle of 47˚ in St. Petersburg. Ok, we’ve narrowed the range of dates to just under three months, but we can do better.
The azimuth of the column’s shadow is pointing to the northeast (an azimuth of 41˚, actually), which means the sun is located to the southwest (an azimuth of 221˚ which is 180˚ from the shadow). Local noon occurs when the sun passes due south of a particular location (for mid-latitude locations in the northern hemisphere) and the sun tracks to the southwest in the afternoon—clearly this photo was taken in the mid-afternoon. The sun reaches its zenith at local noon and then the solar elevation angle gets progressively lower throughout the afternoon. Put another way, if the solar elevation angle in the afternoon is 47˚, it’d be even higher at local noon.
There is an explicit relationship between the solar elevation angle (Ψ) and the azimuth angle of the sun (α) that can be determined for any latitude and time. It’s given by two cumbersome trigonometric formulas (not shown), but if one’s latitude and the solar declination (date) are known we can calculate when the sun rises and sets and its position throughout the day.
For our picture of Palace Square, we know the solar elevation angle (Ψ) at that moment = 47˚ and the azimuth angle of the sun (α) = 221˚. There are only two days of the year that yield that combination of elevation angle and azimuth—June 3rd or July 11th at 14:54 (2:54 p.m.)*. I suspect it’s the earlier date—June 3rd. Look closely at the people in square: they are bundled up, suggesting it is a cool day in St. Petersburg, I’d expect cool weather in early June more so than July.
I forwarded my answer to Mike and Bella: Mike responded “Not bad. June 3, 2004 at 2:49 p.m.”. I was off by 5 minutes, but this is where the equation of time and the analemma come to the rescue.
The north-south component of the analemma illustrates the solar declination throughout the year, but the east-west component illustrates the equation of time and whether the sun is ahead or behind clock time. Because the Earth’s orbital path around the sun is elliptical, and the Earth’s rotational axis is tilted relative to the ecliptic plane, there is a difference between mean solar time and apparent solar time that varies regularly throughout the year. On June 3rd the sun is 3 minutes ahead of clock time—so my final answer is 14:51.
Did I only give credit for June 3rd at 14:51 local time? No, full credit answers went to anybody that put the date between June 1st and July 15th and called it mid-afternoon. Collectively, the class did fine. As I’ve mentioned in earlier posts, I want students to be able to think spatially: questions like this are meant to get spatial thinking skills into high gear.
Why pose a question like this? Who, especially in a first-level non-majors course, is likely to employ this type of analysis (photogrammetry) somewhere down the road? Very few I expect, although photogrammetric analysis is regularly employed by intelligence agencies (how do you think the Russian missile sites in Cuba were sussed out back in 1962?) and has been utilized to offer insight on controversies such as who first reached the North Pole. But more importantly, I want William & Mary students to use their own observations to understand the world. These types of questions might appear to come from ‘left-field’, but neatly demonstrate how much can be discovered about the Earth’s environmental systems from close inspection of everyday scenes.
*St. Petersburg is on Moscow time.
October 26, 2012 by Chuck Bailey
Water gaps are intriguing and iconic landforms that have long drawn humans to them. We are all familiar with streams and rivers flowing in valleys; a water gap is dramatically different- it’s a place where a river cuts though a ridge or mountain range. Thomas Jefferson discusses the Potomac River water gap in his Notes on the State of Virginia (1785), declaring in an often-quoted passage: “This scene is worth a voyage across the Atlantic.”
For me, the prose that comes earlier in the same paragraph is even more vivid.
The passage of the Patowmac through the Blue ridge is perhaps one of the most stupendous scenes in nature. You stand on a very high point of land. On your right comes up the Shenandoah, having ranged along the foot of the mountain an hundred miles to seek a vent. On your left approaches the Patowmac, in quest of a passage also. In the moment of their junction they rush together against the mountain, rend it asunder, and pass off to the sea.
The Appalachian Mountains are flush with water gaps (e.g. Delaware water gap, Cumberland gap), but water gaps are common features in many mountain ranges worldwide. Water gaps are important as they typically form a route of conveyance through steep and mountainous country and they’ve long been utilized as routes for wagon trails, railroads, and highways. The Potomac River water gap through the Blue Ridge Mountains has been a pivotal place in American history since Colonial times.
Southwest from Harpers Ferry the Blue Ridge Mountains form an unbroken topographic barrier for 240 km (150 miles). The next water gap is near Lexington, Virginia, where the James River has carved a 10-km (6 mile) gorge through the Blue Ridge, a range with peaks over 1,200 meters (4,000 feet) in elevation. This is an impressive gap- see for yourself by playing this Google Earth tour through the James River water gap (kmz).
How can a stream cut a path across a mountain ridge or range that lies in its course?
At the Potomac River water gap Jefferson opined:
…this scene hurries our senses into the opinion, that this earth has been created in time, that the mountains were formed first, that the rivers began to flow afterwards, that in this place particularly they have been dammed up by the Blue ridge of mountains, and have formed an ocean which filled the whole valley; that continuing to rise they have at length broken over at this spot, and have torn the mountain down from its summit to its base.
TJ is certainly entitled to his opinion, but he’s not the only one to wax poetic on this topic.
For another take on the landscape consider John Denver’s famous lyrics in the song Country Roads:
Almost heaven, West Virginia
Blue Ridge Mountains, Shenandoah River
Life is old there, older than the trees
Younger than the mountains, blowing like a breeze
So according to John Denver the trees are younger than the life, and both the trees and the life are younger than the mountains (i.e. old mountains).
But what about the Shenandoah River? Is the Shenandoah River older or younger than the Blue Ridge Mountains? In Jefferson’s landscape model, the mountains formed first, creating a topographic barrier that was later breached by the river carving out a water gap. But there is another possibility: what if the rivers were there first and the mountains formed later? In this model, the rivers are older and maintain their courses while the mountains are uplifted. These rivers would be antecedent streams that pre-date the current topography through which they flow.
What do you think? Are the Blue Ridge Mountains older than the rivers (Potomac/Shenandoah and James systems) that flow through these impressive water gaps? Or do these rivers pre-date the formation of the Blue Ridge and Appalachian Mountains?
Let me know and we’ll return to this question in a follow up post.