Last weekend the Geology Department set forth on our Fall departmental trip for a geological field trip down the James River in canoes. The late summer weather was glorious, and the James River’s flow was just right– not too high and not too low.
On Friday evening, our crew of 22 camped along the James River floodplain at Hatton Ferry. While marshmallows toasted on the campfire, we discussed the origin of the floodplain upon which our tents were pitched. With the James River’s waters well within the stream banks, it was a fine evening for camping on the floodplain.
Aerial image of our campsite located on the James River floodplain at Hatton Ferry, Virginia. (photo by Jim Kaste)
Early Saturday, we piled into the outfitter’s magic school bus and were shuttled upstream to the put-in at Howardsville. By 9 a.m., we were underway and, soon thereafter, heading at full steam down the James River. As I’ve written in previous posts, the canoe is the perfect craft for navigating flowing waters such as the James. Our crew consisted of ambitious paddlers and we quickly arrived at our first outcrop in mid-river.
At an outcrop called the Alligator, as its long and low profile resembles a ~100-meter partially submerged alligator, we discussed the depositional environment of this clastic sedimentary rock deposited ~220 million years ago in a newly formed rift basin.
Arrival at the Alligator, an exposure of conglomerate/breccia, a sedimentary rock cropping out in the Scottsville Basin, an ancient rift basin that was a forerunner to the Atlantic Ocean during the Triassic.
At other locations, we estimated the James River’s flow rate (discharge), measured the river waters’ conductivity and sediment load (both quite low), and mused on the residence time of pebbles on the bottom of the channel.
From a paddling perspective, the highlights of the trip are three rapids which throw up some modest whitewater and present challenges for proceeding downstream completely dry. On this trip, we had 11 canoes, and our collective scorecard for successfully navigating the three rapids was 32/33 — not bad. Unfortunately, the one calamity involved an overturned canoe and the cooler with lunch supplies going in the river. Lunch was recovered, but the humus-spreading knives went to the bottom of the James.
A fluted outcrop of diabase with potholes along the James River in central Virginia. Fluvial erosion/abrasion of the bedrock created these features.
The three sets of rapids are all developed where the James River crosses diabase dikes. Diabase intruded into the older sedimentary rocks of the Scottville Basin, with these dikes likely serving as conduits for deep-seated magma to be erupted to the surface. Diabase is a hard and durable rock. Smack a diabase sample with a rock hammer and it will produce a distinctive ring and sparks, but even a strong blow does not always break the rock. Along this stretch of the James, fluted/scalloped surfaces and even potholes are common on the diabase outcrops–these are distinctive erosional features that develop during floods as water-washed sediment abrades and sculpts tough bedrock.
A geologic map of the Goosby Islands area along the James River in central Virginia. The colored dots are outcrops. Two north to northwest-trending diabase dikes (Jd) cross cut the older Triassic sedimentary rocks (Trs). Class I-II rapids are developed on the diabase outcrops.
From a geomorphologist’s perspective, the rapids are knickpoints. Effectively, stretches of the channel where the stream gradient is noticeably steeper than both up and down stream. What causes knickpoints? It’s not unusual for knickpoints to develop on harder rocks, because, to erode harder rocks, a stream’s gradient must increase, thereby creating faster flows and more power.
After lunch as we finished our discussion of knickpoints, I cheekily asked whether these are stationary or migratory knickpoints. In the post-lunch torpor, nobody much wanted to answer my question and some students took to riding partially submerged logs as a means of escape. Learning has its limits :).
But from the comfort of my office, I’ll ask the question again: are the rapids along this stretch of the James River stationary or migratory knickpoints? Why not post a comment and enrich the debate!
Conceptual models of stationary and migratory knickpoints over time.
From my perspective, the departmental trip was a great success. These trips build community by growing camaraderie amongst both students and faculty. William & Mary is an institution that values experiential learning and our going with the flow down the James River is a perfect example of how learning and fun can be coupled together.
Curious to see more from our adventure? If so, then checkout these photos and another video from the trip.
Toasting marshmallows by the campfire.
On the bus and en-route to the put in.
Pondering an outcrop of metamorphic rocks from canoes.