Terraces and Deltas

Just at the end of the week as we finished our collecting season, we logged our 2000th road mile within our study area of the Kennebec River corridor. We’ve now seen more backroads and small highways in this area that most people do in decades. We’ve collected just over 100 samples, mostly from gravel pits and river banks. My expectation based on doing field work in Maine and New Hampshire in the ‘80’s was that we would encounter moderate reluctance from land and pit owners due to fear of liability and a strong sense of private property ownership. Instead, I’ve been really impressed by how open and willing almost all pit operators have been in allowing us access. Whether this is due to my shorter and thinning head of hair, an improved legal climate or some other social factor, I can’t be sure, but it certainly has been welcome.

The effects of processes operating on a truly large scale were evident this past week. As we moved our sample-collection efforts southward and closer to the coast, the Quaternary stream terraces that were so prominent in the Farmington area (see Jeff's entry of Big Terraces) have diminished greatly. In fact, we’ve had difficulty finding good exposures of stream terraces closer to the coast because they have become so thin (locally just 1-4 meters thick) and are often covered by farmer’s fields. This southward thinning of the stream terraces is not due to greater erosion or even to less time for accumulation in the south. Instead, what we’ve seen is the effect of the greater weight of the thicker glacial ice in the northern areas. The greater weight depressed the land further in the north than in the south. The end of the Ice Age in Maine and the melting of the glaciers occurred faster than the land buoyed up (termed isostatic rebound). This same process continues today in parts of the Arctic and Scandinavia, where the land is still slowly but obviously rising more than 10,000 years after the ice melted away. Because the land was depressed more in the north, there was more accommodation space in which to accumulate sediment (essentially, a deeper hole to fill). Now that the land has effectively fully rebounded, we find thick, eroded stream terraces in the north, and thin, vegetated terraces in the south.

Glacial-era marine deltas contrast in several ways from the slightly younger stream terraces. These deltas formed after the glaciers had partially melted away, exposing lots of unconsolidated sand, cobbles, silt, and clay generated by the grinding action of the glaciers. Water from the melting glaciers carried the sediment to the ocean, where it accumulated in deltas. As the marine shoreline rapidly regressed southward due to isostatic rebound, new deltas formed at progressively more southern locations. These are the source of many of the numerous sand and gravel pits we’ve sampled (see photo).

Patterns

In gravel pit after gravel pit, we’re seeing a similar pattern in sediments and depositional environments. Dr. Erikson has pointed out that almost every good exposure over about 10 m tall has a base of sand and cobble beds that gradually become finer grained upward in the section. This is typically overlain by more than a meter of bluish-grey marine clay, and then by the rapid return of sandy beds. As he explained, this is evidence of a history of dropping energy levels as sea level rose (yielding finer grain sizes), until only the finest particles (clay) accumulated in an offshore location, and then finally increasing energy levels as sea level fell (producing the return of sand above the clay). Occasionally, we’ve found large boulders dropped in the middle of the clay or sandy layers (as seen in the picture); the boulders are far too large to have been transported by ocean currents, so we can tell that glaciers were floating around in the ocean above the gravel pits dropping boulders and other debris as they melted.

We’ve made a number of observations this week and a half that have nothing to do with science. Our live animal sightings have extended to include another moose, a mouse, deer, groundhog, porcupine, blue herons, three fox kits, and dozens of alewives and herring in a fish lift. Other interesting sightings include two large windmill blades (allegedly 150-ft) pulled by semis. There has been a dramatic change in lifestyle in this upper region of Maine. We have found that people have been universally friendly and they have allowed us unrestrained access to their land, the traffic among the towns is nearly nonexistent, and everywhere we have eaten the portions have been plentiful. Our campground for the week located in Madison was much bigger than the last, but just as quiet. Our tent site includes an Adirondack structure which is a 3-walled shelter, which came in handy this week as we saw a drastic change from sunny and high temps, to rainy and low temps.

Big Terraces

Being new to this part of Maine, today held several new encounters. Northeast of Rangeley, we found a massive glacial till quarry with walls over 30 meters tall. To see how much stuff the glaciers can pick up, move around, and deposit is amazing. “Glacial” was the feeling of the Dead River, where I decided not to jump in but let Dr. Erikson get a sample of bedrock poking through the surface. Appropriate for the area, we saw lots of moose sign, plus a good-sized moose in a swampy area. Before lunch we stopped along the Carrabassett River near Sugarloaf Mountain. This spot was very nice with lots of bedrock exposed above water which were eroded smoothly and contained potholes and crevices allowing the river the flow swiftly through unique paths. We also made a couple passes across a wire bridge in New Portland, which was built in 1846 with oxen! And it still supports the weight of vehicles.

The most exciting part of the day came late in the day. From the road we could spot a giant, 25-meter tall, eroded Quaternary stream terrace. We were pretty happy to see this, the only downside was its location … across the Carrabassett River. The river actually wasn’t as cold as we thought it was going to be. I thought it was refreshingly nice, especially on the cuts and bug bites on my legs. When we finally reached our destination, we had to climb way up the steep wall of cohesionless layers of sand and cobbles (see photo). The layering reflects changes in river-flow intensity, with the cobbles being higher flow and the sand being lower. Since we had to climb from the river bed to the top of the terrace, it’s amazing to think that the entire valley was once at that height, but now almost all of this once extensive terrace has been eroded and washed downstream.

Load Casts and Flames

The sedimentologist in me got excited, yesterday, when we found load casts and flame structures in a silt and sand outcrop far north of Farmington. Typically, these sedimentary structures are associated with rapid loading of more dense sand on soupy, less dense silty mud, which leads to balls of sand sinking into the silt and silt squirting up into the sand. This is the sort of thing we look for in order to interpret the conditions of deposition.

Today, we left the confines of surrounding hills and headed out onto the broad floodplain created by the ancestral Sandy River. This area is rich with farming activity now, but its low relief makes it a challenge to find outcrops. We do find two good outcrops along the river’s edge. The first was also the location of our biggest startled moment of the day. While walking through tall grass, I sensed something moving at my feet and peered into the grass. Just as Jeff became aware that I was cautiously peering into the grass, a turkey hen burst out of the grass within a few feet of us. Turkey chicks were scrambling every which way . . . we turned around and found another route.

First-day Clay

First field day! After packing up on campus, we drove to Farmington and found this week’s base camp at the Troll Valley Campground. Intrepid scientists that we are, we were not intimidated by the prospect of the trolls being offended by our modern, albeit rustic, pursuits in their valley.

The major discovery of the day was on our first foray, northward from Farmington along the Sandy River. In a black fly-infested tributary valley, we found an outcrop with the most impressively plastic grey clay. Saint Joseph's student Jeff Boudreau holds up a sample of this Quaternary marine clay that a potter would covet. The presence of the clay overlain, first by silt then by sand and pebbles, indicates environmental conditions that changed from tranquil, relatively deep water (clay) to an increasingly energetic nearshore setting (sand and pebbles). This sequence is consistent with sediments accumulating as relative sea level was falling and the shoreline was moving southward through the area (termed a regressive sequence).

Field Area

The research project is based here in Maine. The region of interest broadly covers the land between the White Mountains, Katahdin, Bangor, and Portland. I expect that we'll be spending a great deal of time in the greater Farmington area where we’ll be looking at dozens of locations with early post-Ice Age sediments. One of our first logistical tasks? Securing a campsite as our base camp.

Research Objectives

"To see the world in a grain of sand..." William Blake, Auguries of Innocence

When William Blake penned this line, he wasn't thinking about geology, but his phrase reveals a great deal about this project. Sand is typically comprised of 2-3 common minerals and another ~20 less common to scarce minerals (plus hundreds of rare minerals). The identity, proportions and total amounts of the less common minerals are widely used in academic, governmental and industrial geologic investigations to interpret climate, rates of erosion, general physiographic setting, rates of sediment accumulation, and basin-water chemistry and to correlate rock bodies over long distances.

The relative abundance of each of the ~20 less common minerals in any particular sand layer is affected by a variety of factors, the most important of which are:
1) the mineral composition of the source area supplying the sand grains;
2) chemical alteration of the minerals in the source area prior to eventual erosion and transportation to their depositional site;
3) chemical alteration of the minerals in the depositional area prior to consolidation of the loose sand grains into solid sandstone; and
4) sorting based on size, shape, and density of the different minerals during transport from source area to depositional site due to differences in the flow characteristics of streams, rivers, ponds, etc.

This research project addresses the fourth of these factors: how different environments result in measurably different mineral assemblages, even when supplied by the same source area. The project involves analyzing and correlating geologic environment (for example, low-energy stream, below wavebase marine shoreline, high-energy river, etc.) with mineral composition. Consequently, there are two major investigative components to the study:
1) interpretation of the environment of deposition, and
2) determination of the identity, proportions, and total amounts of the less common (but diagnostic) minerals.

Grant Award

Welcome to this new Mineral Sorting blog, a blog designed to document a faculty-student research project based at Saint Joseph's College!

Recently, I received word that my two-year $50,000 grant proposal to the American Chemical Society was accepted. This funding will allow me, and student researchers from the Department of Natural Sciences at Saint Joseph's College, to examine geologic processes affecting the mineral composition of early post-Ice Age sediments in Maine.

The research has implications for interpretation of long-term climate changes and their effects on the landscape. This is basic research on how the rock record preserves a history of events on the Earth’s surface.

The grant supports three weeks of field work during two summers to collect sediments, which will be brought back to the campus lab for mineral identification. Our research team also will figure out the size distribution of the collected sediments in order to shed light on how natural geological sorting processes affect the distribution of minerals in the rock.

I hope that you will consider following our progress.