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).

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.