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).