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Photo: Landslide on Riverside Ave 2019 By the UVM Spatial Analysis Lab
The purpose of this project is to gain insight to the structure, composition, and processes that formed the soils behind Delehanty Hall at UVM in Burlington, Vt. This location sits at the top of the drainage basin that flows down to Riverside Ave, at the location of the 1955 landslide.
Figure 1: Soil Pit Location by GPS. Topographic Map from USGS shows the locality of the drainage basin to Riverside Ave. Inset from GoogleEarthPro shows site relative to Colchester Ave and Riverside Ave.
Figure 2: Cartoon illustrating the soil profile.
Our soil profile was dug off the trail that runs parallel to the drainage basin behind Delehanty Hall, UVM Campus. Burlington Vt. USA (See Fig 1), in a local low slope zone between two trees and a fallen log. The O horizon in the forested area is well developed into the autumn season, the layer is 8 cm thick and consists of decomposing leaf litter, pine needles, twigs and fallen branches. The A horizon is 16 cm thick, and is composed of dark brown-to-black topsoil rich in decomposed nutrients mixed with coarse, visible quartz sand grains. Small surface level plants have roots growing throughout this layer. Near the 26 cm depth is a wide gradient transition from horizon A to B, marked by a contrasting shift from dark brown/black (enriched in organics) to light brown and orange (depleted in organics) with localities of red around primary tree root systems. This B horizon is roughly 43 cm thick, contains the majority of the roots of the surrounding plants and ground cover to a depth of 30 cm, while smaller deep tree roots persist to nearly 50 cm. Incorporations of ~1 cm diameter pebbles from the parent material are scattered throughout, though this layer is dominated by silt, rather than sand sized grains. At 69 cm depth the C horizon appears rather abruptly, though this can be hard to observe as this layer became too tough to excavate after 80 cm. The C horizon is light grey, dominated by moist clays and unsorted pebbles and cobbles 1-4 cm in diameter. Likely glacial till.
Figure 3: West (Left) and North (Right) soil pit profiles with delineated horizons, blurry white bands show gradient transitional areas. The O Horizon is composed of pine needles, decomposing leaf litter, twigs and fallen branches. The A horizon is dark brown to black and well developed, with coarse sand grains well mixed with organic material. The B horizon is light brown with patches of red near root systems showing depletion of nutrients, rather than sandy, this layer is composed primarily of smoother silt and clasts of pebbles ~1 cm in diameter. A dense root system pervades the A and B horizons, with the largest roots intersecting the profile at 30cm, smaller roots sparsely continue to 50 cm depth. The C horizon appears at 69 cm, as it marked by a clear transition to light grey (left), this layer is dominated by poorly mixed moist clay and pebbles and cobbles 1-4 cm in diameter. The C horizon is extremely dense and halted our dig - likely glacial till. Photos by Mark Hehlen. Saturation added in post image to more clearly define transitions.
This appears to be a fairly normal alfisol (soil from younger, temperate, primarily deciduous plant cover regions) profile for a forested area post-glacial cover. The glacial till composing the parent material was likely sourced itself from the surrounding geologic strata, such as the Munkton quartzite (due to the presence of coarse sand), Dunham dolomite (due to the characteristic grey in the C horizon indicative of calcite/dolomite), or older formations farther north on the the craton in Canada. ~80 cm depth between the surface and parent material correlate vaguely with 8 kyr of soil development at a rate of 0.1 mm yr-1. This is slightly slower than expected due to the Laurentide Ice Sheet's retreat past the area 12 kyr ago. This is likely due to the nearby slope as a sink of eroded and weathered parent material during early pedogenesis (soil formation), and potentially past slope failures.
As quartz and clays were weathered from the glacial till, and organic matter was added to the surface soil formation began. Over the last 8-10 kyr decaying and leached organic matter was distributed down through the soil, while chemical weathering and redox allowed parent material to percolate up.
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