Stratigraphic Reconstruction of Holocene Paleogeography and Paleoclimate, Little Falls, MD
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In the Appalachian piedmont region of the eastern United States, the proliferation of milldams and their ponds greatly altered the landscape during early American settlement. Widespread upland deforestation coupled with other land clearing processes increased sedimentation behind the milldams. This sediment, referred to as legacy sediment, buried the original valley bottom topography, soils, plants, and deposits. The focus of this research is to reconstruct the Holocene landscape that existed in part of the Little Falls watershed, northern Maryland, before it was buried by millpond sediment. The stratigraphy, sedimentology, and spatial relations of sedimentary units exposed in stream banks along Little Falls, now a deeply incised channel due to 20th-century milldam breaching, are the basis for this reconstruction. Radiocarbon dates from organic material in the sediment provide age control, while magnetic susceptibility indicates whether or not sediments were disturbed by anthropogenic activities. Historic and recent air photos, combined with light detecting and ranging (LIDAR) data, reveal the incision and subsequent stream bank erosion that have occurred since 1952. Five distinct sedimentary units overlying bedrock are exposed along Little Falls. From bottom (oldest) to top (youngest), these are: 1) late Pleistocene (?) cobble- to boulder-sized rubble of angular to sub-angular schist with some quartz; 2) a thin (0.2 to 0.6-m) layer of late Pleistocene sandy cobble gravel that consists predominantly of oxidized, sub-angular to sub- rounded quartz; 3) a Holocene organic-rich loam that contains wetland seeds (primarily sedges) and varies in thickness from 0 m near valley margins to 1 m in the lowest parts of the valley bottom; 4) a mid- to late-Holocene(?) light grey sandy loam with occasional angular pebbles and cobbles that fines upward and varies in thickness from 0 to ~1 m; and 5) historic laminated silt loam that coarsens upward to sandy loam. The quartz-rich sandy cobble gravel is older than ~10,500 yrs based on two radiocarbon dates of wood at the top of the gravel. The organic-rich loam is a wetland soil with radiocarbon ages that range from ~5000 yrs BP to 1700 AD. The sandy loam is rich in kaolinite and thickest near valley margins, supporting the interpretation that it is derived from upslope erosion of saprolite. In places, this transported saprolite merges down slope with the mid- to late-Holocene wetland soil. The laminated silt loam has a high magnetic susceptibility, indicating that the sediment has been extensively disturbed by anthropogenic activity. This sedimentary unit has in-filled the pre-existing topography, supporting the interpretation that it is mill pond sediment upstream of a milldam. In places, it overlies Holocene wetlands, whereas in others it overlies the saprolite-derived deposits that mantle the toes of hill slopes. The origin of the rubble layer between bedrock and the sandy cobble gravel is uncertain, but it is likely to be Pleistocene in age, and possibly related to full-glacial climatic conditions. It is proposed here that this rubble might have formed as a result of late Pleistocene para-glacial conditions, whereas latest Pleistocene peri-glacial conditions mobilized the overlying sandy cobble gravel. It also is proposed here that cyclical episodes of cold, dry conditions throughout the Holocene led to intermittent deposition of clay-rich sand derived from eroded saprolite along the toes of hill slopes. Intervening warm, wet intervals resulted in thin brown soils on these clay- rich sands. Results of this work indicate that Holocene wetlands began to form and spread along the Little Falls valley bottom after the Younger Dryas (~12,900 to 11,500 yrs BP). For most of the Holocene, the Little Falls valley bottom was occupied by wetlands, and part of it remained a tussock sedge meadow for the entire mid- to late-Holocene.
Franklin and Marshall College Archives, Undergraduate Honors Thesis 2009
- F&M Theses Collection