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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Late Pleistocene stratigraphy and morphology of the lower Northern Neck, Virginia

Farrell, Kathleen M. 01 January 1979 (has links)
No description available.
42

Tectonic and climatic influences on bedrock channels traversing the Central Andes, Bolivia

Syrek, Jonathon F. 09 January 2013
Tectonic and climatic influences on bedrock channels traversing the Central Andes, Bolivia
43

THE EFFECTS OF RECENT AND RELICT PERMAFROST DISTURBANCES ON TUNDRA VEGETATION, CAPE BOUNTY, MELVILLE ISLAND, NUNAVUT

CASSIDY, ALISON ELIZABETH 10 August 2011 (has links)
Permafrost disturbances, including active layer detachments (ALDs), have occurred both recently and historically at Cape Bounty, Melville Island. These recent and relict ALDs were studied to determine their short- and long-term landscape effects. Six relict detachments showed altered vegetation and site characteristics, despite 60 or more years of recovery. Of the environmental variables studied, including soil moisture, soil temperature, and active layer depth, soil moisture showed the greatest changes in disturbed zones. These were attributed to the concavity of disturbance, which allows for more snow accumulation. Two vegetation types influenced by moisture regimes, polar desert and mesic heath, were compared to determine the role of moisture in recovery. The best indicators of disturbance differed in areas of different moisture regimes, with canopy height showing increases in disturbed mesic heath zones, while Normalized Difference Vegetation Index (NDVI) values increased in polar desert disturbances. Ultimately, the comparison of vegetation in disturbed and undisturbed zones revealed differences to be highly localized and minimal. Remote sensing was utilized to compare the effects of recent and relict disturbances as seen on satellite and airborne imagery. Vegetation was classified using NDVI, and zonal separation of these values in ALDs revealed the upper scar areas and lower toe zones contained significantly different NDVI values. The upper scar values were similar to undisturbed control areas, as blocks of vegetation in these areas often contain unmodified vegetation, which later helps with revegetation. The lower toe zone displayed both elevated and lower NDVI values, as material accumulates in these areas but often in a complex with bare soil. Remote sensing techniques also allowed for site characterization of disturbances, with physiographic factors including slope and flow properties determined through satellite imagery. Slope values ranged from 3 to 13 degrees in each disturbance, but were found to be slightly lower in relict disturbances, as recovery and revegetation have reduced these slopes. Flow paths were identified in ALDs, however similar patterns were also identified in surrounding undisturbed landscapes. Given the small scale of many ALDs, it is difficult to characterize the nature of the changes associated with these events, particularly for relict ALDs. / Thesis (Master, Geography) -- Queen's University, 2011-07-28 11:42:47.636
44

Holocene glacier fluctuations and tephra fall inferred from lacustrine sediment, Emerald Lake, Alaska

LaBrecque, Taylor S. 16 October 2014 (has links)
<p> Downcore changes in physical and biological characteristics of lacustrine sediments from Emerald Lake were used to reconstruct the Holocene glacier history of Grewingk Glacier, which drains the Grewingk-Yalik Ice Complex on Kenai Peninsula, Alaska. Emerald Lake is a threshold lake, receiving meltwater and clastic sediment when Grewingk Glacier overtops the topographic divide that separates it from Emerald Lake. Glacier meltwater discharge is represented in sediment cores from Emerald Lake by distinct light-gray, stony mud, with high density and low organic-matter content. Sub-bottom acoustical profiles were used to locate two core sites: one with a low sedimentation rate (Core 2; 18 m depth) and one with a high rate (Core 3; 50 m depth) to maximize both the length and resolution of the sedimentary sequence recovered in the ~3-m-long cores. Bulk density, sedimentation rate, stratigraphy, organic-matter content, and chlorophyll were used to record environmental changes since ~12 cal ka, with <sup>14</sup>C and <sup>210</sup>Pb for geochronology. Ages were assigned to tephra beds in Cores 2 and 3: 18 and 9 beds respectively. A diffuse transition from the basal inorganic mud to organic-rich mud ~11.4 cal ka marks the initial retreat of the Grewingk Glacier below the divide of Emerald Lake. The overlaying organic-rich mud is interrupted by stony mud that records a brief re-advance as ice overtopped the divide again ~10.7 cal ka, followed by the final glacial-interglacial transition ~9.8 cal ka. The glacier did not spill meltwater into the lake again until the Little Ice Age, from around AD 1350-1900, consistent with documented LIA advances on the Kenai Peninsula. The retreat is estimated from lichen ages on a bouldery moraine on the topographic divide and is consistent with the previously estimated age of the Grewingk Glacier terminal moraine (AD 1858). The retreat of Grewingk Glacier below the divide at 11.4 cal ka took place as temperature and productivity increased across southern Alaska; the subsequent readvance above the divide at 10.7 cal ka corresponds with cooling beginning ~11.0 cal ka in south-central Alaska. Decreased precipitation in southern Alaska from 5.5 to 4.0 cal ka lowered the level of Emerald Lake and sedimentation rate decreased. The initial LIA advance over the divide (AD 1350) and peak meltwater input into Emerald Lake (AD 1660) coincide with documented solar minima, suggesting solar variability influences Grewingk Glacier fluctuations.</p>
45

Island dynamics and their role in regulating sediment flux in the Middle Snake River, Idaho

Thomas, Dai B. 14 August 2014 (has links)
<p> This study was conducted to provide an improved understanding of the dynamics of river islands and to investigate the role of islands in regulating sediment flux within the fluvial system. </p><p> The study showed that the islands in entrenched geomorphic subreaches of the MSR form, erode, and reform in locations controlled by lateral constrictions. The geometry of the islands adjust on a decadal or even longer time scale in response to a disturbance or changes in water and sediment supply, and thus, the islands form part of a temporal and spatial continuum of bedforms. The formation of the islands regulates sediment flux through the reach. </p><p> The study reach of the Middle Snake River (MSR) in Idaho contains over 300 islands within approximately 200 km between Swan Falls Dam and Brownlie Reservoir. The hydrology of the study reach has been significantly altered by upstream dams on the mainstem and dams on tributaries within the study reach. </p><p> Data used in the study include: (1) historical aerial photos (1938/1939) and topographic maps (c1894-1906), (2) topographic and bathymetric survey data collected in 1997 through 1999, (3) flow measurements from 1911 to present, (4) bed material samples, (5) morphostratigraphic mapping of 194 islands and (6) stratigraphic soil profile data collected on 95 islands. The soil profile data included soil stratigraphy, soil samples (used for sediment gradations and pollen analyses), pedological descriptions, historical artifacts and charcoal fragments (used for carbon dating). A previously developed 1-dimensional hydraulic model of the study reach was used to evaluate the hydraulic conditions along MSR and to calculate the overtopping discharges of the islands. </p><p> Comparison of historical aerial and topographic data with 2012 aerial photography showed evidence of the growth and erosion of islands and reworking of island chains to form new configurations, illustrating the dynamic nature of the islands over the last approximately 100 years. The historical document review also showed that the location of almost all larger islands and island groups are controlled by lateral constrictions such as tributary fans. </p><p> Soil profile data, pollen analyses, historical artifacts and radiocarbon dating of soil charcoal were used to determine the approximate age of islands and to evaluate the erosional and depositional activity of the islands. The soil profile data showed an extreme range in age at some islands where the gravel platform of the islands is old (circa 7,000 years), but the overlying sediments are young (on the order of hundreds of years). </p><p> Two-dimensional sediment-transport models were developed to evaluate the baseline conditions and simulate island development. Baseline conditions modeling showed the gravel- to cobble-sized material forming the core of the islands is not mobilized under the current hydrology. The islands formed in response to more recent floods from silt-sand sized sediment supply, which explains the relatively young soils overlying older gravel cores. </p><p> Modeling results showed that: (1) the island geometry adjusts to a disturbance or a change in sediment supply, (2) the formation of islands regulates sediment flux, and (3) the islands form, erode, and reform in the same general locations, which supports the study hypotheses that islands form part of a temporal and spatial continuum of bedforms.</p>
46

Tectonic geomorphology of the San Timoteo Badlands| New insights from OSL and LiDAR data

Wicker, Cary 10 June 2014 (has links)
<p> Optically stimulated luminescence (OSL) dating and stream profile analysis were used to evaluate the main structural elements controlling the formation of San Timoteo Badlands topography. Stream profiles from 16 streams were created using light detection and ranging (LiDAR) and GPS data, providing new 1 meter-resolution stream profiles along the badlands. Ten new OSL dates from sediments of elevated surfaces preserved east and adjacent to the Claremont fault provide five new vertical displacement rates along the badlands. OSL dates and stream profile analysis suggest that basin subsidence within the Casa Lorna pull-apart basin is the main factor controlling the formation of San Timoteo Badlands topography, and that there is currently little uplift in the northern portion. of the badlands.</p>
47

Verification of post-glacial Speleogenesis and the origins of Epigene Maze Caves in New York

Cooper, Max 10 June 2014 (has links)
<p>Dissolutional features called karst exist on the surface, and in the subsurface as caves. In glaciated regions caves were thought to be post-glacial in origin. Work in the 1970s demonstrated that pre-glacial caves existed, but did not answer if a cave could form post-glacially. A model proposed by Mylroie and Carew (1987) states that a post-glacial cave would be controlled entirely by glacial features and the deranged drainage of glaciated terrains. Caves known as maze caves form at maximum rates, and could form to navigable size in the time since deglaciation. Maze caves form in the shallow subsurface, allowing them to be removed in subsequent glaciations. GIS water flow analysis, and calculation of formation times using cross-section data demonstrates that maze caves in the glaciated region of New York are post-glacial in origin fitting in the deranged drainage and forming in the time since deglaciation. </p>
48

The Quaternary history of Mohawk Valley, northeastern California

Redwine, Joanna R. 26 February 2014 (has links)
<p> Mohawk Valley is an inter montane basin with a rich Quaternary record, located at the northernmost end of the Sierra Nevada Mountains in northeastern California. Geologic mapping of surficial deposits, stratigraphy, tephrochronology, geomorphology, and soil development were used to interpret the past 740 ky of Quaternary history of Mohawk Valley. The robust tephrochronologic record within Mohawk Valley includes twenty-six different tephras and sixty-seven tephra beds that range in age from 740 to 7 ka. Geochemical analyses and correlations with previously identified volcanic tephras have resulted in revised age estimates for tephra beds distributed within, and beyond, Mohawk Valley. </p><p> The tephra beds were deposited in lacustrine deposits of Mohawk Lake. Elevations of shorelines and minimum lake-levels based on elevations of waterlain tephra beds were used to reconstruct the history of Mohawk Lake. Mohawk Lake began to fill prior to 740 ka and continued to fluctuate, but overall rise, until after 175-235 ka when the lake reached the sill elevation, began to spill to the west, then incrementally lower and empty by ~7 ka. Throughout this period, there were at least five, and up to nine, different generations of glacial deposits that extended towards Mohawk Lake. These glacial deposits have been mapped, their soil development and weathering properties characterized, and ages estimated based on stratigraphic relations with tephra beds deposited within Mohawk Lake deposits. This mostly continuous, 740 ky record of sedimentation has enormous potential to examine paleoclimate in this area from any of a number of paleoclimate proxies. </p><p> The interpretation that a deep lake existed in Mohawk Valley requires a mechanism to allow for deposition and preservation of organic-rich deposits in deep water. Mohawk Lake was likely a meromictic lake, a setting that leads to an anoxic environment that can preserve organic-rich sediments such as those found in Mohawk Valley. In addition, shorelines around Mohawk Valley and across much of the Mohawk Valley Fault Zone are at consistent elevations suggesting there is not a significant vertical component of faulting since 175-235 ka, and maybe since 570-610 ka. This indicates a change from the history of subsidence since the early Pliocene.</p>
49

Morphological Changes Associated with Tropical Storm Debby in the Vicinity of Two Tidal Inlets, John's Pass and Blind Pass, West-Central Florida

Brownell, Andrew T. 29 January 2014 (has links)
<p> Tropical Storm Debby affected the Gulf coast of Florida in late June, 2012. The storm's southerly approach temporarily reversed the annual net southward longshore sediment transport. The energetic conditions associated with Tropical Storm Debby can be seen in the wind, wave and tidal measurements taken from both onshore and offshore weather stations around the dual tidal inlets system of John's Pass and Blind Pass, approximately 25 kilometers north of the mouth of Tampa Bay. The energetic and persistent southerly forcing, in addition to higher storm induced water levels and wave heights, resulted in atypical beach erosion and sediment deposition on the ebb tidal deltas of the two inlets and the surrounding beaches. The John's Pass ebb delta gained 60,000 cubic meters of sediment and the Blind Pass ebb delta gained 9,000 cubic meters as a result of the storm. Shoreline position, beach profile and offshore bathymetric surveys conducted before and after Tropical Storm Debby illustrate the changes in the coastal morphology such as the development of an offshore bar south of Blind Pass and erosion of the dry beach north and south of John's Pass. The Coastal Modeling System (CMS) was used to simulate wave and tide-driven current fields during the passage of the storm. The modeled wave field qualitatively illustrated the shadowing effect of the Tampa Bay ebb delta in reducing the southerly approaching storm wave energy arriving at the study area during the storm. The tidal flow patterns through the inlets and over the ebb tidal deltas were considerably different during the storm, as compared to normal tidal cycles.</p>
50

Characterisation of a "Bioregional Land Zone", Brisbane.

Heim, J. A. Unknown Date (has links)
No description available.

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