Relevant geomorphological evidence indicative of neotectonic activity in the Acambay graben, a major structure in the central part the Mexican Volcanic Belt, has been derived from topographic maps, in conjunction with morphometric analysis and detailed field mapping, and remote sensing analysis, employing large-scale aerial photos and digitally enhanced Landsat imagery. The Acambay-Tixmadeje, Pastores, Venta de Bravo and Temascalcingo fault systems, which are generally orientated east-west, are associated with linear ridges, sag ponds, offset drainage and shutter and compression ridges which indicate a left-lateral component. Prominent fault scarps and triangular facets show vertical displacement along the faults flanking the graben. The existence and freshness of such morphological features implies that these faults have been active during the Quaternary. Tectonic landforms in the Acambay graben reflect faulting characterised by normal south-facing and normal north-facing faults combined with left-lateral displacement. Geomorphological data for the Acambay graben are consistent with a system of faults which have experienced transtensive, large-scale, left-lateral shear along the Mexican Volcanic Belt. Morphological evidence and the historical occurrence of seismic activity reveal areas of more active tectonics and seismic risk along the Acambay graben. Morphotectonic characteristics of the Venta de Bravo fault system suggest that it is the most seismic and potentially active fault system in the Acambay graben.

Ice sheets are integral to the earth's climate system, both modulating and responding to climatic change. Iceberg calving fronts are the only dynamic interface at which the atmosphere, oceans and ice sheets directly interact. Calving introduces mechanical instability to glacier systems such that the response of calving glaciers to climatic forcing is commonly nonlinear. The interaction between calving dynamics and the ice-marginal environment, notably the topographic geometry of glacier troughs, can partially or totally decouple glacier fluctuations from climate for periods of several centuries. In West Greenland these instability mechanisms appear to have been important both during deglaciation and recently. In the Late Glacial/early Holocene, trough geometry controlled the retreat stages of the ice sheet margin in the Ilulissat (Jakobshavn) area of central west Greenland. During the second half of the twentieth century, the oscillations of 72 outlet glaciers between 61 °N and 72°N show that land- terminating glaciers respond directly to climate change (albeit with variable time lags) but that calving glaciers behave non- linearly. Freshwater calving glaciers have lower calving fluxes and calving rates than tidewater glaciers, and may be the first to respond to climatic cooling. It is not clear whether ice sheet outlet glaciers oscillate cyclically as do calving mountain glaciers, but the instabilities introduced by calving cause many glaciers to respond more directly to topographic than climatic factors. It is therefore hazardous to attach palaeoclimatic significance to the glacial geomorphological record of the fluctuations of former calving margins, or to regard the behaviour of contemporary calving outlets as indicative of climative trends. Factors affecting the stability of ice margins have a fundamental impact on the dynamics of ice sheets, and are important controls on the timing and patterns of ice sheet response to climate change.

No description available.

The aim of this thesis is to reconstruct the dimensions of the Loch Lomond Stadial (LLS) ice cap in Western Lochaber, Scotland, and to make inferences about ice cap dynamics on the basis of the field evidence. There have been no previous detailed studies of LLS ice cover in Western Lochaber, yet an accurate empirical reconstruction of the ice cap is an essential input and constraint to models of the interactions between climate, ice volume, topography and mantle rheology. Three different types of field evidence are used to reconstruct the ice limits. Firstly, geomorphological and sedimentary evidence indicates some lateral and terminal limits and retreat patterns. Secondly, a seismic stratigraphy of Loch Linnhe and the Firth of Lorne provides additional evidence for the distribution of glacial deposits and an ice limit. Thirdly, the glacial and periglacial evidence on 111 slopes marks trimline altitudes reflecting the palaeo-ice surface. These three types of evidence allow a three dimensional reconstruction of the ice surface. The ice cap was up to ˜650m thick, and ice flowed from the main mountain ridges and an ice plateau around the heads of Lochs Eil and Shiel, down the main troughs to the sea lochs. All available chronological controls support the proposition that this ice cap existed during the LLS. Depositional evidence suggests that subsequent ice retreat back towards the mountains was punctuated by stillstands at topographic pinning points in the sea lochs. There are distinct contrasts in the spatial distributions of glacial erosional and depositional evidence. Throughout the west and south of Western Lochaber terrestrial signs of glacial scouring are widespread and glacial till and moraine are thin and sporadic, yet there are thick glacimarine sequences in the proglacial submarine basin in Loch Linnhe, and large proglacial outwash fans around the lochs. In the north east of Western Lochaber and in Eastern Lochaber slopes are mostly mantled with glacial till, there are large terminal and lateral moraines and outwash deposits are common.

The aim of this project is to investigate the influence of climate and topography on ice sheets in maritime environments. Numerical models are adapted to simulate the behaviour of the climate and ice sheets in southern Chile and Scotland during the last glaciation. The climate model relates climatic variables to snow accumulation and ablation using an energy balance model. The ice sheet model is based on the continuity equation for ice thickness and relates surface mass exchange to ice thickness and flow. Subsequently, a simple topography model is developed to examine the critical transition between glaciers and ice sheets. The net mass balance gradient in maritime regions is primarily sensitive to temperature and precipitation. Ice sheet initiation is strongly influenced by the adjacent ocean's temperature, which affects the delicate balance between decreasing precipitation and decreasing temperature. In Chile, expanded glaciation reflects an equatorward movement of the prevailing westerlies, though the postulated migration of precipitation belts implies that the maximum depression of the snowline is unlikely to have been contemporaneous at different latitudes. In Scotland, ice sheets appear to be triggered by the southward movement of the North Atlantic polar front. The configuration and latitude of upland topography determines the point of initiation and the threshold between stable upland glaciers and the growth of an ice sheet. The topography acts as a filter between climate and the response of a glacier, and topographic evolution leads to a powerful feedback between topography, climate and ice over Quaternary time scales.

This thesis has three aims: (1) to construct tephrochronologies on large and small scales at sites in Sweden and Iceland; (2) to assess the effects of erosional and depositional processes on the nature and distribution of tephra in the sedimentological record; (3) to use these case studies to propose a model of the deposition of tephra in lakes. The Swedish Timescale based on the annually laminated sediments (varves) has recently been linked to the present. To test the chronology, traces of volcanic glass (tephra) from five historical eruptions of Icelandic volcanoes were sought within relevant sections of the varve deposits. Difficulties in isolating and identifying tephra to define isochrones in distal deposits led to the adoption of an integrated catchment and lake basin sampling strategy to assess the processes which affect the temporal and spatial distribution of tephra in lakes. A detailed tephrochronology of Svínavatn, a lake in northern Iceland, was constructed by identifying and correlating 95 tephra deposits from five lake cores and twelve profiles situated in the lowland peats, hillslopes and delta areas of the catchment. The tephra record from each site was highly variable due to both uneven fallout of the tephra following the eruption, and later reworking of the deposits in the lake and the catchment. The environmental changes of Svínavatn and its catchment were reconstructed using tephra as a geomorphological tracer. The peats and soils of the catchment were stable throughout most of the Holocene until the deposition of Hekla 3 (2800 yBP). Repeating layers of reworked Hekla and Katla tephra after this period at several terrestrial and lacustrine sites reflect increasing episodic instability of the catchment and the effects of this disturbance on the lake record. Until the arrival of Norse settlers in the 9<SUP>th</SUP> century, much of this disturbance was linked to climate and vegetation changes around the catchment. Significant, but temporally discrete, secondary inputs of H3 and H4 (3800 yBP) into the lake occur several thousand years after the original airfall.

The moraines are made up of stratified glaciofluvial sediments overthrust on their proximal flanks by clay-rich tills composed of reworked glaciofluvial and glaciolacustrine sediments. A single model explains the location, morphology, sediments and structures of the moraines. The key point is that once a lake has formed, it allows the deposition of fine-grained lacustrine sediments which in turn affects the dynamics of the glacier during any subsequent advance and retreat. A relative chronology shows a number of similarities and differences between the behaviour of the Puyehue and Rupanco glaciers. Both show the same broad pattern of advance and retreat but Rupanco lagged Puyehue in its response to climate change. The shorter response time of Puyehue also means it was more likely to reach the culmination of an advance in response to a climatic fluctuation. The empirical evidence of differential response has been used as a strong test of a glaciological model developed in parallel with this research and helps to explain the geomorphological record. The differences in response reflect the more uniform, steeper longitudinal bed slope of the Puyehue glacier basin, and the influence of calving dynamics. Radiocarbon dating has confirmed that the moraines belong to the last glaciation and that the timing of advances demonstrates a similar pattern to other outlets. All the glacier basins show advances at c.21 ka and c. 14.5 ka but the precise timing in individual basins may have differed by as much as 1500 years. There are as number of implications to be drawn from the moraine-forming model and the chronologies of the Puyehue and Rupanco glaciers. In areas where temperate glaciers advance into lakes they are likely to be partly decoupled from climate. Field evidence of these advances will be closely superimposed and detailed morphostratigraphic and lithostratigraphic criteria are needed in conjunction with radiocarbon dating to distinguish different advances. Topography also plays an important role in determining glacier response. These are additional factors to be considered when establishing glacial chronologies from such moraine sequences.

Four beach types have been identified in South Georgia. These are beaches associated with outwash, moraine, relict outwash and cliff. They can be distinguished on the basis of their site, morphology and materials. Generally, the outwash type occupies larger sites and has low, flat and sandy beaches; the moraine type occupies any size of site and has steep pebble beach's of medium height; the relict outwash type occupies medium sized sites and has high yet low angled beaches of mixed sands and pebbles; the cliff type occupies small sites and is characterised by high steep pebble, beaches. A marked contrast in distribution of beach types is evident since cliff-backed beaches are located on any coast whilst the other types, especially the outwash-backed beaches, tend to be located on the north coast. Identification and analysis of beach variables indicates that the beach environment of South Georgia is highly active. Waves derived by hindcasting from meteorological data indicate that 55% of all waves approach from the south-west with dominant periods of 8-14 seconds whilst 32% and 13% approach from the north-west and east respectively with periods of 12-14 seconds. However, refraction of these waves into the bays shows that much of the deepwater wave energy is lost and that the remainder is focused towards specific points on the beaches. The terrestrial environment of South Georgia is also highly active since the cold wet climate allows- vigorous operation of glacial and subaerial processes which both generate and transport much debris to the beach zones. However, the intensity of operation of marine and terrestrial processes varies from place to place. The windward south coast is highly active in terms of wave energy yet of low activity in terms of debris generation and the opposite is the case on the north coast. Variation in the intensity of process largely accounts for the variation in beach form in South Georgia. Classification of beaches by energy and form supports the idea that high energy beaches are located on the exposed sides of islands. From a quantitative and qualitative analysis of form and process, the south coast emerges as a high energy coast with small, steep and high beaches whilst the north coast is mainly a lower energy coast with large, flat and low beaches in the sheltered inlets. This variation is generalised to produce models of beach type and evolution in South Georgia. If South Georgia is assumed to be typical of sub-Antarctic environments then these models suggest that the sub-Antarctic is one of the most active beach environments in the world. Seen in the context of Quaternary climate change, the beaches of South Georgia can be viewed as a model of beach development following deglaciation. If ergodicity in beach development is accepted then the model South Georgia beach may be an analogue of Late glacial mid-latitude beach development.

Soil erosion is a widespread phenomenon in certain parts of upland Britain and is considered by a number of authors to be a neglected area of study worthy of considerably more attention since in many cases it results from bad land use management and poor conservation practices. One major land use which is considered by a number of authors to be of prime importance in the formation and maintenance of scars is that of extensive sheep and deer rearing. Examination was carried out in Strathconon of an area utilised for grazing both sheep and deer. The study focussed on two main topics, firstly how widespread erosion scars were in this area and whether or not they constituted a land use problem, and secondly the contribution made by grazing animals as separate from natural phenomena. The study evaluates the complicated elements of topography, climate, soils, vegetation, land use and animal behaviour as they interact in the study area to influence erosion. It shows that the influence of animals in producing and maintaining soil scars in this area is considerable, though the actual area affected by erosion was only a small proportion of the total. Certain types of erosion scar were, however, spreading and are likely to cover a greater area in the future. In spite of the fact, therefore, that apparent soil erosion in an area may be exaggerated without detailed survey, and that in this area little economic loss is incurred as a result, the areas worst affected are both highly visible and fragile. Erosion of fragile soils and plant communities and visual amenity is therefore a factor which must be considered in relation to that of the economics of sheep and deer production in this area. Many of the patterns observed apply to other valley systems in the central Highlands, and in some cases over a wider upland area. It is possible, therefore, to predict that based on the observations in this area, certain patterns and relationships relating to erosion may occur in other areas. Where these are related to land use management it may be possible to modify them to reduce erosion, and encourage more efficient soil conservation.

Natural geochemical and isotopic tracers were used to assess and model hydrological processes in two mesoscale (>200km2) catchments in the Scottish highlands, the Feshie and Feugh, in order to upscale understanding from the traditional headwater catchment scale (<10km2). Gran alkalinity was used as a geochemical tracer to distinguish acidic, organically enriched soil water from more alkaline groundwater. Spatial variations in alkalinity reflected the influence of different hydrological sources at the sub-catchment and catchment-wide scale, whereas temporal alkalinity variation at different flows over the hydrological year and over shorter event timescales provided information on the influence of hydrological flow paths. The well-defined relationship between alkalinity and flow meant that two-component end member mixing analysis could be used to quantify the influence of these hydrological flow paths and sources over a range of scales and contrasting catchment characteristics. These techniques were then used to examine more specific groundwater-surface water interactions in the River Feshie's extensive braided section. These interactions were seen to exert a significant and dynamic impact on the hydrochemistry of main stem surface flows and as a result, the hydrological and hydroecological functioning of the catchment as a whole. Stable isotope (18O) variations were also employed as a natural tracer to further investigate hydrological flow paths and provide preliminary catchment residence time estimates. These estimates indicated the relative dominance of catchment characteristics over the more general influence of scale in determining the age sources of catchment runoff. This represented one of the first such assessments of stable isotopic tracers for investigating catchment hydrology other than at the headwater scale. The natural tracer approach therefore provided considerable insight into mesoscale catchment hydrological functioning that would not have been feasible through more conventional small-scale hydrometric investigation. This has direct utility for the sustainable management of such catchment systems as well as highlighting the potential for applying such tracer investigations in order to help structure and validate more accurate hydrological models.