Summary

Subglacial landforms across various scales preserve the history of movement, deposition and erosion by the last great ice sheets and their meltwater. The origin of many of these landforms is, however, contentious. In this chapter these forms are described both individually and as suites that make up entire landscapes. Their interpretations are discussed with reference to the megaflood hypothesis. A description is provided of individual forms via their size, shape, landform associations, sedimentology and the relationship between landform surfaces and internal sediments. The possible origins of each are then discussed. To simplify the chapter the landforms are categorised by their size (micro, meso, macro and mega), although, importantly, it should be noted that several landforms show similarities across scales. Also discussed is the relevant subglacial hydrology associated with the described forms, especially the volume and discharge rates of megaflood flows, and where water may have been stored prior to the megaflood events.

Introduction

As early as 1812, Sir James Hall interpreted the famous Castle Rock in Edinburgh, Scotland, a crag and tail, as a landform created by immense, turbulent floods. Likening the hill to features carved in snow by wind, he could only hypothesise that water was responsible; probably giant tidal waves, as, at that time, he knew of no other mechanism that could conceivably create such streamlining. It is now very clear that the streamlined forms first noted by Hall are part of a continuum containing landforms of many shapes and sizes.

Introduction

A long-term objective of population biology is to explain the spatiotemporal variations in abundance of organisms by understanding the factors that limit both distribution and changes in abundance. In general, theory predicts that a major determinant of distribution and dynamics is the instantaneous population growth rate, presented as r (where r = ln(λ) = ln(Nt+1/Nt). Some models reveal the obvious, such that species will tend not to exist where their population growth is consistently negative and there is no immigration (see Chapter 2). But the models also expose intriguing dynamics; for example, simple single-species nonlinear models reveal that dynamics can vary from stability through oscillatory to chaotic behaviour simply by subtle changes in the population growth rate (May 1976). As such, it is not surprising that when we incorporate interspecific interactions, the stochastic vagaries of environmental conditions and dispersal, we reveal a Pandora's box of dynamical behaviours.

In the empirical literature, the population growth rate parameter does not enjoy the same importance as it does in the theoretical literature. Rarely do workers make an estimate of the intrinsic growth rate parameter (r) or its empirical equivalent, the maximum growth rate (rmax) which is simply the maximum rate of growth observed within a time-series. Changes in the observed growth rate at a specific time (rt) may be recorded along with the factors associated with the reproductive output of individuals, but studies tend not to estimate the extent to which the growth rate is reduced by density dependent regulatory factors.