ABSTRACT: Modelling (horizontal) water surfaces on glacially tilted terrain : the inverse solution.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Paleogeographic reconstructions over areas that have been partially submerged during deglaciation can represent a real challenge. Glacioisostatic tilting, poor dating of shoreline features, misinterpretation of asynchronous features, all contribute to the complexity of the task. A typical example is the Champlain Sea basin. The highest shorelines of the south shore are recorded at about 160 m asl near Covey Hill and 175 m asl near Warwick, Québec. The highest elevations of the north shore, however, vary from 250 m asl northwest of Montréal, 200 m north of Trois-Rivières, and 230 m north of Québec City. Since these two shorelines are not synchronous, any representation of the «maximum extent of the Champlain Sea» is an erroneous temporal view. The availability of digital elevation models (DEMs) has allowed rapid modelling of water surfaces using commercially available software packages. The temptation is great to look at the modelled images as true paleogeographic representations. If no corrections are applied, either to the water surface or the DEM, the most obvious error is the absence of tilted topography. In order to render more realistically the paleogeography of the St. Lawrence Valley during Champlain time, we have modelled water surfaces at various time intervals for which we had reliable elevation, age, and tilt data, assuming uniform tilt. The resulting equation is a four-variable polynomial. Two avenues can then be followed : correcting the DEM (a lengthy procedure requiring large computing capacities), or tilting the water plane according to the calculated surface trend. Each tilted water surface is then projected onto an uncorrected DEM. The intersection between the surface and the DEM defines probable shorelines and islands. Solving the polynomial equation also allows modelling the fall of the water surface as a function of time. As new data become available, the equation can be modified easily and new surfaces generated. The image can be graphically tilted so the water plane becomes horizontal, without actually recalculating the entire grid. The images generated are undoubtedly not the true paleogeographic reconstructions we'd all like to see. It is a tool to help visualize the impact of post-glacial water bodies in glacially tilted areas under known or assumed conditions.

Bibliography of Canadian Geomorphology