C-Change Working Paper: Coastal erosion and deposition in the Cap laRonde-Goulet Beach sector of Isle Madame, Cape Breton Island, Nova Scotia by Eric R. Force Geoscience Department University of Arizona, Tucson AZ 85719 US ([email protected]) C-Change Working Paper May 2012 Published by the C-Change Secretariat (Canada) Telfer School of Management, University of Ottawa C-Change ICURA Working Paper Series No. 40 This document is prepared as a public discussion document among C-Change communities as part of the C-Change ICURA Project 2009-2015 (www.coastalchange.ca) and with the permission of the C-Change Secretariat (Canada). This paper has not been subjected to peer review or other review processes and does not necessarily represent the position of individual C-Change Community Partners or researchers. This work is presented to encourage debate and enhance awareness of environmental change among coastal communities in Canada and the Caribbean. © C-Change Correspondence on this paper should be directed to the C-Change Secretariat, c/o C-Change Administrator, Telfer School of Management, University of Ottawa, 55 Laurier Avenue East, Ottawa, Ontario CANADA K1N 6N5 email: [email protected] Telephone: +1 (613) 562-5800 x2933 C-Change Secretariat (Canada) www.coastalchange.ca C-Change Secretariat (Caribbean) Telfer School of Management, c/o Sir Arthur Lewis Institute of University of Ottawa Social & Economic Studies (SALISES) 55 Laurier Avenue East University of the West Indies, St. Augustine, Ottawa ON K1N 6N5 Canada Trinidad and Tobago, West Indies Tel: (613) 562-5800 Post 2933 Telephone: (868) 662-6965 Email: [email protected] E-mail : [email protected] C-Change Working Paper #40 Force Abstract The study area, a 6-km stretch of coast along the southern entrance to St. Peters Bay, invites study because of its strategic position and its documentation through about 238 years of its history. Cap laRonde and adjacent capes are eroding drumlins, and Goulet Beach/Island has been at various times a gravelly barrier island or a large spit. Erosion of Cap laRonde and other capes, by wave undercutting (“Roman-nose profile”) and by slump of sea-cliffs (“boxer’s-nose profile”), has occurred throughout this history, but at Cap laRonde erosion accelerated about 1970 from 0.8 m /yr or less, to 1.3 m/ yr or more. Concurrently, the tombolo that extends to Cap laRonde has narrowed from 120 m to 60 m by beach retreat. Acceleration of erosion is due to some combination of sea-level rise, mining of gravel and sand from several beaches, and retreat of the Cap laRonde cliff from a bedrock platform, all of which are consistent with the observed timing of acceleration. The study area is especially sensitive to sea-level rise because the surface separating bedrock from glacial deposits is near present sea level and its slope is very low. Mining has been of sufficient magnitude to systematically deprive two important coastal compartments of a supply of gravel and sand since about 1970, suggesting great importance of this factor in governing local headland erosion rate. The evolution of Goulet Beach is out of step with the erosional chronology, and has changed little since the 1970’s, but extended, translated, and reshaped itself markedly in previous periods. It is likely that evolution of this beach was internally controlled in part by erosion and reworking of a nearby glacial feature that has been eroded. Extrapolation of current erosion rates with adjustment for changing geometries and processes suggest that Cap laRonde will vanish as a glacial geomorphic feature by 2040, and subaerial parts of the tombolo soon thereafter. Accelerated alterations in adjacent areas may follow. MAY 2012 Page | 2 C-Change Working Paper #40 Force Coastal erosion and deposition in the Cap laRonde-Goulet Beach sector of Isle Madame, Cape Breton Island, Nova Scotia 1. Introduction Context of the area and this study Rapid modification of drumlin-dominated shorelines is common on the coasts of Atlantic Canada and New England, and poses problems not only to coastal navigation, wetland protection, etc., but to increasingly valuable land holdings there. This paper presents a time- series study of such modifications along a 6-km stretch of coast on Isle Madame, Nova Scotia (fig. 1) based on personal observation, air photos, and maps as old as 1770; the changes are related where possible to observed modern processes. The “drumlin coasts” of Nova Scotia were the subjects of early attention by Goldthwait (1924) and Johnson (1925), and Nova Scotia has remained a center for such studies (Wang and Piper, 1982; Boyd et al., 1987; Carter et al., 1990; Manson, 2002). However, between Framboise, 46 km to the NE of the study area (Wang and Piper, 1982) and Chezzetcook and Musquodoboit Inlets, over 200 km to the SW (Boyd et al., 1987; Carter et al., 1990), there are no detailed descriptions of coastal change. The stretch chosen for study centers on Cap laRonde and Goulet Beach/Island at the northeast corner of Isle Madame (fig. 1), at a point of transition from conditions facing SE toward the open sea, and those to the NW sheltered by the Lennox Passage between Isle Madame and Cape Breton Island. To the NE stretches St. Peters Bay, which in turn gives access to the Bras D’Or “Lake” system, via a narrow isthmus (now a canal) that was already strategically important for Indian trade and French colonial and military interests in 1650 (and much later for ships seeking to avoid German submarines). Throughout this area, Cap laRonde is a navigational landmark. It is its position re navigation into this approach that resulted in the study area being described and mapped so early (fig. 2). The area is also blessed with unusually early air photo coverage and high-quality landscape photos, and is thus well suited for time- series analysis. Cap laRonde and adjacent capes seemed likely to function as a linked system with intervening beaches, including those extending as far as Goulet Beach/Island (fig. 3). However, seemingly simple questions of sediment budgets, etc. prove to be complicated by several factors, among which are sediment inherited from now-vanished glacial features, changes in rates of shore retreat, and mining of gravel and sand from the beaches. Accordingly this study has focused on those factors. Physical setting The topography of the study area is low-lying and dominated by drumlins, many of which have become partially submerged, intersecting sea level to form an anastomosing archipelago (fig. 3). The area is entirely under 30 m elevation, and sparsely populated, with most land forested. Bedrock consists of north-dipping clastic rocks of Carboniferous age (Force and Barr, 2006; Giles et al., 2010). Rock exposure is entirely along the shore; no inland exposures have been found despite extensive development of valleys at 5 m elevation or less. Thus the general slope on the bedrock surface must be exceedingly low, probably averaging 1 in 150 or less. This MAY 2012 Page | 3 C-Change Working Paper #40 Force surface of course shows reversals in the direction of surface slope where exposed in whaleback outcrops along the shore. The bedrock surface where exposed shows striations indicating glacial movement in several directions, with those to the ESE predominating (Stea et al., 1992; Grant, 1994). Drumlin elongation in the study area is consistent with these directions (fig. 3) and glacial till there predominantly represents the older stages of Wisconsinian glaciation. Drumlins in the study area are typically as much as 300 m long, 100 m wide, and 25 m high. Glacial till is exposed where these drumlins are eroding as cliffed headlands, and is a semi- indurated reddish aggregate of boulders to pebbles, grit to sand, and reddish silt to clay in roughly equal proportions. Most clasts are derived from sedimentary rocks, some local. Cap laRonde is the most prominent coastal headland, flanked by two others called Gull Cape and another I will informally call “Mine Bluff” (labeled 620 on fig. 3; it was not a cape until adjacent beach mining made it so). Farther to the south at Cap Petit Nez (fig. 1) are several more sea cliffs formed from drumlins. Drumlin headlands of the study area, especially Cap laRonde and “Mine Bluff”, are perhaps unusual among those described in the literature in their steep slopes; overall angles of 70 degrees or more are characteristic, and overhanging turf at the top and large wave-cut notches at the base are common. Such headlands are those eroding fastest; headlands with lesser angles, such as about 45 degrees for Gull Cape, are vegetated and fairly stable, although subject to slumping and rill erosion. Of these cliffed headlands, only Cap laRonde shows indications of being a drumlin that was formerly cored by rock. Its bedrock is now exposed only offshore at low tide in the direction of former drumlin elongation, as part of a large shoal of residual boulders. The present cliff face, however, consists entirely of eroding till. Cap laRonde is topographically isolated from neighboring headlands, and is tied to the shore by a tombolo about 600 m long (fig. 3). This tombolo consists of gritty to cobbly shingle; the shore facing south is the more active as shown by its washover fans. Some of this material has been mined as described below. Similar steep shingle beaches link all the eroding-drumlin headlands of the study area. Locally and seasonally they may be sandy or gritty as surface veneers, but they are underlain by coarse-grained material and backed by similarly coarse beach crests and washover fans. The beaches are occasionally breached to form inlets, generally short-lived.