Journal ofCoastal Research 1168-1169 Royal PalmBeach, Florida Summer 1998 .tttlllll". ~ DISCUSSION - Discussion of: Johnson, M. E. and Libbey, L. K., 1997. Global Review of Upper Pleistocene (Substage 5e) Rocky Shores: Tectonic Segregation, Substrate Variation and Biological DiversityJournal of Coastal Research, 13(2), 297-307. Stephen K. Donovan Department of Geography and Geology University of th e West Indies Mona, Kingston 7, J amaica The rece nt review of rocky shorelines of th e Pleistocene to be tectonically active, thi s is due to their positi ons adjacent substage 5e by J OHNSON and LIBBEY (1997 ) is a welcome to the North Ca ribbean Plate Bound ary. Simil arly, Gra nd contribution to a field that is somewhat under studied. How­ Cayman is a subae rially exposed summit of the Cayma n ever , although I warmly appla ud the initiative of the authors Ridge, whi ch form s the northern ma rgin of the Cayma n in compiling and interpreting this fascinating database, I do Tren ch (JONES, 1994 ). The Cayma n Islands have been th e query whether the desire to det ermine a consistent tectonic site of limestone deposition since the Oligocene. classification of such shore lines has not led to the true range In conclu sion, J OHNSON and LIBBEY (1997) have classified of settings becoming partially ma sked by "shoehorn ing". Thi s four Antillean islands, from ea ch of which Pleistocene sub­ is particularly noticeable to a Caribbean work er like myself, stage 5e rocky shorelines have been re porte d, as occur ring who sees substage 5e rocky shorelines in Barbad os, Grand within an "island arc" tectonic setting. In fact , of these, only Cayman, Haiti and Jamaica (JOHNSON and LIBBEY, 1997 ) all Barbados can be regarded as bein g assoc iate d with the acti ve being classified together under th e broad tectonic setting of Lesser Antilles island arc. The other three examples lie in "island arcs". tectonically active regions adjacent to th e Northern Carib­ What is an island arc? I choose the following definition bean Plate boundary. In consequence, it is suggeste d th at "is­ from a recent publication, although most physical geology land arc" is an inad equ ate and erroneous description for textbooks and dictionaries will say much the sam e th ing; "A these occurrences. Rather, they represent occur re nces adja­ group of islands usually with a curving archlike pattern, gen­ cent to an acti ve plate margin, that betw een th e North Amer­ erally convex towards th e open ocean, having a deep trench ican and Caribbea n Plates. Thus, they are closer in tectonic or trough on the convex side and usually enclosing a deep setting to the "active contine ntal margin" setting of J OHNSON basin on th e concave side" (PARKE R, 1997 , p.186). and LIBBEY (1997 ), which might constructively be re-n am ed Thus armed with a working definition of an island arc, we as the "active plate margin s" setting to include such exam­ are in a position to assess the claims of the various Antillean ples. islands mentioned above . The only volcanically active island arc in the region is in the Lesser Antilles (WADGE, 1994 ), ACKNOWLEDGMENTS such as is demonstrated by the ongoing eruptions in Mont­ serrat (WILLIAM S, 1997 ). Although none of the listed islands This comment was written during the peri od of National falls precisely within the arc sensu stricto, Barbados is the Geogr aphic Society grant #5722-96, which is gratefully ac­ closest geogr aphically. It represents the exposed apex of an kn owledged. accre tiona ry prism, the Barbados Ridge (SPEED, 1994), and is entirely sedimentary in origin , but is nevertheless part of the broad island arc environment. However , thi s is not true LITERATURE CITED of the other three islands. Both J amaica and Haiti (which cons titutes the western end of th e island of Hispaniola) orig­ J OH NSON M.E. and LIB BEY, L.K., 1997. Global review of Upper ina te d as part of an earlier island arc system (LEWIS and Pleistocene (substage fie) rocky shores: Tectonic segregation, sub­ strate variation, and biological diversity. Journal of Coastal Re­ DRAPER, 1990; MONTGOMERY et al., 1994 ), but this was a search, 13, 297-307. Cretaceous phenomenon and both islands have been volcan­ J ONES, B., 1994. The Cayman Islands. In: DONOVAN, S.K.andJACK­ ically quiescent since the Paleogene. Although both continue SON, T.A.,(eds.), Caribbean Geology: An Int roduction. Kingston: Discussion 1169 Univers ity of th e West Indi es Publisher s' Association, pp. 87- 109. SPEED, R.C., 1994. Barbados and th e Lesser Antilles Forearc. In: LEWIS, J .F. and D RA P ~;R , G., 1990. Geology and tect onic evolutio n DONOVAN , S.K. and JACKSON, T.A,(eds.), Caribbean Geology:An of the northern Caribbean margin . In: DENGO, G. and CASE, J .E., Intro duction. Kingst on: University of the West Indies Publish er s' (eds.), The Geology of North America, Volume H. The Caribbean Association, pp. 179-192. Region. Bould er : Geological Society of Ameri ca, pp. 77-140. WADGE, G., 1994 . The Lesser Antilles. In : DONOVAN, S.K., and MONTGOM ERY, H.; PESSAGNO, E.A, JR., and PINDELL, J .L., 1990 . A J ACKSON, T.A, (eds.), Caribbean Geology: An Introduction. Kings­ 195 Ma terr an e in a 165 Ma sea : Pacific origin of th e Caribbean ton: University of the West Indies Publishers' Association, pp. Plat e. GSA Today, 4(1), 3- 6. 167-177. PARKE R, S.P., (ed.), 1997. McGraw-Hill Dictionary of Earth Sciences WILLIAMS, AR., 1997. Montserrat: Under th e volca no. National Geo­ (International Edition). New York: McGraw-Hill, xii + 468 pp. graphic, 192(1), 58-75. Journal of Coastal Research, Vol. 14, No. 3, 1998.