The emergent sand-beach system at Gisborne consists of six recognisable groups of ridges and swales. It is about three miles wide and four miles long and rises gradually from 15ft above sea level at the coast to 40ft inland. From time to time the emergent beach has been mantled with air-borne volcanic ash including ash beds of the Waimihia Lapilli, Taupo Sub-group Members 9 - 13, Taupo Pumice, and Kaharoa Ash Formations. As the dates of these eruptions are known, the times of formation of the groups of beach ridges and wales have been determined as follows:
Group 1: c. 9000 B.C. - c. 1400 B.C.
Group 2: c. 1400 B.C. - (?) 850 B.C.
Group 3: (?) 850 B.C. - c. A.D. 131
Group 4: c. A.D. 131 - c. A.D. 1020
Group 5: c. A.D. 1020 - c. A.D. 1650
Group 6: c. A.D. 1650 - A.D. 1956
Evidence of recent earth movements has been noted in ridges and swales of Group 1, and of possible movements in those of Group 3. Changes in sea level could not be established and were taken from Wellman and Schofield. No attempt was made to distinguish directly wind-blown sand from wave-deposited sand; instead, a shell layer (assumed to be associated with the intertidal strand) was used as a marker bed to indicate the approximate sea level at the time when the shells were deposited.
Elevations of ridges and swales in each group were measured on a 15,000ft transect across the beach system. Then, overall linear and quadratic regressions as well as linear regressions for each group separately were computed. For both of the overall linear and quadratic regressions the trend lines show a fall seaward, but the separate trend lines for each group are as follows:
Group 1: Highly significant seaward decline.
Groups 2 and 3 combined: Very highly significant seaward decline.
Group 4: Highly significant seaward incline.
Groups 5 and 6 combined: No significant change.
The departure of the regression trend lines within Groups 1 to 6 from the overall linear and quadratic trend lines suggests that the trends of elevation across the emergent beach at Gisborne should be regarded more as a series of discontinuous trends rather than as one overall continuous trend of seaward decline. Though the overall trend of declining elevation is seaward, the corresponding fall in sea level is likely to be more apparent than real because of compounding of fall in sea level with earth movements.
