Delta Of Waimakariri Active In Recent Past

Press, Volume C, Issue 29604, 29 August 1961, Page 20

Delta Of Waimakariri Active In Recent Past

The ancient delta of the Waimakariri, in the middle of which Christchurch now stands, was active possibly, up to less than 100 years ago —not 2000 to 2500 years ago, as was previously supposed, Mr J. B. Cox, of the Soil Survey Division of the Department of Scientific and Industrial Research, said yesterday. One arm of the delta ran down through the present Islington area to the Halswell river, and thence to Lake Ellesmere. Other branches spread through to the sea through where Papanui, Styx, and Belfast now are. While every branch of the delta may not always have contained permanent streams, the branches all carried considerable volumes of gravel and silt-laden water at flood times. The delta was probably active for about 500 years, down to the 1868 flood in the European settlement era, said Mr Cox. This renewed activity after a period of down-cutting by the river resulted, he believes, from the widespread burning by the Maoris of a forest which, he and his co-workers have discovered, extended over practically the whole plains as well as the foothills and lower mountains up to a few hundred years ago. The discovery of the extent of the ancient forest, and the unsuspected recency of the river deposits in the delta, came as a result of field work for a soil map of the Paparua county, now in preparation. The field work was done and the results evaluated by Mr Cox in conjunction with the head of the Soil Survey office in Christchurch (Mr C. G. Vucetich), and a former member of the staff, Mr C. B. Mead. The tree species in the forest were identified from charred remnants by Mr H. R. Orman, of the Forest Research Institute, Rotorua. Specimens of this wood charcoal are with the Nuclear Science Institute in Lower Hutt awaiting carbon dating, so' that more accurate dates can be put to the destruction of the forest and the renewed delta activity. “Last Fling” Mr Cox believes that, by the time the Europeans came to Canterbury, the deltabuilding phase was reaching its end. The 1868 flood may well have been “the last fling” of the river before settling down to another period of down-cutting, he thinks. Although the flood waters burst through to the Avon, no record is found in the newspapers of the day of any depth of silt, yet in slightly earlier times there were great areas of alluvium spread out, which now form the surface deposits over large parts of the Christchurch area. A delta-building period 2060 to 2500 years ago seems to have taken place as formerly believed, said Mr Cox. Soils formed on the deposits from that time have long been known as the “Waimakariri series.” Similar soils have been identified around the lower reaches of other Canterbury rivers, where they are called by the same name. The Soil Survey group has termed the soils on the most recent deposits as the “Selwyn series,” and has identified them on several other rivers. New Cycle Although the most recent delta-building period was probably at an end by 1868, many people believe that a new cycle has been set in motion by further destruction of plant cover by the settlers, Mr Cox added. The North Canterbury Catchment board was keeping a close watch on the position. The idea that the Canterbury Plains may have originally been forested was first put forward in 1911 by Professor R. Speight, of Canterbury University College, Mr Cox said. His evidence did not extend, however, to welldrained areas of low rainfall. The proposal was taken further by J. T. Holloway, of the Forest and Range Experiment Station, Ashley, who found evidence of an extensive forest on some deep soils and assumed on general considerations that there must also have been forest cover on the more shallow soils on the terraces. The Soil Survey group, however, has found the first direct evidence of this shallow-soil cover.

Mr Cox is in general agreement with the theory put forward by Mr Holloway that, although the Maoris destroyed the forest, a climatic change about 1200 A.D. led to conditions in which native trees were unable to regenerate on the plains (except in certain favoured places). Charcoal Nuggets The evidence found by the group was a scatter of woodcharcoal in soils throughout practically the whole of Paparua County. The potential importance of charcoal as indicating the former presence of forest had been indicated by the pioneer work of Mr B. Molloy on beech charcoal at Porter’s Pass. In most cases in the county, the charcoal was discovered in tiny nuggets no larger than apple-pips, which had to be washed out of large quantities of soil. The charcoal was usually at or near the surface, but in the delta area it was buried by more recent alluvium which did not contain wood-charcoal—one of the clues which led Mr Cox and his associates to the conclusion that these deposits must be much more recent than had previously been suspected. Charcoal has also been tracfed by the group on similar soils around the lower reaches of other Canterbury rivers, and in the Mackenzie Country near Lake Tekapo, where at one representative site large quantities of matai (black pine) charcoal have been identified. In some areas, the former presence of forest has been borne out by logs reported by the early settlers or the occasional discovery of a buried tree-stump. Most of the wood remnants have decayed long ago, the charcoal being generally much more resistant to disintegration. The wood from which the charcoal came was identified by Mr Orman as from kanuka (white manuka) scrub on the poorer land, and from a mixed forest dominated by matai on the deeper soils. Mr Cox thinks from general considerations that the kanuka may have been a second-growth replacement of beech; but has so far been unable to substantiate this.