APPENDIX No. II. THE COAST NEAR THE MOUTH OF THE RIVER ASHBURTON.
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APPENDIX No. II.
THE COAST NEAR THE MOUTH OF THE RIVER ASHBURTON.
AFTER having written the Report on the Canterbury Plains, Mr. H. Cridland made me acquainted with some facts concerning the mouth of the River Ashburton and the sea shore near it, which I considered so interesting that I at once proceeded there in order to convince myself of the difference between the mouth of the Ashburton and that of the Rakaia, as pointed out to me by that observer.
The cliffs which form the sea shore near the first-named river are, as stated previously, about fifty feet high. They consist entirely of river-shingle and gravel closely packed together and imbedded in yellow somewhat ferruginous sand and loam, with here and there a layer of sand, from one inch to two feet in thickness between them, but only of very small horizontal extent.
This shingle is capped by a bed of loam or loess often four to six feet thick, whilst the sea in some localities near highwater mark has disclosed the existence of patches of a harder brownish conglomerate, which seems sometimes to dip towards west.
A closer inspection of these beds has shown that they are composed exactly of the same river gravel and shingle as the higher beds are, and that the matrix or sand which fills up the interstices between the shingle and gravel is more highly ferruginous than the upper beds, and that this alone may account for their greater hardness.
The sesquioxide, and probably the protoxide, of iron, which form the hardening matrix of these conglomerates, may have been derived from the ferruginous water of springs, or of swamps and marshes, where a formation of similar chemical combination is still going on without interruption to the present day.
Thus, for instance, we observe along our rivers, where a small watercourse of a ferruginous nature runs over the gravel and shingle banks of the main river at its confluence, that these latter become cemented by the same process. They become at the same time impermeable to water, and the existence of such beds in other parts of the plains below the upper shingle may be of great importance in well-sinking or boring, as the surface or leakage waters, not being able to sink lower, will collect upon them.
These cliffs near the Ashburton not only are generally vertical, but at some places overhanging, and are washed by the sea in high tides, the beach shingle reaching to the very base of the cliffs.
Thus it is evident that whilst at the Rakaia the land has risen so far as to bring the vertical banks of the sea shore out of the reach of the tides, here, near the mouth of the Ashburton, the land has been either sinking, or been at least stationary, since the latest changes in the level of Banks' Peninsula have taken place.
Consequently this, for the latest era at least, whilst it ought to modify somewhat our views concerning the rising of the east coast line, shows the more distinctly, as the rising of the coast near the Peninsula north as well as south is undeniable, that this volcanic system, emerging from the sea, forms the centre of a local oscillation.
The knowledge of such a fact is of the greatest importance, as it will give additional strength to the deductions offered in the foregoing Report concerning the rivers which have their mouths close to that Peninsula.
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CHRISTCHURCH:
Printed, under the authority of the Provincial Government of the Province of Canterbury, at the 'Press' Office, Cashel-street, by James Edward FitzGerald, Official Printer for the time being to the said Government.
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