504 Tranmctions. 



When the river had reduced its bed to an easy grade, and had wandered 

 over its completed fan and valley deposit, changing its course perhaps for 

 centuries, the country began to sink. This subsidence affected the whole 

 of New Zealand (which was, immediately prior to this depression, of con- 

 tinental dimensions), and, by reducing the elevation of the country, put an 

 end to any permanent snow^fields or glaciers which may have existed in 

 this part of the North Island during the " glacial period." In the North 

 Island this reduction of elevation certainly amounted to more than 500 ft., 

 the base of the Ohau fan deposits being fully that depth below the 

 present sea-level. If during the period of great elevation Cook Strait was 

 occupied by a river, the Ohau must have been one of its tributaries, and the 

 inclined Pliocene plain over which it flowed the side of the old Cook Strait 

 River valley. As Cook Strait is at its southern end (where the mouth of the 

 lost river probably lay) considerably over 100 fathoms in depth, it would 

 seem that an uplift of 1,000 ft. would be necessary to restore it to its 

 original form — i.e., an open and spacious valley. It is therefore safe to 

 assume that the elevation of the southern part of the North Island was 

 reduced by at least 1,000 ft. 



The downward movement of the land was not sudden or convulsive, 

 but slow, gradual, and perhaps imperceptible. The sea began to inundate 

 the western border of the Pliocene plain, and in the course of time washed 

 the edge of the Ohau fan. The subsidence continuing, the exposed area 

 of the fan became less and less, until it was finally covered, and the sea had 

 advanced a little way up the Ohau Valley. The subsidence then ceased. 

 The usual marginal deposits of sand were laid down against the western 

 flanks of the Tararua foothills, marking- the limits of the waves, mud and 

 the finer material being deposited in deeper water at some distance from 

 the shore. 



In those days four small islands — Florida Islet, and Muhunoa, Kuku, 

 and Poroporo Islands — lay off the coast of Horowhenua. Their approxi- 

 mate areas were 20, 70, 260, and 670 acres respectively. Their former 

 insularity is shown by the presence upon their flanks of fragmentary patches 

 of uplifted sea-beach deposits. On Florida Islet the latter has been best 

 preserved, and is now elevated 360 ft. above sea-level. At the present time 

 the former islands exist as isolated hills of the ancient rocks of the district. 

 In the early Pleistocene, Florida Islet appears to have projected through 

 the Ohau fan in a manner exactly like the isolated hills of the Upper 

 Canterbury Plain. 



A period of repose was followed by one of elevation, a movement which 

 probably continues at the present day. The land rose with the same slow, 

 gradual motion as it had previously sunk This elevation, however, differed 

 from the previous subsidence in that it was local, the uplilted sea-margin 

 lying between Paekakariki and Wanganui. The sea receded and the sandy 

 beach widened, so that in the course of time it formed a plain of marine 

 deposit sloping toward the sea. 



At that time the Ohau River flowed in a north-westerly direction across 

 the newly uplifted coastal plain. It flowed in a wide, shallow valley 

 between banks of the soft marine sandstone, and had for its bed the surface 

 of its old fan. After cutting away the soft sandstone, the river formed 

 in the surface of the fan a large number of shallow channels, varying from 

 1 ft. to 5 ft. in depth, and averaging 1 chain in width. These channels wind 

 about a good deal, but their general trend is to the north-west. 



