Adkin. — The Horoirhenua C'oasfal I'lain. ]09 



South of the Manawatu Gorge the former shore-line [i.e., the eastern 

 or inner margin of the coastal plain) lay along the western border of the 

 old land, and was very irregular and indented, the sea wiudinii in and out 

 round the projecting spurs. Sea-cliffs were cut along the old-land margin, 

 and these in places are in a good state of preservation, notably near 

 Paekakariki, Otaki, and Ohau, and between Shannon and Tokomaru. 



As shown by the altitude of the upper edge of the coastal plain on the 

 Palmerston-Pahiatua Road (see above table), it is evident that prior to its 

 initial emergence an arm of the sea ran through the Manawatu Gorge and 

 spread out in the Woodville-Pahiatua-Dannevirke district to form a shallow 

 harbour or estuary, into which the Manawatu River flowed. " Lacustrine 

 deposits " are reported to exist in the district defined, the supposed former 

 lake being due to the ponding of the Manawatu River by an unusually rapid 

 uplift of the mountain axis.* This theory is now shown to be erroneous ; 

 the supposed lake was in reality an estuary, and the so-called " lacustrine 

 deposits " are doubtless estuarine deposits. 



Dissection and Topographic Development of the Horowhenua 



Coastal Plain. 



Since its uplift above sea-level the coastal plain has been very thoroughly 

 dissected by the rivers and streams that extended their courses across it, 

 assisted by others that take their rise within its borders. Extensive 

 stretches of plain unbroken by the marks of stream erosion, which at first 

 must have been the predominant feature, no longer exist. Instead, irregular 

 strips and areas of flat open plain are intersected by a network of water- 

 courses. These flat areas, however, are not the original surface of the 

 coastal plain, but denuded surfaces corresponding to the original one. The 

 erosion by rain, for instance, uniformly lowered the surface of large areas, 

 the resultant silt being washed into the channels of the then less-developed 

 stream-systems. 



The rivers, the larger of the minor streams, and the lesser ones have all 

 left their distinctive marks upon the plain, producing a diversity of surface 

 forms. The present topography therefore varies from youthful to mature. 



The rivers, the former fan-builders, in their passage from the hills to 

 the sea rapidly cut into the soft marine beds (or sandstone, as they may 

 briefly be designated), and swept them clear away, exposing their fans, 

 without excejjtion, to a greater or less extent. 



The usual work of the larger of the minor streams was to carve wide 

 shallow cliannels in the sandstone, and later to aggrade their bottoms 

 with fine alluvium, forming flood-plains often of considerable fertility. 

 Bordering the courses of these streams the topography of the sandstone 

 has often reached a mature stage — of low relief and smooth curved 

 contours. 



The lesser minor streams are for the most part tributaries of the larger 

 ones. Many take their rise in the plain, and occupy narrow youthful 

 channels divided by broad flat-topped sandstone ridges. Others of brisker 

 flow, with their sources in the hills, have cut in the sandstone formation a 

 descending succession of wide-flaring, flat floors, usually free from even 

 a veneer of alluvium, and opening out at different levels into the valley 



* D. Petkie, Account of a Visit to Mount Hector, &c., Trans. N.Z. Inst., vol. 40, 

 p. 290, 1907. 



