212 Transactions. 



Art. XXIII. — The Geomorphology of the Coastal District of South- 

 western Wellington. 



By C. A. Cotton, Victoria University College, Wellington. 



[Bead before the Wellington Philosophical Society, 12th December, 1917 ; received by 

 Editors, 31st December, 1917 ; issued separately, 10th June, 1918.] 



Plates XIV, XV. 



Contents. 



Introduction. 



The Foundation or Old Land. 



The Coastal Lowland. 



Theoretical Discussion of the Growth of a Coastal Lowland 



under Conditions of Fluctuating Waste -supply. 

 An Alternative Explanation. 

 The Paekakariki Coast. 

 Another Alternative Explanation. 

 Subdivisions of the Lowland. 

 The Otaki Series. 



Topography of the Otaki Series. 

 Distribution of the Otaki Series. 

 Lithology and Structure of the Otaki Series. 

 The Fans or Gravel Plains. 

 The Delta of the Manawatu. 

 The Modern Dunes. 

 Lakes and Swamps. 



Introduction. 



The coastal district of south-western Wellington (fig. 1), forming part of 

 the fertile " Manawatu " and largely comprised within the limits of Horo- 

 whenua County, presents a considerable variety of physiographic features 

 all of comparatively modern growth* and explicable on an assumption of 

 a geological history of a somewhat unusual kind. 



I have been informed by Dr. L. Cockayne and Mr. A. H. Cockayne that 

 the ecology and agriculture of the district are closely related to the physio- 

 graphy, and I am tempted to present this somewhat sketchy description 

 and attempted explanation of the forms in the hope that it may be of use 

 as a basis for further studies. 



The Foundation or Old Land. 



The skeleton or foundation' of the coastal district of south-western Wel- 

 lington is the upland block of somewhat old rocks sculptured into strong 

 relief that forms the Tararua Mountains and the lower ridges farther south. 

 Lithologically these rocks are greywacke-sandstones with occasional bands 

 of argillite, the latter much sheared by earth-movements. f All are closely 



* Geologically the whole history of the lowland is comprised within a portion of the 

 Notopleistocene period. 



f The early geologists termed these rocks the " Rimutaka series," a local name that 

 might be now appropriately used for them. Subsequently to the issue of the geological 

 map of 1873 the name " Rimutaka " gave place to " Maitai," as it was believed that 

 the two formations were identical ; but it was used occasionally by McKay in the 

 " seventies " for the whole or part of the rocks of the Rimutaka and neighbouring ranges. 



