Speight. — Terrace-development of Canterbury Rivers. 23 



Overlying the outer portion of the flat valley are the gravels 

 of the Canterbury Plains. They rise in the Waimakariri fully 

 200 ft. above the wide floor of the upper gorge, and are found at 

 places in the gorge itself.. The plains are formed by the over- 

 lapping and coalescing of the fans of great Pleistocene and 

 Post-Pleistocene rivers, and have covered up nearly all irregu- 

 larities in the solid floor of the land ; though at such places as 

 Gorge Hill, Burnt Hill, View Hill, the older rocks are visible 

 above the level of the plain. Owing to the rivers cutting down 

 through the gravels the solid floor has been exposed in other 

 cases. In this gorge portion the terrace- development is most 

 perfect. In the Rakaia, sixteen terraces maybe counted from 

 the top of the heaps of morainic matter down to the level of the 

 river — that is, in a height of 500 ft. The other river-valleys 

 show similar phenomena, though perhaps the sequence is not so 

 complete. 



It seems highly likely that this portion of the river-valley 

 was filled by gravels up to a certain level previous to the glacier 

 maximum, as the moraines and fluvio-morainic deposits overlie 

 the gravels at the mouth of the gorge. This filling-up might 

 have happened several times during the Tertiary era, as our 

 valleys were largely excavated before the Oligocene period, as 

 emphasized by Captain Hutton, and it is possible for a glacier 

 to override even loose gravels at its terminal face without dis- 

 placing them. Some of the lower gravel-beds just below the 

 Rakaia Gorge are so highly coloured by hydrated iron- oxide 

 as to afford an easy means of distinguishing them from the 

 upper gravel-beds. This points to a considerable lapse of time 

 in order to allow for this oxidation, and suggests a much older 

 date for the lower gravels. However, this evidence is by no 

 means conclusive. The fact that the glacier deposits overlie 

 the gravels of the plain is important, as showing that the exten- 

 sion of the glaciers was subsequent to the deposition of the 

 gravels in this upper portion of the plains. 



Terraces near the Gorge. 



An examination shows that a great majority of the terraces 

 in this part of the river's course are connected in some way 

 with obstructions met with by the river as it cut its bed through 

 the lacustrine silt just above the gorge proper, or through the 

 gravels of the plains just below it. A number in the gorge 

 itself are rock-cut terraces covered with a thin veneer of gravel. 



As nearly all terraces are the remains of former flood plains — 

 whether they are cut terraces or built terraces, or formed by a 

 combination of both processes — any circumstances which tend 

 to preserve these flood plains will favour terrace-development. 



