46 THE ROYAL SOCIETY OF CANADA 



Belts of Lower Cretaceous sediments occur at both the upper 

 and lower ends of the canon. 



Tertiary rocks occur only in the lower part of the river valley 

 and constitute an ancient delta formation extending from Chilliwack 

 to the sea. 



Glacial and recent deposits of unconsolidated material occupy 

 relatively small areas in the valley bottom and in patches on its sides. 



Topographic Description 



The portion of the Fraser valley known as the caiion lies in 

 southwestern British Columbia, the broad physiographic features of 

 which have been so well described by Dawson and others in papers and 

 the reports of the Geological Survey. 



The main features of this part of the province are briefly as follows. 

 Commencing approximately at the International Boundary line 

 and extending northward through the central part of the province 

 is the great Interior Plateau region. In its southern part the Plateau 

 has a width of about 100 miles and a general average elevation of 

 about 4,000 feet above the sea. Its northward extension has been 

 defined for about 500 miles where its altitude declines to about 3,000 

 feet. The period at which the plateau features of this region began to 

 be developed has been referred to the early Tertiary times, and its 

 subsequent modification is due to the great extrusions of lavas and 

 later still to the dissection by river erosion consequent to re-elevation 

 of its surface. The plateau is traversed in all directions by deep valleys 

 which have been incised to a depth of 2,000 feet below its general 

 level and which carry the drainage of the southernmost part of the 

 plateau southward through Okanagan valley to the Columbia river, 

 and of the central part westward by the Fraser river to the sea. 



To the west of the Interior Plateau region lie the Cascade and 

 Coast ranges. The former, continuing northward across the Inter- 

 national Boundary line from the State of Washington, extends up 

 along the eastern side of Fraser valley gradually decreasing in height 

 until at Nicola river it merges with the plateau region. 



The Coast range, commencing on the opposite side of the Fraser 

 river and running northwesterly parallel to the Pacific Coast lies in 

 almost the same axis as the Cascade range, and is overlapped by it 

 at the Fraser river for a length of about 75 miles. 



Between the overlapping parts of the two ranges is a narrow gap 

 by which the Fraser river finds its way through a deep narrow cafion 

 from the interior to the coast. This gap is the only break of any signifi- 

 cance in the continuity of the mountain ranges bordering the Pacific 



