SEDIMENTS IN THE DELTA OF ERASER RIVER 127 



laminated, but not so definitely as the sandy bands. The rate of 

 deposition in the Pitt Lake Delta is not definitely known, but 

 probably is very small, for a comparison of a number of soundings 

 taken in 1920 with those taken in i860 shows no great difference. 

 The soundings taken at the point, about i mile from the front of 

 the delta, where sample No. 16 was obtained, showed that deposi- 

 tion had taken place to a depth of 5 to 10 feet during the past 60 

 years, or at the rate of i or 2 inches a year. The rate of deposition 

 near the front of the delta is probably much greater. The beds 

 probably show therefore both tidal lamination and tidal banding. 

 They apparently do not show seasonal banding. From the fact 

 that there is very little current in the entrance to Pitt Lake during 

 parts of May, June, and July when the Fraser is in flood, it might 

 be expected that this period would be indicated in the bottom 

 samples by a thick layer of silt. During this period, however, the 

 turbid water of the Fraser does not enter Pitt Lake to any extent 

 and the lake water is nearly clear. 



Bottom sample No. 16, taken at a depth of 50 fathoms, is from 

 the bottom-set beds of the delta, in the deepest part of Pitt Lake, 

 near the south end. It is composed of silt and clay and differs 

 markedly in the character of its stratification from the fine-grained 

 samples from the bottom-set beds of the Fraser Delta in the 

 Strait of Georgia in that it is thinly and gradationally laminated. 

 The lamination is probably due to tidal action and to the absence 

 of flocculation of the fine material in suspension in the fresh water, 

 which permits the silt and clay to settle to the bottom at different 

 rates. 



Bottom sample No. 17, taken at a depth of 35 fathoms, is from 

 the bottom-set beds or from near the contact of the fore-set bottom- 

 set beds of the delta at the north end of Pitt Lake. It shows in 

 places a tidal lamination and probably also a seasonal banding. 

 There is a fairly regular recurrence of silty layers J to i inch, 

 separated by sandy layers i to 2 inches thick. The silty layers 

 were probably deposited during the freshet when the surface of 

 the lake did not rise or fall appreciably with the tides, and the sandy 

 layers during the periods when the lake was affected by the tides; 

 for at ebb tide the currents in the streams flowing through the 



