32 THE ROYAL SOCIETY OF CANADA 



The Advance Movement. 



In the two periods during which an advance of the sea is recorded 

 the surface of the land area was probably well covered by a dense 

 vegetation. The earlier advance at the close of the Colorado stage 

 was not across a very wide area in Canada. In Montana a somewhat 

 broad land area seems to have risen to sea-level. In the second 

 advance in which the upper part of the Belly River series was covered, 

 a wide area of this surface had been for a long time slightly above the 

 sea and is now marked by a very important coal horizon, This 

 material had been but lightly covered by coarse grained deposits pre- 

 vious to the advance of the sea. There is little trace of brackish- 

 water beds between these coal seams and the marine shales. The 

 surface indicates a long period of tranquillity in which a belt at least 

 150 miles in width settled to about sea-level. The advance of the sea 

 across this plain with but slight further subsidence would be rapid, 

 if not there should be some erosion or further deposition on the original 

 surface. Evidences of this have not been observed except in the 

 western portion of the advance, that is, in the foothills. 



The Retreating Movement. 



After a period of subsidence, the erosion of the land was revived 

 by a further elevation which stimulated the shifting of the detritus 

 seaward. This elevation seems to have been of some magnitude 

 as it resulted in the return to shallow water conditions over a wide 

 area in the marine basin which was at some distance from the country 

 affected by the maximum uplift. The deposits which then covered 

 the marine silts in both of the retreating movements of the Montana 

 sea were distributed in brackish water. The period represented by 

 these conditions was not of short duration. In the first retreat the 

 brackish-water beds of the yellow portion of the Belly River forma- 

 tion are in places about 300 feet thick. In the second or final retreat 

 the brackish-water beds are found at intervals throughout 700 feet 

 of beds. It thus seems that while a vast amount of detritus was 

 carried eastward and many coal seams were formed on the flat plains 

 (of the Edmonton and Belly River formation,) the sea had not 

 made much progress in its retreat, so that unlike the advance move- 

 ment the retreat was very slow. 



Considering now the correlation of the shore deposits with 

 those of land or marine formations it would seem that the deposition 

 might, on the above assumption, i. e. slow retreat and rapid advance, 

 be illustrated by diagram somewhat as below. 



