igoo] DowLiNG — Red River Valley. 117 



great lapse of time. Along- the margin of the sea the waves and 

 currents were breaking up and carrying away the loosened parts 

 of the former land surface. The heavier material was left near the 

 shore to form the lower rocks which are mainly of sand, while 

 above are the deeper sea deposits; limestones. 



That this sea remained for a long time is evident from the 

 great thickness of the limestone beds laid down over its bed, for it 

 is generally supposed that limestone is not formed very rapidly. 



The commencement of an upward rise was probably about the 

 time of the great coal period. Traces of rocks formed at this time 

 are found in Minnesota but none so far in Manitoba. As this part 

 rose above the water it probably presented a very even surface or 

 that of a great plain sloping to the south-west, but the fact that 

 near the shore the beds were thinner than elsewhere would cause 

 them to be more easily fractured by any unequal movement of the 

 crust in the general elevation. 



There was a long lapse of time during which this part of the 

 continent remained above the sea and it is probable that in this 

 interval the surface of the limestone was worn away and brought 

 near its present contour. Along the eastern margin there was 

 probably a line of cliffs facing the east, and in front of this a line 

 of lakes or a river system the fore-runner of the Lake Winnipeg 

 basin. 



The next evidence of change in the elevation shows that the 

 next advance of the sea was caused by a much less depression 

 than in the previous case. In this instance the sea was shallow 

 and apparently the waters very muddy if we are to judge by the 

 amount of silt that was left by this submergence 



A preliminary sandy deposit showing the advance of the seals 

 succeeded by a great thickness of shale or hardened mud which is 

 characteristic of this later submergence. These shales not being 

 here subject to any great pressure except the weight of the upper 

 beds, are not hardened to any degree. 



Above these dark shales there is a lighter coloured series of 

 sands and clays holding a few seams of lignite, but as these de- 

 posits have been removed from most of the area in the vicinity of 

 the Red River valley they are merely referred to ; farther west they 

 are better developed and are of great econoniic importance. 



