November, 1918] 



The Ottawa Naturalist 



85 



or a little above low water. But it is seldom found 

 in water of any depth. Exposure to the air between 

 tides appears to be as essential to the vigorous growth 

 of this shell as it would be fatal to the majority of 

 shells. The presence of M. edulus in these beds in 

 abundance thus affords evidence of their origin in 

 very shallow water. Macoma baihica is also a 

 shallow water species occurring according to 

 Whiteaves "usually at or a little below low water 

 mark." The presence of Saxicava rugosa in the 

 fauna has but little significance regarding the depth 

 of water in which the fauna lived since this species 

 is now found living in Canadian waters from low 

 tide level to a depth of 50 feet. Temperature 

 appears to be the chief factor which controls the 

 vertical distribution of this shell. While it is never 

 found in very shallow water in the Gulf of St. 

 Lawrence, in Arctic and sub-Arctic waters it has 

 been found living near low tide level. The associa- 

 tion of this cold water-loving shell with such a 

 typically intertidal species as M. edulus suggests 

 colder climatic conditions, since water sufficiently 

 shallow to be a satisfactory habitat for M. edulus 

 in our present climate would have too high a tem- 

 perature to accommodate 5. rugosa. 



A noteworthy feature of this fauna is the absence 

 from it of most of the shells which are most common 

 in the fossil fauna found in the widely distributed 

 Pleistocene blue clay. The blue clay fauna repre- 

 sents species which were contemporaneous with the 

 fauna of the sand beds but which occupied a differ- 

 ent bathymetric zone relatively deeper as well as 

 distinctly different in the character of the bottom 

 materials. The species most commonly met with in 

 the clay beds include the following shells: Pori- 

 landica arctica, Saxicava rugosa, Macoma calcarea, 

 and Nucula tenuis. 



The collector of fossils is often puzzled by the 

 very marked contrast in the relative abundance of 

 fossils which the same type of sediments display at 

 different localities. While the marine sands may be 

 extremely rich in sea shells at certain localities like 

 the Rideau river sand pits they may at other 

 localities be entirely barren of fossils. One of the 

 reasons for such barren areas is doubtless the ten- 

 dency of sands to move rapidly under current action 

 and smother the marine life which attempts to live 

 on them. Dr. G. A. Huntsman who has been 

 engaged in studying the conditions under which 

 marine animals live in the Gulf of St. Lawrence 

 has directed attention to another factor in producing 

 lifeless zones. He states*: 



"By means of these traps we discovered that a 

 barren zone existed off the Cape Breton shore, 

 comprising the part of the sloping bottom between 



Canadian Fi^iherman, May, Uill 



the depths of 10 and 20 fathoms. In this zone the 

 temperature at the bottom underwent violent fluctu- 

 ations often in the course of a day or so, at one time 

 being as high as 65 F., and at another as low as 

 39' F. This was caused by the winds for when 

 the wind was blowing on shore it drove the surface 

 water against the coast and heaped it up, forcing the 

 deeper colder water down, then when it changed 

 and blew off-shore the warm surface water was 

 driven away from the coast and the cold water 

 welled up from below to take its place and so 

 flooded the zone. The effect of this on the slow 

 moving bottom animals may be imagined. Few of 

 them would be able to stand such changes, but the 

 active fishes are able to move up and down the 

 slope and avoid these changes." 



It is probable that we can safely ascribe to the 

 variable temperature factor some of the paucity of 

 life which in many places characterizes the Pleisto- 

 cene clays as well as the sands of the Ottawa 

 valley. 



THE instability OF SHORE-LINES. 



The advance or retreat of shore-lines results from 

 two distinct causes. Elevation or subsidence of the 

 land through the action of deep-seated forces within 

 the earth is a very slowly acting but potent agent 

 in changing geographical features. The second 

 great factor in making new shore lines is the sea 

 itself which is everywhere either cutting away or 

 adding to existing shore lines. The rapidity of this 

 constructive and destructive work of the sea varies 

 enormously according to the hardness of the rocks 

 and the behaviour of waves and currents, as well as 

 the topography of the shore. 



Everywhere along the Atlantic coast of America 

 the first named factor has wrought enormous 

 changes in the shore line since the close of the 

 Glacial epoch. A profound subsidence of the land 

 ill eastern America which accompanied or followed 

 the disappearance of the glacial ice sheet brought 

 the sea far inland along all the great valleys leading 

 to the sea coast (fig. 2). At one time during this 

 marine invasion it has been estimated that the sea was 

 at least 200 feet deep over Parliament hill. The 

 re-elevation of the land and withdrawal of the sea 

 in eastern Canada and New England was the last 

 great geological event of the Pleistocene epoch. 



Elevation or subsidence of the land, although 

 the most powerful factor in producing the major 

 features of coastal geography operates with extreme 

 slowness and is subject to long periods of inactivity, 

 while the sea in revising its boundary never ceases 

 work for a single year. Enormous changes are 

 sometimes wrought in a very short period where the 

 shore line is composed of sand or clay. Sable 

 island off the east coast of Nova Scotia furnishes 



