POSTGLACIAL BIOTA OF THE GREAT LAKES REGION 179 



Insecta 

 Tenebrio calcidensis Fornax ledensis 



Byrrhus ottawaensis Phryganea ejecta 



At Pakenham, about thirty miles west of Ottawa, a deposit of fresh and 

 brackish water mollusks occurs. The section in which these shells occur con- 

 tains the following strata: 



1. Sand and surface soil, about 10 feet inches 



2. Clay 10 " " 



3. Fine gray sand with shells of Valvata, etc " 2 " 



4. Clay 1 " v 



5. Gray sand, laminated, with Tellina " 3 " 



6. Clay " 8 " 



7. Light gray sand, with Valvata, Spkaerium, Amnicola, Planorbis and 



TeUina " 10 " 



8. Clay 1 " 2 " 



9. Brown sand and layers of clay with Planorbis, Sphaerium " 4 " 



In the bank of a brook emptying into the Mississippi River, two miles 

 below Pakenham, shells occur in a deposit of sand and gravel corresponding to 

 number 7 of the above section. Eight species have been identified. 202 



Valvata Iricarlnata Planorbis parvus 



Amnicola limosa porata Galba elodes? 



Planorbis bicarinatus{ = anlrosus) Pyramidida cronkhitei anthonyi 



Irivolvis Tellina groenlandica( = Macoma balthica 



var. groenlandica) 



These strata doubtless represent different periods of time as well as varying 

 physical conditions, facts attested by the alternation of beds of clay and 

 sand. The locality was evidently at the mouth of the Mississippi River and 

 the water was fresh or only slightly brackish. The large volume of fresh 

 water which was constantly discharged thru the Nipissing outlet doubtless 

 freshened the sea for a long distance. This whole region possibly formed an 

 estuary. 



B. MONTREAL AND VICINITY 



At Montreal fresh water shells occur mixed with marine mollusks, in sandy 

 strata, immediately above the Leda clay. As suggested by Bell 203 these shells 

 might have been washed down from fresh water ponds on the top of Montreal 

 Mountain, which rose over 200 feet above the level of the Champlain|Sea. 

 This mountain is now 700 feet above the level of the sea and a number of 



JM Dawson, Can. Nat., V, p. 194; IV, p. 36. 

 SC3 Can. Nat., VI, p. 42. 



