63a W. A. JOHNSTON 



area a number of beaches rising well above 1 .200 feet have been found 

 by Mr. Leverett, who states that bars of gravel and sand formed 

 by the waters of Lake Agassiz oeenr on the highest points of Bel- 

 trami Island. 1 



The lacustrine deposits of Lake Agassiz in the district occupy 

 areas oi considerable extent and are in places at least ^o feet thick. 

 They are generally even-bedded but not strongly laminated. In 

 places they are characterized by an irregular alternation of sandy 

 and clayey layers and occasionally thin gravelly layers. The 

 beds are in places more sandy toward their base than in their upper 

 portion, and are frequently ripple-marked but not cross-bedded. 

 The material is more oxidized than that of the older laminated 

 stony clays, and there can be little doubt that the material was 

 derived from erosion of land surfaces by wave and stream action. 

 The sandy ripple-marked beds underlying clay, and the occurrence 

 of gravelly layers interbedded with sandy and clayey layers are 

 explained by the fact that the sediments were deposited in a rising 

 body of water. The lacustrine beds are also characterized by the 

 presence, in their lower portions at least, of fossil fresh-water shells. 

 Fossil fresh-water shells also occur in some of the beach ridges at 

 various altitudes up to 1,149 feet, or 88 feet above Lake of the 

 Woods. 



Unconformity at the base of Lake Agassiz sediments. — The evi- 

 dence found in Rainy River district, which confirms Tyrrell's con- 

 tention that Lake Agassiz had at first a rising stage, is based largely 

 on the fact that the sediments deposited in Lake Agassiz rest 

 unconformably upon the underlying deposits; that is, a period of 

 erosion intervened after the deposition of the calcareous till and 

 associated laminated stony clay, and before the later lacustrine 

 sediments were laid down. 



This is well shown in numerous sections exposed along the 

 Rainy River and around the shores of the southern portion of Lake 

 of the Woods. Fig. 1 illustrates the character of one of these 

 sections which lias been exposed by wave erosion on the present 

 shore of Lake of the Woods at its southern side. At the base a 



'Frank Leverett, "Surface Formations and Agricultural Conditions of North- 

 west Minnesota," Minn, Gtol. Soc., Buil. No. 12, 1015. p. 37. 



