Abstract

Artificial lowering of Steep Rock Lake, Ontario, over 100 feet exposed thick glacial clays, deposited in a lake, named Lake Johnston, which was dammed in the west by an ice lobe. Lake Johnston stood mostly 100 feet above Steep Rock Lake and was in existence for more than 1250 years. The clays were laid down in water depths of 125 to 275 feet. During midsummer, glacial Lake Johnston may have had three temperature zones, viz. a proximal belt with the coldest water (below 39.2°F.) at top; a middle one with uniform temperature of about 39.2°F. (4°C.), or about the greatest density, from surface to bottom; and a distal zone with the warmest water (above 39.2°F.) at top. The middle, isothermal, belt had different width during the different lake stages.

The clays fall naturally into eight series. Sixty feet of them consist of typical simple varves (annual deposits); 40 feet are abnormal with greatly variable winter clays and with subordinate clay laminae in the summer deposits. The laminations were determined by many factors and conditions. The coarse, silty mud fractions settled rapidly. The winter clays consist of material which remained suspended in the lake at the end of the melt season; they are at times missing. The clay laminae in the summer component were probably deposited during cold spells with so little ice melting that no coarse mud, no silt, was brought to the site. The mud of such clay laminae was brought previously and was carried to great depth by circulation in the isothermal lake belt, so that it had only a short way to sink. Summer clay laminae may thus have required alternate marked warm and cold spells and uniform temperature and deep circulation of the water in a zone of the lake. A summer clay lamina and an underlying silt lamina form a couplet which often resembles a thin true varve. Both winter and summer clay laminae vary greatly in absolute and relative thickness.

There are also current laminae of coarse and fine silt which represent day-and-night or weather changes of a few to several days.

Four major laminations have been distinguished: Simple varves, drainage varves, composite varves, and unvarved laminations. The two latter types are, generally speaking, rare and abnormal, and consequently a broad isothermal lake zone and greatly variable, or intermittent, mud inflow must have been exceptional.

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