The thickness of the ice during the time of maximum glaciation in New England was sufficient to fill up the valleys and overtop the highest mountains. The Green Mountain range was completely submerged by a gigantic ice stream flowing southeast from the Adirondack region, across the Champlain valley, over the mountain range and on across New Hampshire to the Atlantic coast. The upper surface of the ice, as in the case of the continental glaciers of Greenland, may have had a uniform slope; and as this surface had an elevation so far above the mountain tops that the direction of the flow was not affected by the topographic features of the country, bold as they were and still are, the depth of the ice over the valleys could not have been less than 7,000 or 8,000 feet. Dana estimates the height of the ice over the Laurentide mountains at 10,000 feet. In Iowa we have no mountains to aid in estimating the thickness of the Kansan ice. But this is known; the flow of glacial ice depends on the slope of its upper surface or rather on the average gradient of the entire mass. The Greenland glaciers have a surface slope varying from 30 to 200 feet to the mile. It is scarcely possible that ice would flow if the gradient of the surface were much less than 30 feet to the mile. Now the Kansan ice flowed across Iowa and down to the latitude of Jefferson City and Saint Louis, Missouri. The movement from central Iowa was toward the southeast, nearly in the direction of a line drawn from Des Moines to Saint Louis. The distance from Des Moines in that direction to the southern margin of the drift is about 250 miles, and a gradient of 30 feet to the mile would make the surface of the ice at Des Moines 7,500 feet higher than at the margin. The present difference in elevation between Saint Louis and Des Moines is about 250 feet. The difference may have been greater then than now, but it certainly never much exceeded 500 feet, which would leave the thickness of the ice over the present site of Des Moines 7,000 feet. Reduce the gradient to 20 feet per mile, which is below the limit at which energetic ice flow is possible, and the thickness of the ice at Des Moines cannot even then be estimated at less than 4,000 feet. Carry this same

slope northward to the sources of the ice streams, and instead of feet, the thickness will be measured in miles.

An immense amount of detrital material, varying in dimensions from finest rock flour to bowlders eight or ten feet in diameter, was transported by the Kansan ice and strewn over the whole glaciated area to form the present mantle of Kansan drift. In Iowa the Kansan drift is largely composed of blue clay, but other materials such as sand and gravel, disseminated pebbles, and multitudes of bowlders, enter into its composition. A large proportion of the pebbles and bowlders are fragments of crystalline rocks derived from Archæan and Algonkian areas in northern Minnesota, Wisconsin and Michigan. Some may have come from the Hudson Bay region away beyond the national boundary. A few are of local origin and represent the limestones and sandstones of more immediate neighborhoods. One of the most interesting that has been seen by the writer is a mass of native copper, thirty-two pounds in weight, the property of Lt. Gov. Dungan, which was found in the drift of Lucas county. This copper was brought by the glaciers from Keweenaw Point in the upper peninsula of Michigan. There are two flattened sides to the mass, and both show the effect of abrasion consequent on its long journey beneath a sheet of ice thousands of feet in thickness. The distance from Keweenaw Point to Chariton is about 500 miles in a straight line. But it is known that the glaciers did not follow straight lines. Their course from Keweenaw Point was southwest, following the basin of Lake Superior, into northern Wisconsin and Minnesota, then southward and finally southeastward to Chariton. The direct course was not the direction of least resistance nor the direction of maximum slope.

The bowlders of the Kansan drift are small as compared with those of some of the other drift sheets of Iowa. Not many exceed a foot in diameter, and specimens ten feet in diameter are exceedingly rare. Many of the bowlders are granites, but those that are most common and most character

* This piece of copper was presented by Lt. Gov. Warren S. Dungan to the Historical Department of Iowa, where it is now on exhibition.

Granite bowlder of Iowan age near the southeast corner of the Public Park, Nora Springs, Iowa Length 30 feet, width 25 feet, height above ground 12 feet.

istic of the Kansan stage are dark colored, basic eruptives popularly known as greenstones. Furthermore, a large proportion of the Kansan bowlders are planed and scratched on one or two sides as a result of having been dragged along over the subjacent rocks while firmly imbedded in the lower surface of the moving ice.

There is no direct measure of the length of the Kansan stage, but it is certain that the time must be expressed in thousands of years. It was long enough for bowlders embedded in the lower surface of the ice to be transported through a distance of 600 to 1,000 miles. The Lucas county copper travelled more than 600 miles. The rate of flow of a glacier depends on a number of factors, such as the gradient of the surface, the depth of the ice and the temperature of the air. When the ice has great depth, as in the case of the Pleistocene glaciers, the flow at the surface may attain a rate of speed equal to 50 or 100 feet a day. But the base of the glacier, retarded by friction of the bed, moves more slowly than the surface. Depth of ice, by increasing the weight tends to increase the friction at the base. The multitude of rock fragments with which the lower surface of the glacier is studded, cutting into the underlying rocks, tends still farther to emphasize the effect of weight and retard the flow at the bottom. On the steep slopes of the Alps, where the temperature is relatively high, and the thickness and consequent weight of the ice is comparatively small, the daily motion at the base, as shown by Tyndall, does not exceed three or four inches. Allowing that the movement at the base of the Kansan glaciers equalled the rate observed in the Alps, allowing the rate to be doubled, or more than doubled, it still took many thousand years for sub-glacial bowlders to accomplish a journey of 600 or 800 miles.

5. Buchanan Stage. The extreme severity of climate that characterized the period of domination of the Kansan glaciers, at length relaxed. The glaciers melted and Iowa was once more released from fetters of ice. Then followed a second interglacial stage which is here provisionally named Buchanan, but which may be found equivalent to that called