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Superior and Des Moines Lobes

By
H. E. Wright, Jr.
H. E. Wright, Jr.
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Charles L. Matsch
Charles L. Matsch
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Edward J. Cushing
Edward J. Cushing
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Published:
January 01, 1973

The Superior Lobe of the Wisconsin glaciation was initially localized by the deep lowlands of the Lake Superior Basin, cut in relatively nonresistant late Precambrian red sandstone. It advanced southwest out of this lowland and crossed a low divide leading to the Minneapolis Lowland, which is underlain by Precambrian and Cambrian sandstones. The conspicuous drumlins of central Minnesota, and the rugged St. Croix Moraine that borders the drumlins on the west and loops across the Minneapolis Lowland, delimit the major stillstand of the Superior Lobe. Discovery of red drift with diagnostic rock types from the Lake Superior Basin (agate, amygdaloidal basalt, red and purple felsite, red sandstone) in southwestern Minnesota indicates that the Superior Lobe once extended farther southwest down the Minneapolis Lowland to the Minnesota River valley and beyond during a pre-Wisconsin or early Wisconsin glaciation.

As the Superior Lobe wasted from the St. Croix Moraine, a series of sharp subparallel tunnel valleys were cut into the drumlin plain and even into the underlying bedrock by subglacial streams driven to high velocity by the hydrostatic pressure resulting from the load of many hundreds of meters of active ice. Subsequent thinning and stagnation of the Superior Lobe opened the tunnel valleys to atmospheric pressure and converted the subglacial streams from major erosional streams to small depositional streams, which formed discontinuous eskers along many of the tunnel valleys.

After distant retreat the Superior Lobe readvanced three times out of its basin, twice after proglacial lakes had produced a supply of red clay to be overridden. These latter two readvances may represent surges of the ice lobe resulting from the buildup of basal meltwater behind the frozen toe of the ice lobe.

The Des Moines Lobe, originating in the Red River Valley of Manitoba and western Minnesota, moved southeastward down the Minnesota River valley and thence northeastward up the Minneapolis Lowland, overriding a segment of the St. Croix Moraine and extending across the state to Wisconsin in the form of the Grantsburg Sublobe. Its drift is characteristically gray to yellowish-brown and highly calcareous. The ice incorporated masses of Superior Lobe drift as it overrode the St. Croix Moraine, stringing them out to produce a complex of foliated red and gray drift.

The Grantsburg Sublobe blocked the Mississippi River and other drainage from central Minnesota to form glacial Lake Grantsburg about 16,000 yrs ago. By that time the Superior Lobe had withdrawn completely from central Minnesota, for it supplied meltwater (and red clay) only on the east, down the St. Croix River and its upper tributaries.

Meanwhile, the main Des Moines Lobe, which thus far supplied ice only to the Grantsburg Sublobe, thickened sufficiently to spill southward out of the Minnesota Valley across a low divide into Iowa. This produced the lobe that reached Des Moines 14,000 yrs ago. Beheading of the Grantsburg Sublobe in this manner caused the stagnation of the latter, which then wasted to form the Anoka Sandplain in its stead.

The St. Louis Sublobe protruded from the Des Moines Lobe in northwestern Minnesota at a later date (about 12,000 yrs ago). Its meltwater flowed down the St. Louis River toward Lake Superior, but it was diverted southward into the St. Croix drainage by the still-existing Superior Lobe. Final wastage of the entire Des Moines Lobe produced glacial Lake Agassiz in northwestern Minnesota and adjacent North Dakota and Manitoba.

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Contents

GSA Memoirs

The Wisconsinan Stage

Robert F. Black
Robert F. Black
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Richard P. Goldthwait
Richard P. Goldthwait
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H. B. Willman
H. B. Willman
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Geological Society of America
Volume
136
ISBN print:
9780813711362
Publication date:
January 01, 1973

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