A Fossiliferous Till Found in New Ulm, Minnesota

• Main Author: Moe, Amy P.
• Format: Text
• Language: English
• Published: 1998
• Subjects: Cottonwood River; Frozen Lake; Ice Shelf
• Online Access: http://archives.gac.edu/cdm/ref/collection/irstudents/id/3481

A till exposed along an unnamed tributary of the Cottonwood River in New Ulm, Minnesota is found to contain a number of unbroken microfossils including numerous species of freshwater gastropods, pelecypods and oslracodes as well as marine forams and fish teeth. Also contained within the till are plant fossils and wood fragments ranging in size from small seeds and twigs to entire logs. The freshwater fossil assemblage and preliminary identification of the plant fossils indicates that the animals lived in a small lake or pond in a temperate, interglacial environment with temperatures similar to those of modern-day South Dakota. It is most likely that the marine microfossils and fish teeth were reworked from older Cretaceous sediment found to the northwest. Analysis of the sediment found at the New Ulm site indicates that it is a till that can be correlated to the pre-Wisconsinan tills of the upper mid-west such as the Whetstone Till, the Kandiyohi Till and the "old gray" till. The Whetstone Till of Grant County, South Dakota, overlies the Gastropod Silts (Gilbertson, 1990). At least three of the gastropod species found within the Gastropod Silts (Valvata tricarinata, Valvata sincera, and Gyraulus parvus) are the same as those found within the New Ulm site till. Three models of deposition for the till found at the New Ulm site have been proposed based on the results of this research. The first, the entrainment model, involves the entrainment of large chunks of frozen lake sediment into the shear planes of an advancing glacier. The sediment is deposited and mixed when the glacier retreats and the sediment melts out. The second model is an ice shelf model in which the glacier advances, creating an ice shelf over part or all of a small lake. Till is deposited beneath the ice shelf, through the water and mixes with the lake sediment on the lake bottom. Finally, the solifluction model is essentially a large land slide of till into the lake basin. The till, once again, mixes with the lake sediment on the lake bottom.