Abstract

Quaternary geology in St. Croix County has never before been mapped at the detailed scale undertaken in this project. There is currently tremendous development in the western part of the county due to expansion of the Minneapolis-St. Paul metropolitan area. Mapping and characterization of surficial Quaternary deposits will assist development of an intelligent and resourceful zoning and development plan. Quaternary deposits in western St. Croix County are dominated by Pleistocene glacial deposits. The three main units of Pleistocene deposits in the county are Copper Falls, River Falls and Pierce Formations. Each formation contains glacial diamicton (most considered to be till) and associated proglacial sand and gravel, and most units also contain associated silty lake sediment. These Quaternary deposits were derived from several glaciations that took place from at least 430,000 years ago for Pierce Formation deposits to as recently as 15,000 years ago for the Copper Falls Formation, in addition to modern alluvial and organic sediments. The deposits of these formations are subdivided into members and further subdivided by landform type.

The till of each unit has a unique set of material properties and characteristics. These properties depend on the source of the material, glacial flow path, mode of deposition, age, and stress history. Each unit also has a characteristic range of geotechnical and hydrogeological properties. Copper Falls and River Falls Formation deposits have the same provenance and were deposited in similar ways. The till of these Lake Superior basin-derived formations has a hydraulic conductivity of about 10^-3 cm/s, and has moderate strength with an effective angle of internal friction of 14-37 degrees. Cohesion is almost lacking in these units, and there is some minor cementation. Till of the Pierce Formation is very cohesive (over 200 kPa), and contains a significant amount of clay with high activity, yielding plasticity indices between 17 to 54. Pierce till is over 450,000 years old, and was deposited in a period of reversed polar magnetism, comes from the northwest in contrast to the younger red till units from the northeast, and has a very low in situ saturated hydraulic conductivity of around 10^-7 cm/s.

Mapping of the deposits and determining characteristic properties allows visualization of the spatial distribution of the materials and their properties. This document will be useful for the county government and others who need to make educated decisions about land use planning.