I propose that Minnesota has three basic raw material resource regions -- western, eastern and southern (Figure 1). Each contains a different set of raw materials and different combinations of raw material sources. The boundaries between these regions are not clear, and no attempt need be made to define them clearly. The three region concept is a radical simplification of a complex situation, but it is the complexity of the situation which makes such a simplification useful. The three region concept is an analytical device, to be used with appropriate caution.

Click on Figure 1 to see the full size map.

This three region concept is based on Minnesota's geological history. The state contains a variety of rocks and sediments which range from recent to over 3 billion years in age. The surface throughout most of the state has been shaped by the geologically recent actions of glaciers, which tore rocks from their original contexts, mixed them together and spread them over the state. In addition, in some areas they exposed bedrock by scraping it clean of soil and other sediment. Locally the landscape has also been altered by subsequent erosion and deposition, which have both redistributed raw materials or exposed once buried raw material sources. Additionally, relatively small areas of southern Minnesota were not affected by recent glaciations; these areas contains bedrock exposures and lag deposits of raw materials. (2)

Of the several major episodes of glaciation in North America, the most recent had the most substantial effect on the present surface geology of Minnesota. This episode, known as the Wisconsin glaciation, ended here no more than 10,000 years ago. During the Wisconsin glaciation, several large ice masses advanced across the state. They originated primarily from two directions: northwest and northeast. The ice which came from the northwest (Des Moines and Wadena lobes) crossed Paleozoic sedimentary rock, mostly limestone and related carbonates. These glaciers deposited grey and brown drift across northwestern, western, central and parts of southern Minnesota. The ice which came from the northeast (St. Louis and Rainy lobes) crossed older volcanic, metamorphic and sedimentary rocks. These glaciers left reddish drift in northeastern and east central Minnesota (see Goebel and Walton 1979; Sims and Morey 1972:16-17; Wright 1972:520-546).

Because the surface materials in the western and eastern parts of the state have different origins, they contain different lithic raw materials. This leads to the division between eastern and western lithic resource regions. In the Western Resource Region, bedrock exposures are uncommon and glacial drift provides the only substantial raw material source. In the Eastern Resource Region, however, and especially in the northeastern part of the state, there are both glacial drift deposits and extensive bedrock exposures which serve as raw material sources. The Southern Resource Region is separated from the other two primarily because it contains additional raw materials. Both bedrock and drift sources are present. In this region, the bedrock sources tend to be more important than in other parts of the state, and glacial redistribution appears to be less extensive.

Briefly, the characteristics of the three regions are as follows. In the Western Resource Region bedrock outcrops are almost unknown and glacial drift represents the only significant local source of knappable lithic raw materials. The drift is derived from the northwest and consists principally of pulverized limestone and dolomite. These carbonates yielded a variety of raw materials. The drift also contains a smaller complement of raw materials derived from the west. At least some of these are present as a result of the incorporation of preglacial (or interglacial) fluvial sediments. (3) The Western Resource Region is characterized by extremely diffuse raw material sources. Primary geological contexts are known for only some of these materials.

The Eastern Resource Region contains both bedrock and glacial drift, both of which may have been significant sources of knappable lithic raw materials. The drift is derived from the northeast, from geological formations which still exist as outcrops in that direction. The lithology of this region is ancient, even in geological terms. Because of this long history, it is also complex. The drift is derived from a variety of igneous, sedimentary and metamorphic rocks. It contains a limited number of raw materials, but these raw materials tend to display an uncomfortable degree of variation. Primary geological sources may be inferred for most of the materials. It might initially seem that, because of the occurrence of bedrock outcrops, the Eastern Resource Region has both focal and diffuse raw material sources. While this may be literally true, in practice it is probably irrelevant. It is likely that any accessible bedrock source has also contributed material to glacial drift. For example, even in the presence of clear trace element signatures connecting an artifact to a particular source, in most cases it would be impossible to determine if the raw material for the artifact was derived directly from the primary geological context or from a secondary (via glacial redistribution) geological context. Therefore the Eastern Raw Material region is also characterized by diffuse raw material sources.

The Southern Resource Region presents yet another combination of raw materials and sources, one which is -- regrettably -- even more complex. The Southern Resource Region also contains both bedrock exposures and drift. Both represent important raw material sources. Much of the drift in the region originated from the northwest, and this region therefore contains many of the same raw materials that are available in the Western Resource Region. Collections of raw material samples suggest, however, that the drift also incorporates matrix derived from the northeast. The Southern Resource Region therefore also contains small amounts of the same materials that are available in the Eastern Resource Region. The materials which occur in regional bedrock were also incorporated to some extent in glacial drift. The drift distribution, however, tends to be less extensive for any given material because the materials have not been moved as far from their primary context; this may be because the region is on or near the southern margins of the most recent glaciation. The primary geological context of many raw materials can be deduced. In addition, the southwestern and southeastern corners of the state contain either no glacial drift or very old, dissected and eroded drift. These regions display a complex mosaic of loess mantled terrain, localized bedrock exposures, and possibly even localized exposures of older glacial drifts of uncertain origin. The Southern Resource Regions thus presents a mixture of focal and diffuse raw material sources. Essentially, in this region the distribution of each raw material must be examined separately. This also means that the context of raw material utilization for sites in this region is more particularistic, and must be examined almost on a site by site basis.

This concept of three raw material resource regions is obviously a simplification of a complex situation. The three regions blend and overlap in an intricate fashion (e.g., Chernicoff 1983); there are no clear boundaries between them, and no attempt is made to define clear boundaries. But, as mentioned above, it is just this kind of complexity that makes an appropriate simplification useful. The terms "western," "eastern" and "southern" are deliberately chosen to accommodate the unavoidable ambiguity, yet still provide a meaningful and manageable way of viewing raw material resources in context.

Within a region, all lithic raw materials are not of equal importance. Some materials are common, others are rare. Some materials were commonly used in prehistory, others only infrequently so. In the following discussion and in Tables 1, 2 and 3, "primary materials" are those which are commonly available as raw materials and also commonly found at archaeological sites. "Secondary materials" are less abundant as raw materials, and also less frequently found as artifacts. Finally, "other materials" are relatively uncommon either as raw materials or at sites.

(2) Because Minnesota's prehistoric inhabitants gathered lithic raw material from near the ground surface, we should be safe in following surface geology in shaping our understanding of raw material availability.

(3) A documented example of this kind is the Souris gravels of nearby southern Manitoba, which contain materials of Rocky Mountain origin (Klassen 1969:2-8). The redistribution of such sediments by glacial action may account for the rare occurrences of noncharacteristic lithic raw materials. This is, however, a minor raw material source.

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