Glacial deposits in the Mesabi-Vermilion Iron Range area consist of four major till units and associated glaciofluvial sediments. Particle-size data and pebble, heavy-mineral, clay-mineral, and percentage-soluble content were used in addition to field description of color and texture to describe and correlate the drift units.
The lowermost till unit, basal till, occurs in only a small number of mines, but the mines are scattered across the entire Iron Range. The till is dark gray to dark greenish gray and brownish gray, sandy, silty, and calcareous. Pebbles are largely granitic and metamorphic rocks of local origin, but include limestone, dolomite, shale, basalt, felsite, agate, and gabbro. Clay-mineral content is largely illite. The age of the till is probably middle or early Wisconsin but could be pre-Wisconsin.
The middle till unit, bouldery till, is the thickest and most widespread of the four tills. It is gray, yellow, red, orange, or brown, sandy, silty, noncalcareous and contains abundant cobbles and boulders. Pebbles are largely granitic and metamorphic rocks of local origin, but also include gabbro, basalt, and felsite in minor amounts. Montmorillonite is the most common clay mineral. Colored bouldery till below gray bouldery till may be a separate subunit distinguished largely on particle-size differences, but the overall characteristics are similar to the other bouldery till. The till was deposited by the Rainy lobe, which has a minimum age of 14,000 to 16,000 years before present.
The uppermost, or surficial, tills were deposited contemporaneously by two minor sublobes of the Des Moines lobe about 12,000 years ago. Brown silty till occurs in the western and north-central part of the study area. It is light to medium brown, sandy, silty, and calcareous. Pebbles are largely granitic and metamorphic rocks of local origin, but include limestone, dolomite, shale, basalt, felsite, and gabbro. Clay-mineral content is largely mixed-layered montmorillonite and illite.
Red clayey till occurs in the south-central part of the area. It is red to reddish brown, clayey, silty, and calcareous. Pebbles are similar to those in the brown silty till, but contain less limestone than the brown till.
Glaciofluvial sediments are common between the various till units throughout the area, but are thickest near the east and near the west ends of the Iron Range.
