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At Austin clays that appear to be Cretaceous overlie the eroded edges of the Devonian rocks. A quarter of a mile northwest of the Chicago, Milwaukee & St. Paul Railway station the Austin Brick & Tile Co. has opened a deposit 1 to 20 feet thick that underlies perhaps 93 acres. Although the clay is described as Cretaceous, it is somewhat modified and is disturbed apparently by glacial action, and its Cretaceous age is questioned by Sardeson.1 The clay is variegated in color and contains a few pebbles and rocks, especially where most disturbed near the surface. Glacial drift overlies it but is only a few feet thick. The irregular surface on which it rests is composed of
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sandstone and dolomite with a thin upper layer of blue clay rarely over 6 inches thick. (See fig.30.) The clay slakes [quality by which a dry lump of clay tends to absorb water and fall to pieces when immersed] in three minutes and has a very high plasticity [capability of being molded], requiring 31 per cent of water for molding. It shrinks 7 per cent on drying and has a tensile strength [maximum stress it can withstand before breaking]
of about 100 pounds to the square inch, even when rapidly dried. As burned at the Bureau of Standards it gave the following results:
| Cone No. | Color. | Shrinkage. | Absorption. |
| Per cent. | Per cent. | ||
| 013 | Salmon | High | 25 |
| 010 | …do | …do | 20 |
| 08 | …do | …do | 9 |
| 06 | Red | …do | 1 |
| 03 | …do | …do | 1 |
There was a little trouble from checking, but it was not serious. The porosity was low over a good range of temperature. The clay is hard but not melted at all the specified temperatures.
The factory has a capacity of 60,000 brick a day. Experiments with the different materials available for making high-grade products have yielded excellent results. The clay commonly burns red, but when mixed with the underlying Devonian rock it yields buff and cream-colored ware of equally high grade. The Devonian rock alone burns to a lime rather than to a clay product. To obtain the light colors, a fairly high proportion of the rock must be used. The lime may lower the melting point so much that the mixture is undesirable for making vitrified brick, but it is good for common brick. The 6-inch layer of blue clay between the bedrock and the Cretaceous has also been burned, but the deposit is too small to be of much value. It does not differ greatly from the main body of
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the clay. Both contain a little organic matter and need thorough oxidation in burning.
Source:
Clays and Shales of Minnesota
By Frank F. Grout
With Contributions by E. K. Soper
United States Geological Survey, Bulletin 678
Washington, Government Printing Office, 1919