482 EUGENE WESLEY SHAW 



The upper surface of the clay forms a terrace which is generally 

 so broad and so low that it is scarcely perceptible, though it is 

 commonly separated from the flood-plain by a low scarp. This 

 terrace is almost perfectly horizontal, and since the flood-plain 

 rises up stream the terrace and flood-plain finally merge. However, 

 since the flood-plain itself on the tributaries is nearly horizontal 

 (for the streams have but little fall) the flood-plain and terrace 

 on some rivers are distinct for 40 miles or more, although vertically 

 they are almost nowhere more than 40 feet apart. 



Another characteristic of these valleys is that in places they 

 anastomose. Many valley floors connect through divides with 

 neighboring valley floors. Some of the connecting parts are broad 

 and resemble bays in the sea; others are narrow and strait-like; 

 and the severed parts of the divide are massive. In many places 

 the flat valley floor surrounds hills that stand up sharply like 

 islands. These features of the lower parts of valleys tributary 

 to the Mississippi and Ohio — the broad bottoms in hilly country, 

 and the irregularly branching valleys — point toward valley filling. 

 And well-sections and exposures support this indication, showing 

 that bed-rock is far below the present streams. 



Detailed description of the clay. — The clay varies from greenish- 

 gray to purplish-gray in color and from medium plasticity to 

 "gumbo." The lower part is evenly stratified and in places 

 finely laminated. The upper part has less distinct stratification 

 and is characterized by irregular concretionary masses of lime. 

 Around the border and in the up-stream parts of the deposit 

 there are lenses of fine sand, but considering the formation as a 

 whole, sand forms a remarkably small part. With the exception 

 of the concretionary lime, some particles of which are as small as 

 sand grains, most of the deposit is without perceptible grit. In 

 ground plan the bodies of clay are very irregular and even anas- 

 tomosing — shapes that would be expected of valley fills in a country 

 of medium to low relief (see Fig. 1). The surface of the clay in 

 each valley is horizontal and lies from 5 to 75 feet above low water. 

 But the altitude varies from valley to valley. Near Cairo the 

 surface of the clay is 345 feet above sea; at Galena, Illinois, 400- 

 miles up the Mississippi, it is 650 feet; and there is a corresponding 



