Stoux City dcademy of Science and Letters. 139 
passed maturity and is advancing toward old age. Its 
main streams meander broadly, and its general level has 
begun to be lowered, while yet it has many sharp, narrow 
divides, and steep, narrow, crooked valleys. This condi- 
tion is the result of the erosive forces acting upon the 
surface in conjunction with the peculiarities of the under- 
lying materials. 
Materials on which Topography has been developed.— 
The former plain, perhaps gently undulating, sloped 
very slightly to the southeast. Its surface was treeless, 
the material covering it being a mantle of loess, 30 to 60 
feet thick. Underlying this loess is a clay, gravel, or 
sand, and in some places a shale or soft, friable sand- 
stone, below which alternate clays, sandstones, and 
shales for more than 500 feet. A vigorous drainage has 
been actively at work upon this plain, and, finding that 
the surficial materials were easily cut and removed, the 
major stream has reached a depth at which it is “graded” 
and is widening its valley, co-operating with its tribu- 
taries in the removal of the waste that is gradually being 
brought down from the upland plain. 
The loess is noted for its characteristic ability to 
cleave vertically and to remain standing thus for a con- 
siderable time if not undermined by erosion or solution. 
It is thus capable of receiving and retaining every line 
and curve of the carving of its drainage. An experiment 
that illustrates the capability of the loess to stand verti- 
cally may be performed with loess itself or with flour or 
some such fine, homogeneous material, by packing a heap 
of it upon two adjoining pieces of cardboard, 8x12 inches, 
lying upon a table. If the cards be moved cautiously 
apart, the mass will separate, and except for a slight 
avalanche of material from the looser top parts, the 
divided portions will retain almost perpendicular faces 
where broken apart. 
The sides of the broad trough that the Missouri has 
cut into the plain remain throughout much of the ex- 
tent as almost perpendicular bluffs or as an abrupt 
escarpment, rather than as a broad, open flare such as 
would result in a valley cut in easily broken down mate- 
rials. It is to the characteristic properties of the loess 
that are due the narrow, crooked divides, the sharp, nar- 
row spits extending out into the bottom land, and the 
