LONGITUDINAL STUDIES 109 
The obstruction of the hard layer encountered always produces local 
swift water. Above this the water may be sluggish and the area reduced 
to the general level of the obstruction. In the case of rejuvenation the 
head of erosion proceeds upstream; the part of the stream above the point 
to which erosion has reached is sluggish and is sometimes called the pre- 
erosion stream. 
Of the rivers and creeks which we have considered, nearly all the 
larger ones are sluggish or pre-erosion in their upper courses. This is 
true of the DesPlaines, which is held in this condition largely by rock 
at Riverside. Hickory Creek (Fig. 66) is also of this type, the head of 
erosion being at Marley. In passing from source to mouth of such a 
stream we find formations arranged as follows: In the upper sluggish 
courses of all the streams mentioned we find (1) sluggish creek or 
river formations, (2) chiefly ‘swift-water formations below the sluggish, 
(3) chiefly gravel bottom formations below the swift-water formation, 
C 
ALP ge 
Fic. 66.—Diagrammatic profile of Hickory Creek: A, source; B, mouth; C, head 
of erosion; D, rock outcrop. The figures below refer to the columns in Table XXI 
and represent parts from which fish were collected. 
and (4) typical sluggish river formations farthest downstream where 
the vegetation, silt, and sand formations are arranged much as in 
the Fox River. 
Tables XVIII, XXI, and XXII and Figs. 67-69 show the longi- 
tudinal distribution of fishes in six streams. A few moments’ study and 
comparison of these tables will make the following facts evident: 
a) The only species in the youngest stream of the North Shore 
series is at the headwaters of all the others. 
b) The species found in County Line Creek are found in the same 
order in the upper courses of Pettibone Creek and Bull Creek; additional 
species are found farther downstream in the larger streams. 
c) The same species are at the headwaters of Thorn-Butterfield and 
Hickory creeks and in the upper courses of the North Shore streams. 
Other species are with them. The species of the North Shore streams 
are crowded together in these large streams which have permanent 
