Physical features of the Des Plaines Valley . lls to be long maintained. By the recession of the rapids upthe run, a steep-walled gorge was cut in the rock. At its mouth thegorge is equal in depth to the cliff cut by the main river in the sill,but toward the head of the gorge its depth decreases. Where so coarsea load is gathered and must be carried by the stream, it is forced to 58 THE DES PLAINES VALLEY. [BULL. NO. 11 maintain a steeper slope than that of the old bed rock surface. Inthe park the bed of the stream has been obscured by the constructionof two dams to form artificial ponds. Near
Physical features of the Des Plaines Valley . lls to be long maintained. By the recession of the rapids upthe run, a steep-walled gorge was cut in the rock. At its mouth thegorge is equal in depth to the cliff cut by the main river in the sill,but toward the head of the gorge its depth decreases. Where so coarsea load is gathered and must be carried by the stream, it is forced to 58 THE DES PLAINES VALLEY. [BULL. NO. 11 maintain a steeper slope than that of the old bed rock surface. Inthe park the bed of the stream has been obscured by the constructionof two dams to form artificial ponds. Near the high cement bridgethe gorge is cut about 20 feet deep in rock. The limestone there is thin-bedded, cherty, and very much cracked hy joint planes. Just above thebridge, near ithe upper dam and the trestle of the scenic railway, is afine natural exposure of the limestone, coated with lichens. The over-hanging cliff is due to a slightly inclined joint crack. The strata dip10° to 15° toward the southwest, witli local warpings. One may see. Fig. 13. Diagram illu-^trating the way in which the up-river valley migrationof falls or rapids on a river affects its tributaries. The falls ha\e been worn backfrom near the front of the diagram past tributaries a and b. and are now-ap-proaching c. When tliey pass c. the mouth of the side stream will be suddenlylowered from the level of the crest of the falls to that of the base, forming a fallor rapids there. This has just occurred at b. These falls or rapids will then re-cede up the side valley, forming a branch gorge. This stage in the process hasjust been reached at ft, but has been passed at a. here the way in which the much jointed rock on the cliff face is wedgedand loosened by frost and decay, falling, piece by piece, into the the process seem too slow to account for the sculpturing of thegorge, we must remember that the age of the gorge is to be measured,not in tens, but in thousands of years. Farther up the run, be
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