Physical features of the Des Plaines Valley . river, which was determined thusearly by original inequalities of slope on the newly exposed bay floor, ac-counts in large measure for its present course and the width of the valleywhich it has excavated. While the lake stood at the Calumet level, theriver began its work of excavating a channel along its course, graduallyentrenching itself below the bay plain. Later, as the lake waters fell andthe base level of erosion for the river was lowered, the entrenchment ofthe river progressed to greater depth. The consideration of this matteris left for a
Physical features of the Des Plaines Valley . river, which was determined thusearly by original inequalities of slope on the newly exposed bay floor, ac-counts in large measure for its present course and the width of the valleywhich it has excavated. While the lake stood at the Calumet level, theriver began its work of excavating a channel along its course, graduallyentrenching itself below the bay plain. Later, as the lake waters fell andthe base level of erosion for the river was lowered, the entrenchment ofthe river progressed to greater depth. The consideration of this matteris left for a later section. GOLDTHWAIT.] HISTOBY OF THE UPPEE DES PLAINES. to The Calumet shoreJine.—Tlie margin of the lake at this stage crossedthe plain of the Des Plaines hay ohliquely from Austin to portion of the lake shore is marked in some places hv a beach slopeOi moderate definition. South of Twelfth street (where it crosses Austinavenae near the ^McKinley school) the ridge-like form of the beach be-comes more and more marker!.. Fig. 18. Part of Lake Chicago during the Calumet stage. (Modified fromAlden.) An interesting geographic response to the shore topography of thisold lake stage is seen in the many old farm houses and barns along thecrest of the ridge. The early settlers on this poorly drained Chicago 76 THE DES PLAINES VALLEY. [bull. no. 11 plain located their homesteads on the dry, sandy beach ridges, A char-acteristic old-time farm house, now almost in ruins, stands at the cornerof Austin avenue and Twelfth street. Numberless examples of this sortmight be given. At Charles Beckers picnic grove (Twenty-second street), where theLa Grange trolley car crosses the beach, the characteristic profile—along, gentle front slope and a short, moderately steep back slope, isplainly seen. Here, as is frequently the case, the ridge is lined with oldoak trees, and was formerly followed by a highway. From this pointsouth for a mile or more a road follows the beach (See
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