Physical features of the Des Plaines Valley . e the lower part of it would be left in a beheaded con-dition, the Flag creek of today. Ko positive evidence, however, of rivercapture at this place has been found; nor does there seem to have been, any reason for piracy. Salt creek, below Fullersburg, seems to havehad no advantage over Flag creek, either as to the length of its courseor as to the structure it encountered. So the rather singular escape ofSalt creek from the broad valley seems to be best explained merelyas one of the freaks of glacier-made drainage, inspired by an irregu-larity of t
Physical features of the Des Plaines Valley . e the lower part of it would be left in a beheaded con-dition, the Flag creek of today. Ko positive evidence, however, of rivercapture at this place has been found; nor does there seem to have been, any reason for piracy. Salt creek, below Fullersburg, seems to havehad no advantage over Flag creek, either as to the length of its courseor as to the structure it encountered. So the rather singular escape ofSalt creek from the broad valley seems to be best explained merelyas one of the freaks of glacier-made drainage, inspired by an irregu-larity of the newly exposed surface of drift. LAKE STAGE. While the ice sheet was withdrawing from the Valparaiso moraineto its next position, waters gathering along its border in the Chicago <)8 THE DES PLAINES VALLEY, [BULL. NO. U district began to assi;me the outline of a crescentic lake, glacial LakeChicago (Fig. 11.) The overflow of this lake escaped across the Val-paraiso moraine along the line of the lower Des Plaines Fig. 16. Part of Lake Chicago during the Glenwood stage (copied, with somemodification, from Alden). Outline of Des Plaines bay inferred from the 50-footcontour on the map of the Sanitary district. Hachures show wave-cut and river-cut bluffs; dots show beaches and spits. A long shallow arm^ of the lake, two or three miles wide, reachednorthward from Oak Park up the depression between the west ridge andthe Valparaiso moraine to the vicinity of Franklin park (Fig. 16).Further north the inter-morainic depression was probably a long sloughy GOLDTHWAIT] HISTOEY OF THE UPPER DES PLAINES. 69 on which the upper Des Plaines, fed largely by the melting of the iceto the north, and subject to overloading with Avaste, followed an ill-defined, braided course. As the ice border melted back, leaving thislong bay or slough, outwash gravels accumulated on its floor to a depthof several feet. These deposits of stratified gravel are from 10 to 13feet thick ne
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