Physical features of the Des Plaines Valley . between them. On the map, where the direc-tion of the joints near the gorge is shown by a symbol, the discordancebetween the joints and the trend of the stream comes out plainly. Feeds woods ravine.—One of the prettiest and most instructive ex-amples of excavation by a tributary in the glacial drift is the ravinein Reeds woods, above Bush park (See Plate 7). There are severalfeatures to be observed here which, taken together, cannot fail to con-vince one that this deep ravine and its tributaries have been carved outwholly by the activity of rain an
Physical features of the Des Plaines Valley . between them. On the map, where the direc-tion of the joints near the gorge is shown by a symbol, the discordancebetween the joints and the trend of the stream comes out plainly. Feeds woods ravine.—One of the prettiest and most instructive ex-amples of excavation by a tributary in the glacial drift is the ravinein Reeds woods, above Bush park (See Plate 7). There are severalfeatures to be observed here which, taken together, cannot fail to con-vince one that this deep ravine and its tributaries have been carved outwholly by the activity of rain and running water. In the first place, the behavior of the creek and its little tributariesduring wet weather is significant. The main channel at such times maybe filled brim full or even to overflowing, so that the little flood-plainwhich forms the floor of the valley is under water. In its swollen con-dition the stream may be seen to carrv fine sediment in suspension andto roll sand and fine gravel alonsr the bed of the channel. Around the. GOLDTHWAIT.] HISTOKY OF THE LOWEK DES PLAINES. 61 outside of every sharp curve—and there are many such—the streamhas cut away its bank. At points where the channel swings againstone side of the valley, bare slopes of glacial drift may be seen, severalfeet high and very steep. After a rain it is not unusual to iind littlepillars of clay, capped by pebbles or other protective objects, aroundwhich the rain has excavated the fine cla3^ Obviously, with the wash-ing away of soil from exposed side slopes and from the channel bankthe ravine is changing form, be it ever so slowly. Material thus obtained is washed into the stream and swept down-valley (except such large pebbles and bowlders as cannot be moved),to be deposited sooner or later in the channel on the inside of somecurve, often directly opposite a place where cutting of the outer bankis going on. It is by this cut-and-filP process that the flood-plain hnsbeen built, for even now it i
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