. The top of the continent; the story of a cheerful journey through our national parks. d rock, said Uncle Tom, smiling. Withtools, of course. Tools? cried Margaret. What tools has theriver i Rocks and sand, said Uncle Tom. Sand is therivers principal cutting tool. The hard, angular littlegrains of sand are swept rapidly down-stream by thefast current, each grain scratching the rock on thebottom as they all roll and tumble along. Billions ofbillions of sand grains keep scratching the rocks dayand night, century after century. The river is like astrip of sandpaper two thousand miles long, perpe
. The top of the continent; the story of a cheerful journey through our national parks. d rock, said Uncle Tom, smiling. Withtools, of course. Tools? cried Margaret. What tools has theriver i Rocks and sand, said Uncle Tom. Sand is therivers principal cutting tool. The hard, angular littlegrains of sand are swept rapidly down-stream by thefast current, each grain scratching the rock on thebottom as they all roll and tumble along. Billions ofbillions of sand grains keep scratching the rocks dayand night, century after century. The river is like astrip of sandpaper two thousand miles long, perpetu-ally wearing down the bottom. Then, too, the stonesand loose rocks help by bumping along with the cur-rent, denting the rivers bottom and sides, and break-ing off pieces here and there. These loose rocks arecontinually making more sand, too. Dont you remem-ber those pot-holes in the rocks that we saw in Glacierand Yosemite ? See, theres a big one here in this rock. Oh, yes, Jack exclaimed. I remember you toldme that loose rocks cut those big holes that lookedlike giants 234 THE TOP OF THE CONTINENT **Now, Jack, I was the one who said they lookedlike giants bathtubs, protested Margaret. **Well, I didnt say you werent, did I? Jacksnapped. **This pot-hole, said Uncle Tom, **was made bythe current pushing a loose rock around and aroundinside that hole, making it deeper and wider yearafter year. **But however did the river make this dreadful bigcanyon? asked Margaret. **0h, yes, said Uncle Tom. Now, listen. Amillion tiny streams in Colorado and Idaho and Utahand Arizona are grinding down and scooping out theirvalleys, and carrying each its little burden of muddysediment into the Grand and Green Rivers, whichunite to form the Colorado River. All this sedimentthe river industriously sweeps down into the , right here in the Grand Canyon, are manystreams, like Bright Angel Creek, which we saw fromthe hotel, which continually work their way deeperinto the rocks, and
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