What makes it so hard?
Because it is full of invisible chalk.

In every gallon of that water there are, perhaps, fifteen grains of solid chalk, which was once inside the heart of the hills above.

Day and night, year after year, the chalk goes down to the sea; and if there were such creatures as water-fairies--if it were true, as the old Greeks and Romans thought, that rivers were living things, with a Nymph who dwelt in each of them, and was its goddess or its queen--then, if your ears were opened to hear her, the Nymph of Itchen might say to you-- So child, you think that I do nothing but, as your sister says when she sings Mr.Tennyson's beautiful song, "I chatter over stony ways, In little sharps and trebles, I bubble into eddying bays, I babble on the pebbles." Yes.

I do that: and I love, as the Nymphs loved of old, men who have eyes to see my beauty, and ears to discern my song, and to fit their own song to it, and tell how "'I wind about, and in and out, With here a blossom sailing, And here and there a lusty trout, And here and there a grayling, "'And here and there a foamy flake Upon me, as I travel With many a silvery waterbreak Above the golden gravel, "'And draw them all along, and flow To join the brimming river, For men may come and men may go, But I go on for ever.'" Yes.

That is all true: but if that were all, I should not be let to flow on for ever, in a world where Lady Why rules, and Madam How obeys.