in Thoreau’s Journal:

Sat awhile before sunset on the rocks in Saw Mill Brook. A brook need not be large to afford us pleasure by its sands and meanderings and falls and their various accompaniments. It is not so much size that we want as picturesque beauty and harmony. If the sound of its fall fills my ear it is enough. I require that the rocks over which it falls be agreeably disposed, and prefer that they be covered with lichens. The height and volume of the fall is of very little importance compared with the appearance and disposition of the rocks over which it falls, the agreeable diversity of still water, rapids, and falls, and of the surrounding scenery. I require that the banks and neighboring hillsides be not cut off, but excite a sense of at least graceful wildness. One or two small evergreens, especially hemlocks, standing gracefully on the brink of the rill, contrasting by their green with the surrounding deciduous trees when they have lost their leaves, and thus enlivening the scene and betraying their attachment to the water. It would be no more pleasing to me if the stream were a mile wide and the hemlocks five feet in diameter. I believe that there is a harmony between the hemlock and the water which it overhangs not explainable. In the first place, its green is especially grateful to the eye the greater part of the year in any locality, and in the winter, by its verdure overhanging and shading the water, it concentrates in itself the beauty of all fluviatile trees. It loves to stand with its foot close to the water, its roots running over the rocks of the shore, and two or more on opposite sides of a brook make the most beautiful frame to a waterscape, especially in deciduous woods, where the light is sombre and not too glaring. It makes the more complete frame because its branches, particularly in young specimens such as I am thinking of, spring from so near the ground, and it makes so dense a mass of verdure. There are many larger hemlocks covering the steep sidehill forming the bank of the Assabet, where they are successively undermined by the water, and they lean at every angle over the water. Some are almost horizontally directed, and almost every year one falls in and is washed away. The place is known as the ” Leaning Hemlocks.”