A River’s Work.

Star (Christchurch), Volume XLIV, Issue 579, 8 October 1932, Page 8

A River’s Work.

Nature Notes

By

James Drummond, F.L.S., F.Z.S.

gO ME ROCKY RIVERBEDS in New Zealand have large round holes, like cauldrons. This geological feature, common in many countries, is more important than appears at first. It is a type of erosion, the holes helping rivers to deepen their channels in hard rock that resists the rivers’ mighty will. Even learned geologists who use hard words and dark sayings, stoop to call this phenomenon by one of the homeliest words in the English language. The holes simply are potholes. Their former name, giants’

kettles, was discarded when old-time fancies were thrown overboard. But they stand for a phase in one of the grandest spectacles the world offers, the continual warfare between water and the flinty rock. A river that wishes to deepen its channel in this way seizes a boulder, a stone, or a large pebble and swirls it in eddies for a long time in one place. The face of the rock is ground, and a hole is made. When the hole is large enough, the river may throw in more grinders. These, by united action, may enlarge the hole until it is many feet deep and several yards wide. Boulders may be worn away by the grinding, but others are available. Many a rocky gorge bears old potholes high above the present level of the river that flows at the bottom. The work, in these cases, seems to have been stopped by the river withdrawing its power, or by a pothole becoming choked with sediment. A river, by the direct impact of swift currents in rapids, may gouge holes in bedrock without tools; but a river’s best work always is done by using the grinders, bombs and cutters it carries along with it.