"When one observes the many and great changes that have occurred in some s since the beginning of the , he is inclined to the belief that a longer period of time was involved than he had been led to believe. is an example where lakes of great depth once existed in a locality that is now a true desert. Segments of giant fans, some faulted, have been built out on the dried-up lake basin. Some large s on the surface are so deeply weathered that a hammer blow will reduce them to . We are rapidly learning that certain are much more rapid than we have been wont to believe. But only recently have s attempted to determine by actual measurement process rates formerly the subject of general reasoning or speculation. Such measurement programs have documented the fact that rates of landscape change are greater than had earlier been suspected."