Geology has always centered around an effort to decipher records. Until 50 years ago these records were almost exclusively those of the constructional processes, conspicuously those made by sedimentation. Erosion has always been a great destroyer of records. Down to the last half century it was scarcely thought of as offering any compensation by recording a history of its own.

Within that time its records have been analyzed with increasing insight. They are made rapidly and in great abundance but always at the surface. Hence, they are much like characters written on wax tablets, always destroyed to make way for newer letters. Only occasionally is a tablet discarded and buried and the writing thus preserved. Such a record is an unconformity. Erosional history mentions relatively few large facts of early geologic date, but of recent events the account is very full, even more so than the parallel account written in . . .

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