ABSTRACT

Small circular pit-like depressions mostly in fine-grain sediments have long been interpreted as rain-drop impressions. As early as 1850, however, Desor expressed the opinion that most, if not all, supposed rain impressions are made by air bubbles rising through sediments. From experimental studies, Twenhofel (1921) reached "the opinion that the impressions which have been ascribed to falling rain are of such origin in only a small percentage of cases." Twenhofel (1932) also found it extremely doubtful that they really occur so abundantly as the literature indicates.

Despite these views, the practice of ascribing these impressions to the action of falling rain drops has continued and the geologic literature abounds with examples of such interpretations. Observations on drying muds both in the field and in the laboratory show, that most, if not all, so-called rain-drop impressions are, as Desor correctly interpreted them in 1850, the result of air bubbles rising through sediments. Most of the processes that have been invoked to account for these impressions are improbable.