Examination of the criteria employed by ALLEN (1963) in his classification of cross-stratified units suggests (a) that an additional criterion, the three-dimensional shape of cross-strata, should be used in describing and recognizing varieties of cross-stratification, (b) that the character of the lower bounding surface of sets should be expressed in descriptive rather than genetic terms, and (c) that in describing the relation between cross-strata and lower bounding surfaces, an additional term, asymptotic, should be used.
The recent discussion of the morphological features of cross-stratification by ALLEN (1963) represents a significant advance in our appreciation of the variability of this type of sedimentary structure. The valuable classification of McKEE and WEIR (1953) generated a concern for precise description of cross-stratification which has, in 10 years, made the classification inadequate. Allen's proposed classification, based as it is on observable morphological criteria and utilising a nongenetic terminology, should promote further rapid progress.
Allen has discussed the problems that arise in naming varieties of cross-stratification. He very properly avoids terms with genetic implications and his use of Greek letters results in commendably brief names. Almost inevitably, his chosen nomenclature is not readily applicable in the field due to the difficulty of remembering which letter is associated with which assemblage of morphological features. This difficulty will increase as the number of defined varieties increases. It seems probable that workers will prefer for the most part to give names to varieties of cross-stratification only in the laboratory where reference to literature can be made. The basis for recognition of varieties will be detailed field notes and photographs of morphological features. Nor is this undesirable, for it necessitates the making of detailed observations in the field which will lead to the recognition of new varieties. An easily remembered nomenclature too often leads to the suppression of significant differences in the interests of “pigeon-holing” the structure in question.
It is therefore important that all significant morphological features should be noted. Consideration of the differences between certain varieties of cross-stratification.