In many cases, both the impression and the counterimpression of a given trackway were found and were photographed separately. In these cases, an asterisk is given next to the UCM number in column 1, and in this column the UCM number of the matching slab is given. In most cases, the impression and counterimpression were photographed in succession. However, sometimes different people found the impression and counterimpression of the same trackway, or the two halves were not photographed in succession. As many of these were matched up as could be noticed. Pairs of impressions and counterimpressions constitute about 30% of the database.