All is not lost if the ice core does not contain an ash layer, or if the sediment core has shells that are too old for the 14C method. If a relative stratigraphy can be established, it may be possible to tie that relative stratigraphy to an absolute time scale.
Fossils are one way to establish a relative stratigraphy.
Specific fossils have lived at specific times. If this second fossil group has been tied to the absolute time scale, the scientist now has a date for his assemblage. This other fossil assemblage may not have been dated directly, but it may be in association with an ash layer. This carrying of time from one location to another by identifying the same fossils in both locations is called biostratigraphic correlation, and is an extremely valuable tool for dating sediment cores, and other sedimentary features. The microfossils contained in sediment cores are particularly useful because the assemblages change quickly through time.