We thank Gong et al., Heckert et al., and Bassett et al. for their comments on our discussion paper. Their varied responses are typical of the range of responses we have received informally and that have been published elsewhere (e.g., Ogg, cf. Edwards in Pratt, 2004), from enthusiastic support to outright disagreement. This represents, within the geological community, a surprising divergence concerning the relationship between two such fundamental phenomena as rock and time.

In response to Heckert et al. and Bassett et al., we agree that rock and time are separate phenomena, but then we reach different conclusions than them. Thus, we assume and safely believe that our sense of the passage of time and of the succession of processes that have created Earth as it is now derive directly from the rock record. This assumption holds whether we use the evidence of fossils, paleomagnetic properties, contained isotopes (stable or radiogenic), or even astronomically produced characters (for it is the orbital variations of a solid, spinning Earth that are captured by accumulating sediment).

Thus, stratigraphic geology is essentially a forensic science, and we use the evidence contained in the rocks to interpret how Earth has changed through time and to devise a time scale within which past events from different parts of Earth can be placed and interpreted. Geologists have found it most convenient, and most precise, to set up such a relative time scale using the wealth of evidence of elapsed events—biological, paleomagnetic, and so forth—in stratal successions: hence, the origin of the term stratigraphy, a term which now, though, encompasses all rock on Earth (Salvador, 1994; Rawson et al., 2002) and, as study of our nearest planetary neighbors continues, beyond. Further, the practical development of the global stratotype section and point (GSSP) principle is allowing the creation of a time scale with no gaps and no overlaps throughout the Phanerozoic and potentially embracing much of the Precambrian (Gradstein et al., 2004; see also Nisbet, 1982).

GSSPs, as noted by Heckert et al., are not immutable, nor is their position always determined by the purest academic motives. Nevertheless, whether regarded as spikes of gold, silver, or (to emphasize their potential for obsolescence) rusty iron, they serve as effective temporal reference points, the precision of correlation being the same for time as for time-rock units. If the position of a GSSP is moved, by due process of agreement, (or even if, say, a magnetic chron ceases to be recognized) then this does not mean that an interval of elapsed geological time somehow disappears. It simply represents a change in the way in which the geological community chooses to subdivide the passage of elapsed time. And if we agree that the Silurian and Devonian periods are contiguous, it follows that the GSSP for the beginning of the Devonian effectively represents also the end of Silurian time; the extent to which this should be formally recognized is, of course, a matter for discussion.

Given this, we see no necessity taking this stratal succession and making a separate time-rock scale of it as well as inferring a time scale from it. One can classify the rocks themselves in many ways: by their overall lithological characters (in lithostratigraphy); by fossil content (biostratigraphy); by relative position within an astronomical barcode pattern; and so on (and we agree with Gong et al. that this last approach offers real potential of providing shorter-term units). Any of these classifications may be subject to modification as our knowledge of the rock record is refined. We continue, therefore, to see no fundamental necessity, no imperative of logic to retain the dual time scale and do not agree that its abandonment would provide a fatal chink in our philosophical armor that creationists, for instance, could exploit.

We do, though, recognize that a case can be made for retaining a capacity to define strata by the relative time of their deposition, which is effectively what the chronostratigraphy branch of the dual terminology does. This is particularly the case for the biostratigraphical community, which is used to the dual terminology and finds it provides them with an effective shorthand means of communication. Thus, it is easier, certainly, to say the Arenig Series rather than strata deposited during the Arenig Epoch.

However, for geologists dealing with complex orogenic collages of igneous, metamorphic, and sedimentary rocks, as noted by Gong et al., the dual terminology is at best an irrelevance, and what is necessary here is to fit a diverse succession of geological events into a temporal and spatial framework to say what happened when and where. In such circumstances, a single, numerically calibrated scale of relative geological time is appropriate. Thus we disagree with Heckert et al.'s assertion that igneous plutons (still less, high-grade tectonometamorphic complexes) can easily comprise part of systems, stages, and so on. Similarly, a unified time scale would, we consider, facilitate interdisciplinary use of stratigraphic data by biologists, climate scientists, and others not familiar with the long history of stratigraphic nomenclature. Given the current urgency of providing effective past analogues for future environmental change, this seems no longer a minor consideration.

Thus, the perceived need for, and utility of, a dual versus a unified time scale seems to correlate with a geologist's perspective and context. We maintain that the most fundamental need is a single scale of geological time, abstracted from the rock record, and applicable to all rocks, whether stratified or not, and across all the sciences. It follows that the time–time-rock duality, as currently employed, is not symmetrical: time-rock functions essentially as a means of classifying stratified rocks by the relative time of their deposition and is most useful to those geologists working mainly on such successions. Producing an effective time scale and reconciling the working needs of different groups of scientists is the challenge that we face.

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