Diachronic approaches to text analysis have given rise to a wide range of methods, including historical discourse analysis (Berkenkotter, 2009), historical pragmatics (Jucker, 1995), historical sociopragmatics (Culpepper, 2009), historical speech acts (Jucker & Taavitsainen, 2008), and more recently, diachronic English for Specific Purposes (Alonso-Almeida & Marrero-Morales, 2011; Banks, 2012). Such approaches have long been influential in promoting the idea that socio-historical context is important to consider when describing the linguistic and rhetorical features of specialized writing in English for specific, academic and research purposes. Other noteworthy contributions include Bazerman’s (1988) study on the development of writing in physics, Salager-Meyer’s (1999) study on changes in referential behavior in medical research writing, Atkinson (1999) and Salager-Meyer’s (1994) studies of the evolution of medical written discourse, and Banks’s (2008) analysis of the development of English scientific rhetoric across specific journals. These studies have been instrumental in helping frame our understanding of scientific writing. They have also proposed methods that prove useful for examining the harder-to-see features of specialized writing, such as its implicit, “insider” meaning. Not having access to this implicit content can impede less experienced or non-native writers’ ability to gain a foothold in their respective fields. As argued elsewhere (Dressen-Hammouda, 2008), becoming a proficient writer in a specialized area does not happen just because one has been exposed to the relevant features of specialized genres in the ESP classroom. Rather, learners must also incorporate in-depth contextual knowledge about the socio-historically situated processes by which experts in their fields produce specialized writing. Unquestionably, socio-historical insight into the implicit features of specialized writing can make a considerable contribution to teaching methods, making them both richer and more grounded, thus benefitting ESP teachers and trainee-teachers, and by extension, their students. While ethnographic and qualitative approaches to writing research have become increasingly popular of late as a means of providing such explanations (e.g. Barton and Hamilton, 2000; Lillis & Curry, 2010; Paltridge, Starfield and Tardy, 2016), diachronic analyses also provide important tools, notably by situating current text features within past contexts, and showing how they emerge from those contexts. They can also provide insight into which features become “silenced” over time. For example, while some rhetorical acts, such as making frequent and explicit references to one’s personal research actions, are no longer acceptable in modern scientific writing, diachronic analysis provides evidence that such content may still be present in the writing, albeit tacitly; specialist readers glean it from the text thanks to their ability to “read between the lines” and “fill in the blanks”. Being unable to readily identify and reproduce such tacit knowledge can prove to be a serious obstacle for less experienced or non-native writers. An important task in ESP, EAP and ERP research is therefore to help reveal such hard-to-identify implicit features. The research described in this chapter shows how a diachronic approach can help reveal the implicit content of disciplinary writing. To illustrate the approach, the analysis will focus on the discipline of geology, examining how geologists, both past and present, tend to both write about, and silence, details of their field research. A helpful concept that can also be used to frame the analysis of implicit writing features is indexicality (Dressen-Hammouda, 2014). The next section will briefly describe this concept. Drawing on the comparative, diachronic analysis of two corpora, a four-step methodology based on Hymes (1972) is then described. Indexical analysis, the method used, is useful for identifying specific implicit, indexical structures in geology writing by distinguishing between what is “formally possible”, “contextually appropriate”, “possibly implied” and “actually attested” (see Hymes, 1972). In conclusion, it will be argued that diachronic studies are the foundation for carrying out indexical analysis. In effect, by diachronically reconstructing the emergence of disciplinary and professional discourses and practices, indexical analysis provides meaningful insight into the implicit frames of reference that operate within and give meaning to academics’ and professionals’ written discoursal practices.