Harris, section 15

Rudwick (1985): Chapter 3 – Unraveling the Earth, p. 49-60

The reading is from Rudwick’s book on the “Devonian Controversy” of the 1830s-40s and serves as an introduction to stratigraphic geology in the early 19th century, although it (naturally) emphasizes aspects of significance to the book’s main story.

This chapter discusses the common geological practices and points of consensus among geologists of that era. You can skip to the middle and later parts of this chapter (p. 46-60) – but feel free to browse the rest.

Section 3.2 deals with the kind of visual displays that were developed to communication the sense of three-dimensional array of strata (geognosy).  Section 3.3 points out some of the practical challenges in geological mapping in areas of complex structure. Initially the need was to identify structures and even bedding (in areas with well-developed cleavage overprinting primary beds). As this was worked out, some geologists (most notably Élie de Beaumont) stressed the orientation of major geological features and structures, adding structural patterns to lithology and fossil content as major criteria for unraveling field relations. Note that these features were first conceptualized and identified by mapping rock distributions and subsequently worked into more general interpretations.  The final section (3.4) includes some notes on the development of the geological column and the challenges of the Transition units.

Rudwick mentions various stratigraphic units in the text. Here are some of them, along with their placement in the current geological time scale.

Werner	Unit	Period
Tertiary	Tertiary
Secondary	Chalk	Cretaceous
	Oolite	Jurassic
	New Red Sandstone	Triassic
	Coal Measures	Upper Carboniferous
	Carboniferous Limestone	Lower Carboniferous
	Old Red Sandstone	Devonian
Transitional	“The Greywacke”	Cambrian to Silurian

The next reading presents an account of how stratigraphical geology developed and was practiced. It describes the work by Sedgwick and Murchison on the “Transitional” strata of Wales. These were the great challenge of the 1830s-40s; Secondary strata were relatively easy to correlate and interpret because they were less deformed and formations easier to identify. As you read this selection, please do not get too bogged down in all the details but focus on a few basic issues: (1) the difficulties of field work on interpreting relations in deformed sections of relatively uniform strata; (2) the problem of prioritizing criteria for correlating strata – structural orientation, mineralogy/lithology, and fossil content were all under consideration; (3) the meaning, characteristics and use of stratigraphical “systems”; and (4) the impact of geological frameworks in guiding an individual’s stratigraphic interpretations.

Secord (1986): Chapter 3 – Cambrian and Silurian Established, p. 90-109

Secord’s book examines the development of the Cambrian-Silurian controversy. This selection focuses on the early stage when Murchinson and Sedgwick were working together. It illustrates the difficulty of working with these sections – note the problems involved in evaluating field relations, faunal differences or similarities, and the relative value of different criteria. (He also points out mistakes made – thus on p. 81: “here the fatal error was made.” Well, no one died but it would ultimately be fatal to their friendship.) Again, a lot of detail here – so here are some notes to help guide you through this.

• The early parts of this chapter can be read “lightly” – there is a lot of detail in here about how Murchinson and Sedgwick worked to develop their interpretation. The point is to get a sense of how this was done – what criteria and field relations they used, how they addressed each other’s interest.
  • p. 69-80: Preliminary discussions about linking the work on the upper (Murchinson) and lower (Sedwick) parts of the Transitional interval. Recall that the Transitional strata were largely lumped into one thick package that was hard to correlate due to lithology (miles of slate in spots) and complex structure (in modern terms, this was all deformed during the closure of the Iapetus). The question was how to link work starting at the top (Murchison) and bottom (Sedwick) of the pile
  • p. 80-90: Their critical joint field work and how they managed to reconcile their ideas (setting the stage for future problems) by selecting a limestone bed (the Bala Lst) as the  boundary between their intervals (Secord’s “fatal error”). Later work would show that this bed (which marked the top of Sedwick’s study) actually correlated to a bed in the middle of Murchison’s Silurian – with the result that there was a large overlap.
• The section to be read more carefully
  • p. 90-101: Evolution of thinking that resulted in naming the two “systems”, including how their usage contrasted with that of their contemporary Élie de Beaumont (more on him next time).  The “Devonian Controversy” (p. 93-94 – see my notes for details) played a role in the development of the idea of a “system” by emphasizing the need to break out discrete intervals in the stratigraphic record (1832-35), leading to the Murchison system concept.  Note that in Fig. 3.11 (Murchinson’s idealized section), there are unconformities between the Slaty Grauwacke and Unit 4, and between Unit 1 and the Old Red Sandstone.  Unconformities were commonly placed at “system” boundaries, following the ideas of Élie de Beaumont – the full significance of this will be clearer after our next topic.
  • p. 102-109: Filling in the details – note how each system was characterized using different criteria, in part reflecting their respective frameworks, in part reflecting what was available in the field.  The section about Sedwick’s approach (p. 105-109) is noteworthy because of the usual focus of attention on Murchinson’s successful fossil studies. At this point, the two approaches seemed quite compatible. Of course, later their agreement would fall apart… Also, Figure 3.13 is interesting in light of Lyell’s argument for faunal turnover – note that the Bala Fossils and Lower Silurian Fossils would ultimately become the Ordovician.

Some points to consider:

• Field work and stratigraphy
  • What competency did a geologists need to develop to be successful in this style of field work?
  • What issues arose in sorting out the structure and sequence of Transitional rocks?
  • How did geologists figure this out (criteria)? What was the role of underlying assumptions in framing their interpretations?
• Systems
  • How did the concept of stratigraphical “systems” develop from the older stress on formations?
  • How did Sedgwick and Murchison differ in their concept of a system?
  • How did the British usage compare to the French usage?
  • In a larger (continental) scale, how were strata and systems correlated?
  • Why were the geological systems adopted as a stratigraphic framework by the 1840s?

Links to course homepage and course schedule.