By the early 1860s, the “Ice Age” controversy triggered a major reassessment of landforms that opened up consideration of fluvial and denudation processes as major processes in the development of landforms. This was a major and long-lasting shift in geological perspective and is worth a closer look.  The Davies (1969) reading gives a good introduction to your group’s reading.

Harris sect. 21

General overview of the topic (as usual).  You may want to look back at the “Ice Age” summary (section 17) and keep in mind the findings of the surveys of the western US (section 20).

Davies (1969) ch. 9: Fluvialism Revived

Davies’ work focuses on the development of geomorphological ideas in Great Britain. In this chapter he examines the rise of “fluvialism” – the idea that rivers and associated erosion shaped many of the landforms we see today. What is nice about this account is that he looks carefully at the impact of key case studies – one of which you will be reading. In going over this, you should first read the introductory sections (p. 317-319) that considers why geomorphology was ready for a change by the 1860s. The allusions to Ramsey’s work on rock basins refers to his attribution of what we call cirques (“rock basins”) to the activity of glaciers – previously they had been a major obstacle to the acceptance of any significant role of fluvial processes in denudation and landform modification. Following this, I suggest that you focus on the sections related to the paper you were assigned: Jukes (p. 323-330), Ramsey (p. 337-338), Geikie’s “Scenery of Scotland” (p. 341-344), and the erosion rate estimates of Croll and Geikie (349-351). You will also want to read the section on the counter-arguments to fluvial erosion (p. 345-349) because they reveal the persistence of debates about processes that were noted in your papers. It is interesting to see some of the familiar people cropping up on different sides of the debate (for example, Murchinson being more catastrophic than Lyell).

Overall, this account highlights the significance of key studies that appear at the right time (here just as glaciation had loosened the grip of older theory that present landforms are shaped by marine erosion). Davies also points out some of the reasons that older theories were discarded – an interesting combination of evidence and social considerations.

Jukes (1862), Ramsey (1863), Geikie (1887), and Geikie (1868-69)

You have been assigned one of the Jukes, Ramsey and Geikie articles mentioned in Davies’ text. Davies provides some commentary that will help guide you through them – with very useful maps for the Jukes and Ramsey works. Note that the first and last readings are from technical literature written for professionals, while the second and third are from books written for a general audience.

Jukes (1862) critical study of the rivers of southern Ireland was a key work. After the brief introduction, the paper is divided into three parts of unequal length:

• Part I (p. 379-384) describes the physical arrangement (distribution, elevation, dip) of strata in Ireland in great localized detail – and makes me scream for a topographic map (and remind me why making such base maps were so important in the middle-late 19th century!).. In brief (to make this less painful – skip down to p. 383 “Former extension…”):
  • Southern Ireland is a plain of Carboniferous limestone plain with overling coal measures
  • Older Paleozoic (Old Red Ss, Silurian-Cambrian) hills occur in southern areas with east-west orientations
  • The present-day distribution of the Coal Measures provide evidence of the extent of erosion (p. 383-384)
• Part II (p. 384-390) deals with the present-day river valleys and is best read with the maps of Jukes (following p. 402) and Davies (Fig. 3, p. 324). The details of each of the key rivers is noted, and you can refer to Davies summary as your read this over.  Focus on the sections dealing with the rivers of southwest Ireland (Blackwater, Lee and Bandon) – there are his key examples.
• Part III (p. 390-400) presents a model for forming the “transverse” valleys. Jukes first (p. 390-393) argues against an origin by a structural “distrubance” and marine denudation – two common explanations for this type of drainage. When we get to section C (p. 393) we get to his proposed explanation for the patterns; here again focus on the rivers of southwest Ireland (p. 393-397.  You can just skim the remainder of the section which deals with the general lowering of the land surface (p. 397-398) and the rivers of southeastern Ireland (p. 398-400.

Ramsey (1863) argued that the “Weald” was the result of fluvial erosion. This reading is a later (third) edition from 1872, but presents the same argument.

• The Weald is in southeastern England – see the map inserted between pages 108 and 109 where it is the belt of older (Jurassic) strata (dark green) between the North and South Downs (Cretaceous Chalk).
• Note that marine denudation is the main alternative to fluvial/subaerial erosion – Ramsey states that he accepted this interpretation for years (p. 110).
• There is a lengthy discussion (p. 110-119) of the Weald that includes detailed reasons for favoring fluvial/subaerial processes.
• Finally, he extends this idea to incude the Isle of Wright and the Tertiary (Eocene) (p. 119-top 124) and suggests it can be applied to the older escarpments that extend across the main part of England (see Smith’s map).

Geikie (1865, 1887) book “The Scenery of Scotland” is a highly readable presentation of an argument that the Scotland was formed by fluvial and subaerial processes. This reading is from the second (1887) edition. The selection (chapter 2) summarizes subaerial processes and shows Geikie’s style at its best. (The use of an imaginary journey down a stream is particularly effective.)  The rest of the book applies this view of processes to the interpretation of the landscape. If reading this selection, please list the various processes involved.

Geikie (1868-69) is the work that Davies characterizes as “Geikie’s most important later contribution to the fluvialist cause”. This paper is not written in the same style as his book, and you can skim over some of the details. The paper is divided into six sections:

• The Introduction (p. 154-156) sets the stage by acknowledging the work of Jukes and Ramsey, and narrowing the topic to “existing processes of denudation with the view, if possible, of gathering from them some indications that may guide us in investigating the denudation of past geological times.”
• Part I (p. 156-167) attempts to quantify the volumes of material removed. The first few pages (p. 156-159) are interesting and show an appreciation for both mechanical and chemical weathering.  You can skip over his review of past work (in considerable detail!) to arrive at estimates of the rate of erosion (how long to remove 1 foot), and ballpark estimates of how long to level various areas (unless you really want to read it all). You should start reading at the last paragraph on p. 165 – taking note of the tables on p. 164 – to (finally) arrive at the key point at the very end of this section: an estimate of the time it would take to reduce Great Britain to sea level (p. 167).
• Part II (p. 167-176) deals with the unequal erosion of hillslopes versus valley floors (p. 167-169). The later part of this section (p. 170-176) is a critique of those who deny denudation – skim it over.
• Part III (p. 176-182) turns to the origins of the present landscape with a vigorous argument for erosion. Note the section on “rock-basin lakes” – Following Ramsay, he argues for a glacial origin (today we call these cirques) – illustrating how glacial geology helped clear the way for fluvial arguments.
• Park IV (p. 182-186) deals with marine denuduation, the favorite of the preceeding generation of British geologists.  Read the first two pages and then skip to the last paragraph.
• Part V (p. 186-190) concludes with a look at the history of geology (with the typical British perspective and burst of pride) and an effort to place these erosional processes in a timeframe. In this section, the first paragraph is the one of interest to us – note the issue of actualistic processes and rates comes up. (Not for the last time!) You can skip the rest; the later part of this discussion that veers off into an overall sense of the duration of earth history and the compatibility of the denudation estimate with Thomson’s (aka Lord Kelvin) estimate – more on this topic in a few weeks.

Questions to think about

• The predominant older model for landforms that fluvialism and glaciation challenged older models for landforms that stressed marine inundation (in either brief or long-term variations) and associated erosion.
  • Why was glacial theory important in “making room” for fluvialism?
  • How did the generational “turnover” influence all this?
• What was the role of critical case studies?
• What was the impact of work in the American West so important at this time? Why was it so persuasive?
• In regard to your assigned reading:
  • What was the key contribution of this paper?
  • What is the field evidence used?
  • What argument is presented for fluvial erosion?
  • Why would the author view the marine inundation model as inadequate?
  • Can you explain this paper to your classmates? (A subtle hint perhaps?)

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