May 3. The Old Red Sandstone

Among the things that I collect – and collectively, there’s way too much that I collect – are old geology books. Back in the 1980s at a used book store in Boulder, Colorado, I found a little book by Hugh Miller. It’s called The Old Red Sandstone; or, New Walks in an Old Field. It’s the first American edition, published in 1851. 

Finding this little book was a big deal for me, because Hugh Miller and the Old Red Sandstone were among the names driven into me by my geology teachers. Historically, the Old Red Sandstone was part of a thick pile of rocks in Devonshire, England, that lay between the well-defined fossiliferous Silurian rocks described by Roderick Murchison, and the equally obvious Coal Measures, a package of rocks called the Carboniferous, meaning carbon-bearing. They were between those two well-known sections, but the rocks of Devonshire are tortured, deformed by the Caledonian Orogeny, as well as intruded by igneous rocks and with other complications. So the relationship was not entirely obvious.

When paleontologist William Lonsdale examined fossils from these rocks, he saw them as intermediate between those of the Silurian and the Carboniferous. Ultimately, the shales, silts, limestones, volcanic tuffs and lava flows, and the Old Red Sandstone were defined as the Devonian system of rocks.

Fish from the Old Red Sandstone

The Old Red Sandstone was deposited in intermontane basins, somewhat like the low areas between mountain ranges in Wyoming and Montana today, or like the Great Valley of California, which is receiving sediment eroded off the Sierra Nevada. The basins were within the overall Caledonian mountain belt. Huge thicknesses of sediment accumulated – as much as 25,000 feet or more in places. The fact that the rocks are generally reddish suggests that the wet sediments were at least occasionally exposed to air, so that the iron in them could oxidize and turn to red hematite. It only takes a little iron oxide in a rock, less than 1%, to give it a reddish color, and all of the rocks in the Old Red Sandstone group are not red. Besides the sandstone, the package includes siltstones and shales that must represent the floodplains of Devonian river systems, rivers that were eroding the mountains. The fossils include eurypterids and lots and lots of fish. Those rivers must have been the blue-ribbon crossopterygian streams of the Devonian.

The depositional environment, the rivers and floodplains adjacent to the mountains, in which the Old Red Sandstone and other rocks were deposited, was a terrestrial environment, rather than a marine or oceanic setting. That interpretation has important consequences, because it implies that the fish and other life must have been fresh-water fish. Moving from the oceans to the rivers of the land was a big step for fish and other animals. It may have been a step in the first vertebrates actually coming onshore to live. Another clue to the environment is the presence of wind-blown sand dunes in some parts of the Old Red package of rocks. Such deposits aren’t extensive, but they are found for example in the Orkney Islands off Scotland. In other places, the rocks are very coarse grained, with pebbles and cobbles and boulders, indicating that they were not transported very far from the mountains from which they were eroded.

Old Red Sandstone in Scotland

There were five or six distinct Old Red Sandstone basins scattered across what is now Great Britain and Ireland, some a few tens of miles across and some as much as 100 miles across. Topographically, it’s probably fair to think of it like Wyoming today, with a relatively narrow basin in front of the Teton Mountains, and much wider intermontane basins elsewhere, including the Wind River and Big Horn Basins. There were Old Red Sandstone basins in eastern Greenland and in the Spitsbergen islands which lie above the Arctic Circle today, more than 1,000 miles from Britain, so the system of intermontane basins within the Caledonian mountain belt was quite extensive.

There are similar, but not exactly equivalent rocks in the United States that form the Catskill Delta. We’ll talk about that later this month. And the Old Red Sandstone was called that initially, before the geological periods were defined, to distinguish it from the New Red Sandstone, similar but much younger rocks that crop out in other parts of Britain as well as continental Europe, especially Germany. The Old Red Sandstone is a common building stone around Britain.

Given that the package of rock is really thick, more than 25,000 feet in some places, you might think it took a long time to deposit. That would be the case for the quiet ongoing sedimentation that we think of in a deep marine basin offshore, where it might take a thousand years to accumulate one inch of sediment, but deposition in intermountain basins can be catastrophically fast. Think of the recent landslide in Washington State that was in the news. There, the pile of sediment that came down, may ultimately become part of the rock record a few hundred feet thick – and it was deposited in a matter of seconds. Flood deposits from rivers can accumulate to several feet in a matter of days or weeks. So some of the Old Red Sandstone could have been deposited rather quickly.

Having said that, also recall that the Caledonian Mountains were being built over a vast period of time, from early in the Silurian (or maybe even earlier) well into the Devonian, and they would have still stood high after the Devonian too. Consequently, the age of the Old Red Sandstone ranges greatly depending on where you find it. In some places it might be Silurian in age, in others it might be Carboniferous in age. So its deposition spans many tens of millions of years. But mostly, it’s Devonian, the heart of the time period when the Caledonian Mountains were at their peak, with erosion dumping pile after pile of sediment into the adjacent basins. For comparison, Jackson Hole in front of the Tetons is a down-faulted basin that contains about 24,000 feet of sediment eroded off the mountains mostly in the past 13 million years.

—Richard I. Gibson

Reference: New Perspectives on the Old Red Sandstone, by P.F. Friend and B. Williams, Geological Society Special Pub. 180, 2000.

Old Red Sandstone in Scotland, photo by Graeme Churchard (GOC53), under Creative Commons license 
Fishes of the Old Red Sandstone, from Archibald Geikie: Life of Sir Roderick I. Murchison, 1875 (public domain) 

May 2. Devonian time

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So now, the Devonian time scale.

One thing to understand about the time scales and dates I’m using in these podcasts – they all have some error bars on them. Even when I say, for example, “the Silurian ended 416 million years ago,” that’s not an absolute. The timing of the boundary between the Silurian and the Devonian is even less specific than some of the others, and it’s something like plus or minus 3 or 4 million years. So you might see or hear of the Devonian starting about 419 million years ago while the Silurian ended 416 million years ago. Don’t worry about it – it’s within the error bars of our knowledge. Or it might mean that I’ve checked a time scale that’s a little out of date, but I really have checked. And all these dates are close enough for our purposes, I think. If you have the original book, the History of the Earth Perpetual Calendar, you’ll find that a lot of the dates have been refined somewhat since I wrote it in 1994, usually by some international agreement, and the podcasts do reflect those changes unless I screw up.

So the Devonian began about 416 to 419 million years ago. Its formal subdivisions are simple: Early, Middle, and Late epochs. The epochs are divided into a total of seven ages, three in the Early Devonian and two each in the Middle and Late Devonian. The names of a lot of the ages, such as Famennian, Frasnian, Givetian, Eifelian, come from places in France and Belgium where Devonian rocks are well exposed.

The Devonian lasted until about 359 million years ago, giving it a time span of about 58 million years – more than twice the length of the Silurian. I think we can fill a month with Devonian happenings.

—Richard I. Gibson

Devonian time scale image from Wikipedia. Scale in millions of years ago. 

May 1. The Devonian begins.

I think you’ll enjoy the Devonian. You can look forward to interesting developments in life on land and in the seas, and some interesting rock formations and tectonic events. 

As with most of the early Paleozoic periods, the Devonian was defined in Britain – but not without some controversy. Adam Sedgwick and Roderick Murchison, whose friendship dissolved in a fight about the Cambrian and Silurian, worked together to describe the Devonian in the 1830s. The controversy arose between Sedgwick and Murchison on one side and Henry de la Beche on the other. De la Beche was the first director of the British Geological Survey when it was established in 1835 – a powerful antagonist.

The quarrel was over the stratigraphic placement of some fossils from the younger Carboniferous coal measures – later than the Silurian or the proposed Devonian. This controversy was nothing like the one that developed a few years later between Sedgwick and Murchison over the Cambrian-Silurian boundary, and although it took a few years, and research that took Murchison as far afield as Russia, de la Beche came around to his opponents’ view and the Devonian Period was established and accepted by about 1840.

The period is named for Devonshire, the county in southwestern England where a good section of the rocks crops out.

—Richard I. Gibson