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)