The geology of Lochbrow

To me, as a geologist, one of the most interesting aspects of the Lochbrow Landscape Project is trying to understand the origins of the landscape.  After all, its form, its structure, and its human use is very much dictated by its geological history.  So, I thought I would have a look at what underlies the fields around Lochbrow.

The Lochbrow landscape – what lies beneath?

For simplicity, I will divide this into two posts: one on the hard, bedrock geology, and another on the soft, Ice Age and post-Ice Age geology.  This is fairly standard practice – most geological maps of Britain are published in two versions: ‘solid‘ and ‘drift‘.  Let’s start with the former.

Bedrock geology of Lochbrow

From St Abb’s Head in the east to the Mull of Galloway in the west, a huge band of ancient marine sedimentary rocks dominate the geology of south-east Scotland, forming the distinctive terrain of the Southern Uplands.  These rocks are of Ordovician-Silurian age (around 490 to 430 million years old), and are mudstones and sandstones deposited during the closure of the Iapetus Ocean.  They are of international geological importance, and include the Global Stratotype Section and Point (GSSP) for the Ordovician-Silurian boundary, defined at Dob’s Linn, near Moffat.

Dob’s Linn, home of the Ordovician-Silurian boundary. (Photograph (C) Callum Black, http://www.geograph.org.uk)

In a few parts of Dumfries & Galloway, however, this regional, broadly East-West trending outcrop of ancient ocean is interrupted by narrow, North-South-oriented bands of much younger, non-marine rocks.  These are conglomerates and red sandstones of Early Permian age (~290 million years old) and represent the sedimentary infill of later Palaeozoic depressions.  Lochbrow sits on some of them.

The Permian basins, developed most strongly around Stranraer, Dumfries, and Lochmaben, were giant fissures that opened during tectonic extension of the then young (and very much higher) Southern Uplands.  The sediments that filled them were mainly wind-blown sands and river-borne gravels carried down off the mountains, and are known collectively as the Stewartry Group.

Lochbrow lies towards the north of the Lochmaben Basin, and is underlain by thinly bedded red sandstones of the fabulously named Corncockle Sandstone Formation:

Thinly bedded, aeolian red sandstones of the Corncockle Sandstone Formation, exposed by incision of the River Annan into bedrock at Jock’s Pool, Lochbrow.

The red colour, regular grain size, rounded grain shapes, and sedimentary structures in the Corncockle Sandstone Formation indicate that it was deposited in a wind-blown dune system.  The Permian environment would have been hot and often arid, but these ancient Scottish deserts were not barren.  Fossilized trackways found in quarries near Lochbrow show that many tetrapods inhabited the area.

And did these feet in ancient times walk upon Scotland’s mountains green?

The Corncockle sandstones are the youngest rocks at Lochbrow.  If you were to dig down beneath them, you’d find sandstones and breccias of the Hartfield Formation, representing a slightly earlier period of the Permian when the Lochmaben Basin was wetter.  Perhaps appropriately, geological mapping shows that the River Annan closely follows the boundary between the two formations.

The River Annan around Lochbrow, closely following the boundary between the Corncockle Sandstone Formation (left) and Hartfield Formation (right).

If you dug down further, you’d discover an unconformity, with the Permian rocks overlying the Early Palaeozoic mudstones and sandstones I mentioned at the start.

So, in summary, the geology of Lochbrow is regionally unusual and very interesting.  The Permian sandstones are softer than the Ordovician-Silurian sandstones and mudstones, and they drain rather better, so both the Dumfries and Lochmaben basins have become important river valleys: Nithsdale and Annandale.

In my next post, we’ll see how the last couple of million years of Ice Ages and interglacials have modified the landscape of Lochbrow further.

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One Response to The geology of Lochbrow

  1. Pingback: The geo-archaeology of Lochbrow | Lochbrow Landscape Project

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