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Synopsis:

We traveled to the city where the science of geology was born! Many of James Hutton’s most important localities lie within the city of Edinburgh, especially in the confines of Holyrood Park. This part of the trip provided our best opportunities to walk the outcrops. We were now traveling within the Midland Valley Province. Those of us who live in California’s Central Valley may be surprised to find that the origin of the Midland Valley is very similar. It was part of the forearc basin or magmatic arc of an early Paleozoic subduction zone on the margins of the Iapetus Sea.

Getting on the high-speed train at King's Cross in London

Our trip artist draws St. Giles High Kirk

Sunset over Edinburgh (about 9:30 PM). Twilight lasted a long time, and I couldn't resist the urge to explore the Salisbury Crags where James Hutton found evidence for the molten nature of the rocks found in the Sill.

I've waited many years for the chance to see and place my hands on one of the outcrops where the science of geology was born. The sedimentary rocks below my hand were peeled up by the intruding volcanic rock of the sill. James Hutton argued that there could be no way to explain an outcrop like this using processes known to affect sedimentary rocks, as was mostly believed at the time.

Many historians have remarked how lucky it was that there was an ancient eroded volcano in the middle of Edinburgh where Hutton could explore with ease. This is Arthur's Seat (the Lion's Head), one of the vents for the late Paleozoic volcano. Holyrood Palace lies at the base of the mountain, and the urban areas of Edinburgh completely surround the mountain. It is one of the most extraordinary urban parks in the entire world.

Geological Notes:

The geologic story in the vicinity of Edinburgh is deceptively simple, with rocks from only three periods during the Paleozoic era (400 to 280 my).  Studies of these rocks helped to define the science of geology in the late 1700’s. Today, the same rocks are helping us to understand the larger story of global tectonics.

In Silurian time, muds were deposited on the floor of a shallow sea as the Iapetus Sea shrunk as Europe and Greenland/North America approached and collided. The collision caused the rocks to be bent and folded, and the seafloor was raised high enough to be eroded.

As the Devonian-aged Caledonian mountains rose higher and higher, huge amounts of sediment were deposited as alluvial fans and floodplains across much of the entire region. These sediments are called the Old Red Sandstone. The bounding faults of the Midland Valley, the Southern Uplands fault, and the Highlands Boundary fault were both active, and volcanic eruptions rocked the surrounding region.

By the beginnings of Carboniferous times, the Caledonian mountains had been worn down to low hills and the sea began to encroach on the region once again. For much of the period, the region cycled through marine, deltaic, freshwater and river sedimentation.  Coal beds and oil shales formed in the layers.

The first layers of the Carboniferous, the Dinantian strata of the Cementstone Group, underlie much of Edinburgh. Shortly after their deposition, volcanic activity produced the cones and intrusions that lie exposed in and around the town, including the Salisbury Crags and Arthur’s Seat in Holyrood Park.

During the Pleistocene ice ages, glaciers covered the entire region, and erosion produced the unusually shaped hill upon which Edinburgh Castle is built. This kind of hill is called a crag-and-tail feature. The extremely hard volcanic rock deflected the ice, allowing sediment and loose material to accumulate behind the ‘crag’. Streamlined hills of loose till, called drumlins, can also be found in the region, as well as bedrock roche moutonnees.

Modern tectonic interpretations of the Silurian and Devonian rocks of the Midland Valley emphasize the position of the valley between the accretionary wedge of the Southern Uplands (see Day 7 description), and the volcanic arc of the Midland Valley itself or in the area to the northwest. As such, the Midland Valley can be regarded as a forearc basin (like the Central Valley in California) or as a magmatic arc (like parts of the Sierra Nevada). Because the Midland Valley is small for a forearc basin or magmatic arc (consider the size of the Sierra Nevada batholith), and because the faults bounding the valley are strike-slip (transcurrent), modern researchers consider that the rocks of the Midland Valley were truncated and then moved into their present position. In other words, the Midland Valley block is an exotic terrane, much like the Foothills Terrane or the Calaveras Complex in the Sierra Nevada.