C is for Cumbria

And calcite

“Common Brittonic” was a Celtic language that evolved after about 500 AD into Old English, Scottish Gaelic, Welsh, Cornish, and Breton. The speakers of that ancestral language called themselves kombroges or kombrogis, meaning “compatriots,” and that’s the origin of names like Cambria (and the geologic period Cambrian), Cumbria, Cumberland, and Cymru (Wales).

Cumbria approximates the mountainous Lake District in northern England, a significant source for economic resources including iron. It’s well known for many minerals, but for me, when I think of “Cumberland” I think immediately of the classic and distinctive gemmy, water-clear calcite crystals associated with hematite (iron oxide), also famous as “kidney ore” for its botryoidal, reniform habit.

For most of these calcite specimens I only have “Cumberland” as a locality, but it’s pretty likely that most of them are from the Egremont area in the West Cumberland Iron Field, and a moderately good chance they are from the Bigrigg Mine, a prolific producer of calcite specimens, but I can never be certain of the exact provenance.

Iron was mined here since the 1600s and earlier, but most of the mining in this area ceased by the 1930s. It’s likely that some or most of the specimens below were mined in the 1910s or 1920s, with a reasonable chance they came out in the late 1800s.

Scalenohedrons and prisms terminated by scalenohedrons are among the most common of the many crystal habits for Cumberland calcite. Sometimes the termination is a low angle rhombohedron, giving the crystals a stubby look.

The pair of crystals above (not a twin, just parallel growth) seems to be slightly flattened prisms with rhombohedral terminations, but prisms on calcite are moderately unusual. The longer crystal is 55 mm long, and the pinkish color to the right is caused by iron oxide staining on cleavage planes in the calcite.

Cumberland is also famous for calcite twins – heart twins, butterfly twins, knee twins. All are contact twins on some rhombohedron face that is a surface in common (the composition plane) between the two individual crystals forming the twin. The geometry depends on exactly which rhombohedron is involved, so you can get twins that truly look like hearts, or hearts with barely any re-entrant between the two members, wide-apart butterfly wings, and close-together knees. The twin geometry is further complicated by the presence or absence and size of the myriad of diverse crystal faces each individual calcite crystal may contain.

My crystals above (at least the one on the left) are (I think) twinned on a {10.4} composition plane, a moderately gentle rhombohedron which results in two members whose elongate c axes are at nearly 90° to each other, making a knee-like intersection between them. Note that this is not a Japan-law twin such as you find in Quartz, but the results look similar. More heart-shaped twins often exploit a steeper rhombohedron, a {10.8} face, resulting in c-axis angles of about 54°.

These twins have tiny blades of black specular hematite embedded in their bases, and together with the exceptional similarity of geometry, size, and surface texture to this twin, I attribute them to Frizington in Cumbria.

The most prominent iron mine in the Frizington area was the Parkside Mine, which was most active in the 1870s, and almost all mining was over by 1925, so I’m pretty confident that these specimens were mined long ago. I’ve had them since 1971.

The iron ores in Cumbria are mostly hematite bodies in Lower Carboniferous (about 340 million years ago) limestone where the ore partially fills dissolution cavities and bedding positions in the limestone. The ultimate source of the mineralizing fluids does not appear to be known (Dunham, 1984, Genesis of the Cumbrian hematite deposits: Proceedings of the Yorkshire Geological Society, 45:1-2, p. 130), but it must relate to igneous activity and tectonism during the Variscan Orogeny (late Carboniferous to early Permian, about 320 to 270 million years ago).

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The Variscan Orogeny was more or less a continuation of the older Caledonian Orogeny which also produced some mineralization in the Lake District.

The photo below shows the Wast Water in the Lake District of northern England. As a lake, it’s pretty straightforward – a long narrow glacial lake carved by a valley glacier that descended a few tens of thousands of years ago from the heights of the mountains of Cumbria, where stands the highest point in England. And the Wast Water is the deepest lake in England, at 258 feet.

The rocks that underlie the lake and comprise all the hills in my photo here are much more interesting to me. They are late Ordovician in age (Caradocian, about 450-460 million years) and consist of the Borrowdale Volcanic Group. Those volcanoes were probably part of an island arc system that is now called the Gander Terrane and which became amalgamated with the larger (and probably somewhat more continental) Avalonian Terrane before or as Avalonia collided with North America. Much later, when the modern Atlantic Ocean formed, Avalonia + Gander and other pieces were dismembered, so that what was once the unified Gander Terrane is now scattered from Connecticut through northern New England, to New Brunswick, Cape Breton Island, central Newfoundland (the terrane gets its name from Gander, Newfoundland), southern Ireland, and northern England.

Avalonia + Gander (and two other blocks, Meguma in southern Nova Scotia and Carolinia in the South Atlantic States) broke away from Gondwana as discrete but not-too-far-separated long narrow strips. As they moved away from what is now northwestern Africa, an ocean opened between them and Africa, while they closed the ocean that separated them from what is now North America, northern Ireland, Scotland and eastern Greenland (Schofield and others, 2016, Reappraising the Neoproterozoic ‘East Avalonian’ terranes of southern Great Britain: Gondwana Research 35:257-271). That collision, called the Acadian Orogeny in North America and the Caledonian Orogeny in Europe, climaxed in late Devonian time, around 370 million years ago. In the process the older rocks, including the Ordovician Borrowdale Volcanics, were metamorphosed.

In the photo, the slope at near right is The Screes, rising up to Illgill Head, and at left is the edge of Yewbarrow. In the center in the clouds beyond the head of Wast Water is the Great Gable, at 899 m (2949 ft) among the highest spots in England. And two miles beyond the Great Gable lie the graphite mines at Seathwaite, the source of graphite for writing in most of Europe from the 1500s until the Napoleonic Wars. For more on that story, please see my YouTube slide show (below).

Graphite in the Borrowdale Volcanics is a consequence of the Caledonian orogeny’s metamorphism and related igneous intrusion, but where the carbon came from is debated. Graphite is pure carbon, and there’s a lot of it there, in blobs and outcrops that are meters of pretty much pure graphite. There are no obvious carbonates (limestone) in the rock sequence to derive the carbon from, and its purity is also anomalous (in fact remarkable in the world). Ortega and others (2010, The graphite deposit at Borrowdale (UK): a catastrophic mineralizing event associated with Ordovician magmatism: Geochimica et Cosmochimica Acta 74: 2429-2449) conclude that the carbon came from older carbon-rich shales that were incorporated into the igneous intrusions, essentially as xenoliths (“strange rock,” pieces of the older rock through which the intrusions pushed).

This lake’s name, Wast Water, derives from both Old Norse “vatn,” meaning “water” (often used in the sense of a lake or river, here in the context of Vatnsdair, “water valley”, the old name for the valley which came to be called Wastdale, with the lake called Wastdale Lake), and Old English “wæter,” also meaning “water,” so the name essentially means “water water” or at best “lake water.” So to call it Wast Water Lake would be triply redundant.

The photo is from August 1978 when I was in England with a friend, Dr. Ethel Sassenrath, a professor of psychopharmacology (i.e., she studied effects of marijuana on monkeys) at UC Davis. She was there to give papers and meet colleagues at Oxford and Cambridge; I was on vacation from Gulf Oil.