Slate deposits of Virginia

The slates of Virginia ('Arvonia Slates') are situated in the north-eastern area of Buckingham County, close to Estmond at Arvonia as well as north-east of Lynchburg and lie within the 'Piedmont Province'.

The slates belong to the Upper-Ordovician 'Arvonia Formation' (Evans & Marr, 1988), although a discussion about the exact age is still going on. The primary rock were presumably deposited in a shallow marine environment and then deformed about 300 million years ago (McDowell, 1964).

The minable slates lie within the north western limb of the 'Arvonia Syncline' which is described as a folded anticline with a primarily close folding style (Fig..). To SW it is bordered by the 'Hardware Anticline' and to SE by the 'Whispering Creek Anticline'. Several NW-SW striking shear zones cross through the north-western limb. Although there are no important movements of faults, the faults led to a repetition of stratigraphic units (Evans & Marr, 1988). Within the quarries the fracture cleavage plane steeps 75°-87° to SE.

The low grade metamorphosed 'Arvonia Formation' consist of a slate formation with two about 20 ft. thick intercalated meta-greywackes and show a total thickness of about 3,140 ft. Most of the slate is mined in a 394 ft. thick zone. Certain horizons contain pyrite, leading to a typical speckled brown colouring. The miner call them 'speck' and they are used as marker horizons to find minable slates. To Southwest, along the strike of the 'Arvonia Syncline' the slates gradually change to porphyroblastic phyllites with biotite and garnet ('knotted slate'). This slates show a coarser grain size and are not used as roofing slate.

Except for some cases, the slates show no folding and are crossed by few faults with no recognizable system. The mineralogical composition comprises quartz, muscovite, chloride and feldspar as the main minerals as well as biotite, calcite, graphite, magnetite, pyrrhotite and pyrite as accessory minerals. According to McDowell (1964), the mineralogical composition shows only very little variations (see Table).

Sericite and chloride constitutes a fine grained ground matrix. Quartz has a anhedral grain shape and the deformed and lengthened grains occur in a size of about 0.02 in. Calcite occurs uniformly distributed and the grains can reach a size up to 0.02 in.

The slates have a regular, dark-grey to grey colour which shimmers slightly greenish. McDowell (1964) states that the splitting plane is rough and shimmering and shows lineations. The dominating lineation is caused by the crossing of the bedding and the cleavage plane. Both encompass only a small angle, leading to this typical subtle stripes (McDowell, 1964). In Virginia, this lineation is considered as an attractive surface structure and is named 'strike' or 'ribbon' by miners (Brown, 1969).

A second lineation leads to the so called 'grains' which are caused by tiny lengthened and aligned minerals and which are almost invisible for an unaided eye. This leads to a splitable plane and miners called it 'sculp'. A crenulation foliation can partly recognized. The mica layers seem to have a rough to smooth shape with a varying density of mica layers.

The history of slate mining began in Virginia like in most areas of Northern America: Welsh settlers came to here in the 19th century and started slate mining and -production. The peak was at the beginning of the 20th century with 8 companies and about 40 quarries. Today, only one company mines and produces slate not only for roofing but also for flooring, tables, window-sills etc. as well as filling material for the production of concrete (1988).

According to Brown (1969), compared to other slates in the US the 'Arvonia Slate' shows a higher bending strength and a higher resistance to weathering.