Terfyn is a Welsh word derived directly through the Roman occupation of Wales from Latin terminus. Pont (stone bridge), cwrtil (curtilage), caer (castra, fort) are similar examples. In the context of Dinorwig it is the name of the highest formerly occupied area on Lon Garret around SH59216174. Garret refers to the highest area of Dinorwic Quarry, which runs to nearly 2000 feet high. The highest house on the Ordnance Survey plan of 1891 was Tan y Garnedd, but by 1914 its name had changed to Tan-y-Garret (ie below Garret) at SH59396150. Almost all of these houses were abandoned by the time of the https://en.wikipedia.org/wiki/Town_and_Country_Planning_Act_1947 planning legislation, meaning they cannot be brought back into use now without planning consent, which is very unlikely to be given. They are in varying states of decay, eg Geograph photos https://www.geograph.org.uk/photo/298175 and 298186. The photographer notes the contrast between the two, usually like mine they are built with random stones picked out of the fields, but what looks from a distance like dressed stone on closer inspection turns out to be stone with a natural straight edge.
A few years ago an anemometer and associated electrical equipment were installed at Tan-y-Garret presumably to test its suitability for a wind turbine: subsequently it was blown down in a gale, which I suppose was sufficient evidence of its unsuitability since it hasn’t been reconstituted.
The farmer who used to own this land, who bought it from the Vaynol estate when it was dissolved in 1947 for a nominal price, was very particular about his ownership and put up various signs (whitewash on slate naturally) stating no road, no entry, etc, which is technically correct, even now this is not land subject to right to roam legislation. He told me some years ago that a property company had offered to buy the properties on a speculative basis, in the hope that planning laws would change, but he declined. After his death the farm was taken over by his son in law, and the signs have eventually disappeared. Basically this is upland hill farming, just a few sheep on poor grazing land.
A few of these houses were still inhabited after 1947, so that planning consent isn’t needed for them to be resuscitated. They include Bryn Goleu at SH52916146, which is reached by a steep rather terrifying track from Lon Garret. It passed through several hands until purchased in 2016 by a retired British corporate lawyer from Dubai who presumably had had enough of the heat; it is now the registered address of his “architectural activities” company, having been extensively renovated after he bought it. Even has mains electricity and a new septic tank nowadays.
Another upmarket property nowadays is Groeslon Uchaf SH5888197 which was bought at auction for £150k some years ago, has been extensively upgraded and is for sale at £495K https://www.rightmove.co.uk/property-for-sale/property-68260851.html.
Lon Garret itself is a public highway as far as the gate leading into the old Dinorwig Quarry, after which it is disused. Parascenders park their vehicles in the small parking area by the gate, and climb up Elidir Fach to jump off and attempt to cross the quarry and Llanberis pass over to the the foothills of Moel Eilio. Otherwise it is used mainly by walkers, and motorists who like to drive up a no through road to see what’s at the end. Occasionally you might get more than you bargained for https://www.flickr.com/photos/hamishfenton/4250818356/in/photostream/. I can remember seeing snow like this 30 odd years ago, but it’s pretty much a thing of the past now. A reminder however of how brutal the climate sometimes was in the nineteenth century.
The road is not shown on the earliest large scale map of the area, OS two inch to the mile drawing of 1818, but it must have appeared soon afterwards presumably to gain access to the higher reaches of the quarry. However, the former presence of a Roman milestone at Caer Bythod SH5645 6360 raises an intriguing possibility. It was found in 1798 and is now, it seems, located at the National Museum of Wales site at St. Fagan’s near Cardiff. There has been speculation about why this milestone was found here, since it is currently about 3 miles from the known Roman road to the fort at Segontium, Caernarfon, which comes from Chester and through the northern end of Llanddeiniolen parish. The find site was roughly at the crossroads between the modern A4244 and a lane running from Llanrug through Penisarwaun and Clwt y Bont. This is direct rather than aligned, although E of Penisarwaun it is “warped straight” and is all but level. After crossing the main road it continues climbing gently until it comes to a steep pitch at 5723 6323. After crossing the Dinorwig Tramroad of 1824 at 5746 6309, the main road bears N up hill, minor road goes straight on, and on S side at 5760 6303 there is Tan yr Henffordd, (“below the old road”) which looks as though it was originally a 2 roomed cottage now much extended. Then again, by Deiniolen Silver Band room, the more used road turns N, and lane goes on and immediately swings through a sharp upstream zigzag over river Caledffrwd (5765 6301). Unusually for a minor road in this area this bridge was built up from 2 central piers, rather than a simple arch or horizontal slabs. It was washed away in a flood some years ago, and has now been replaced by a modern construction. There were also slight traces of edge-on paving below the bridge on N side. At 5795 6294 the road veers N round a mound of earth, but the S field wall goes straight on, and there is an old opening for a wicket gate in it halfway along the line. All this stretch from the main road, through what is now the village of Clwt y Bont, and including the zigzag, appears on the Llanddeiniolen enclosure award of 1813 (GAS). On this plan the track then turns abruptly S at 5797 6292, this still exists as a path, the road going straight on is not marked but logically must have been there. The 1818 drawing is similarly the same. A small stream (now culverted) is crossed at 5824 6284, then at 5351 6282 it crosses the road to Dinorwig built by the Vaynol Estate in 1812 and goes very steeply up the side of a tributary of Caledffrwd, where it is known as Lon Bwlch as it climbs to a small windgap at the top. The terraceway is most pronounced at 5862 6254, where a retaining wall some 8ft high is beginning to fall in. Another minor road is joined at 5870 6231 and then continues S but at 5871 6195 there is an awkward T junction and there are no obvious alternatives up hill to the east. We then come to Lon Garrett itself climbing across the mountainside until it disappears under the quarry tips at 5952 6115. It is not obvious why such a line should have been chosen for a quarry road of the Industrial Revolution, when the main C18/early C19 activity is known to have been down below on the Tramroad at Allt Ddu; this upper area of the quarry being as far as is known a later development. It is then referred to by name in a sketch of Dinorwig quarries made in 1836 (GAS Vaynol 4190). Looked at from the opposite side of Llanberis Pass, it is clear the line is the only possible one on the hillside at this high level and has been surveyed to a very regular gradient.
Does this imply there was older quarrying activity up here? There is no way now of getting proof, but the line does suggest the possibility of a Roman “road”, “path” might be a better description, we do know from the fort at Segontium that the Romans had discovered and used slate at least for paving, so perhaps it came from Garret where the beds were cropping to the surface.