Tallis's Topographical Dictionary of England and Wales, published 1860
The county of Brecknock is bounded on the east by Monmouthshire and Herefordshire on the north and north-west by Radnorshire and Cardiganshire; on the west by Carmarthenshire; and on the south by Glamorganshire, and part of Monmouthshire. Its form is somewhat elliptical; in length, about thirty-nine miles, in breadth, twenty-seven, and in circumference about 109. Its area is 754 square miles. The principal rivers are the Usk, which issues from the black mountains on the south-west side of the county, runs north till it reaches Trecastle, and then turning to the east and passing Brecknock, leaves the county below Crickhowell: the Wye, which waters the north side of the county, leaves it at Hay and enters Herefordshire; the Irvon, which has its source in the upper part of the hundred of Builth; and, after a course of no great length, falls into the Wye, a little above the town of Builth. The Tawe rises on the southern side of the black mountains, and enters Glamorganshire at Ystradgynlais, whence it proceeds to Swansea, and falls into the Bristol Channel. The Taf has also its source in Brecknockshire; but it is an inconsiderable stream until it enters Glamorganshire. Other rivers of less note are the Groney, Honddu, Isker Brane, Camalas, and Croy, which fall into the Usk; and the Dylas, Comaick, and the Wevesey, which empty themselves into the Wye; they all abound with fish, particularly the Usk and Wye, which are well stored with salmon and trout. The chief lake in Brecknockshire, and one of the largest in Wales, is Llynsavaddan, or more properly Llynsafeddan, the still or standing lake, which is called also by the names of Brecknock Mere, and Langorse Mere. It covers a surface of about five miles in circumference; is about two miles in length, and one mile in width. It has been stated to be from nine to twelve feet deep; but, in some places, it is forty or fifty. Fish of different kinds are found here in great plenty; especially pike, perch, and eels. Trout is excluded, probably, by the presence of pike. Tradition ascribes the formation of the lake to the following marvellous story, as recorded in the Harleian MSS. 6831:- "A young man pays his addresses to the lady of Llynsafeddan., who rejects him on account of his poverty. He then robs and murders a carrier; bringing and displaying to her his ill-gotten wealth, he urges again his suit, but was interrogated respecting the attainment of the property he had brought. He confesses to her the means under an injunction of secrecy, but she still failed to comply, unless he repaired to the grave of the deceased and appeased his ghost. This he readily undertook, and on his approach, a voice cried, 'Is there no vengeance for innocent blood!' when another answered 'Not until the ninth generation.' Satisfied to find the evil day long protracted, the lady marries him, and their issue multiply so quickly, that they live to see 'even the ninth generation; but the Judgment not following, they made a great feast, when in the middle of their jollity, a mighty earthquake swallows up the whole generation, and their lands become covered with water." Brecknockshire is crossed by two ranges of mountains, which present some eminences of considerable elevation. The first, which is known by the denomination of the Eppynt-hills, rises on the north-eastern confines of the county of Carmarthen, and proceeding in an easterly direction, terminates at Llyswen, on the banks of the Wye, after separating the greater part of the hundred of Builth from the other portion of the county. The second chain, which partly divides Brecknockshire from the two neighbouring counties of Glamorgan and Monmouth, may be said to commence on the west, with two conspicuous hills of abrupt elevation, called Bannau Sir Gaer, or the Carmarthenshire beacons; then stretching in a line nearly parallel with the Eppynt range, it extends into Monmouthshire, and terminates on the southern side of the Usk, below the town of Crickhowell. This county was anciently denominated Garth Madrin; the origin of which appellation is not clearly ascertained, and at what period this tract of country was first peopled, has not been determined. The county of Brecknock is divided into the six following hundreds: Builth, Crickhowell, Devynnock, Merthyr, Pinkelly, Talgarth; and these hundreds are subdivided into sixty-seven parishes, and one part of a parish. The county has four market towns: Brecknock, Builth, Crickhowell, and Hay; six petty sessions, and eighteen acting county magistrates. The iron works of this county form an object of great importance. Most of them lie on the borders of Monmouthshire, and the ore is supplied from the estates of the Duke of Beaufort. Brecknockshire remained in the power of the Welsh princes until 1092. In that year the lordship of Brecknock was granted by the king to Barnard Newmarch, and that he might obtain possession of his rights, and the better to defend himself against the natives, whose hostility and resistance to his authority made it difficult for him to maintain his position in the country, he built the castle of Brecknock as a stronghold for himself and his troops. From that time till 1377, when it fell into the hands of Henry IV, Brecknock Castle was one of the chief defensive posts in the numerous contests between the Welsh barons and the English kings. During Henry's reign, Brecknock was much harassed by the predatory attacks of Owen Glendower. From the time of Henry IV to that of Henry VIII, Brecknock Castle always commanded Brecknock town, and the townsmen suffered much from the continual changes in the possessors of the castle. At length, in 1521, the lordship of Brecknock became permanently annexed to the English crown, and, in 1534, Wales became formally united to England. The climate varies considerably, according to the elevation and exposure; the hilly districts being very cold, and, in some seasons, subject to heavy rains. The principal geological formations of the county consist of grauwacke, slate, trap, porphyry, transition rocks, and the old red sandstone. A few of the Monmouthshire strata of iron and coal extend into Brecknock, and a little copper has been met with. The soil in the more favourable parts of the county yields good wheat, and abundance of apples for cider: in the cold and wet soils, barley and oats are the chief grain crops. In the high land are bred small black and brindled cattle, horses, ponies, and good hill-sheep. In the low lands the Herefordshire breed of cattle predominates, and is on the increase. The farms vary much both in value and in size; they are seldom let upon lease, and are chiefly held at a yearly tenure, at rents from £20 to £100 a-year. There are several cromlechs, traces of British stations, of Roman encampments, and remains of castles in the county.
[Source: page 238 of Tallis's Topographical Dictionary of England and Wales, published 1860 (for further details, see John Ball's website)]