a number of different construction techniques from various ages. The most notable section, behind the buildings on Castle Street, is grade II listed and built of ribbed limestone blocks that may be reclaimed stone from Reading Abbey. Other sections include brick vaults, iron-girder roofing, concrete culverts, and modern corrugated steel construction. At its eastern end, once it has passed under the Central Library, the Holy Brook re-emerges into the open, at a point where it once formed the southern boundary of Reading Abbey. Here the brook provided the driving force for the Abbey Mill, of which only vestiges remain, as well as supplying water to the Abbey's necessarium. Today the brook passes between the backs of modern office blocks before re-entering the north channel of the River Kennet where the latter makes a right-angle turn immediately to the south of the ruins of the Abbey church. Water quality The Environment Agency measures the water quality of the river systems in England. Each is given an overall ecological status, which may be one of five levels: high, good, moderate, poor and bad. There are several components that are used to determine this, including biological status, which looks at the quantity and varieties of invertebrates, angiosperms and fish, and chemical status, which compares the concentrations of various chemicals against known safe concentrations. Chemical status is rated good or fail. Water quality of the Holy Brook in 2019: References Bibliography External links Photo-essay on the Holy Brook, concentrating mostly on the culverted section Geography of Reading, Berkshire Grade II listed buildings in Reading Kennet catchment Rivers of Berkshire