Roman wall, the section had likewise to be prepared again at right angles. It will thus be seen that the diagram produced presents merely an approximate view of the probable course and appearance of the fosses and ramparts situate on the northern side.
The Romans were obliged to carry their wall on the northern part through a stratum of greenish or blueish and sometimes dark-coloured clay, which reappears inside the castrum. The rocky neck or ridge on which Mancunium stands does not cover the whole internal area; in fact, it falls away gradually at a point about 200 feet away from the northern wall, apparently in a slope, the sides of the ridge being covered by a deposit of ancient valley gravel. This gravel rests practically on the under-lying and undulating red rock, which rises and falls at certain points.
The greenish-blue clay, which occurs at trench No. 3, immediately rests in the hollow of this valley or river gravel, and we can follow it right along our section for a distance of 194 feet. It clearly appears to a geological eye to mark the existence of an old transverse valley trending towards the Irwell, which at one time collected the surface-water of the higher grounds to the east. In all probability this dingle or valley stretched from Camp Street on one side, where we are on a constant rise, right across to the ridge on which the Roman station stands. At Campfield and Alport Town we stand again on the outcropping edge of the red rock, as proved by the recent railway excavations, and the rock after plunging away reappears on the southern side of castrum. This miniature hill-and-valley system, which is so characteristic a feature of the new red sandstone in Deansgate and the cause of the abundance of so many wells and springs in that particular locality, is in full evidence from Ivy Street to Great Bridgewater Street,
