As in the case of the River Dove, the Rivers Manifold and Hamps accumulate on the Namurian Grits and Shales to the west. To the north of Wetton, adverse folding together with the shaly nature of the limestones keeps the Manifold on the surface. On reaching reef limestones at Wetton Mill, however, the river sinks underground, and during the summer months, the river bed is dry as far as the risings at Ilam. As water levels increase, the river migrates downstream, successively overpowering the various swallets. Thus for the majority of the year, the swallet caves in the river bed are inaccessible. The Wetton Mill sinks have been dye tested to the Main Rising at Ilam.
The river Hamps sinks in a similar manner at Waterhouses, and this water has been tested to the Hamps Spring and the Upper Rising, both near Ilam. The sinks along the limestone/shale boundary to the south west have been dye tested to the Hinkley Wood Risings, although there appeared to be a connection between these risings and the Upper Rising at Ilam.
The Wetton Mill - Ilam system is unlikely to be penetrable for great distances. The gradient is shallow, and the known caves are all terminated by sumps, although determined pushing in drought will undoubtedly reveal more open passage.
The Waterhouses - Ilam system is a rather better prospect. The straight line distance is roughly 4km, and the fall is in the order of 70m. This gives a gradient of just under 18m per mile, which suggests a significant length of vadose cave passage carrying the entire River Hamps. The intermediate Waterways Swallet was always regarded as a promising site, and significant discoveries have been made here.
The hydrology of the area is fascinating, and the reader is referred to the cited references for further information on the structural geology and geomorphology.
Included in this chapter is a small area of limestone which drains into the River Churnet. The stream found in the Ribden Mine probably drains to the Weaver Hills Rising, but this is not known to have been tested. Also included is the small area of limestone which drains south eastwards to the lower River Dove, where there are small sinks and risings about which little is known.