mboeringa
I am not disputing there are nature reserves that can be considered geological monuments / features as well, in fact, I think the biologically most interesting and valuable nature reserves are those that are - or were - under the strong influence of some active geological process, whether something as benign as an upwards seep groundwater flow, river flow or volcanic / tectonic activity. However, if the biological aspect is entirely missing, as seems to be the case with this specific cave system, I think a classification as geological monument is more appropriate than "nature reserve". The word "nature" still has the connotation of at least some form of biological involvement as well (and no, Homo sapiens visiting the feature don't count ;-)).