I am told that, far from being three distinct terroirs, the alluvial zones of Cahors are now classified by as defined by Ferme Experimental Anglars Juillac as:
T1 - 1st terrace
T2 - 2nd terrace
T3 - 3rd terrace
T4 - limestone scree
T5 - limestone covered slopes: limestone slope
T6 - upper quartenary
T7 - limestone plateau: siderolitic
T8 - white marly-limestone
T9 - limestone plateau
Does it matter to us as consumers? The answer is almost certainly "no". Most estates claim to be in T1-3 (made up of alluvial deposits from the Massif Central) or on the causse. Altitude and soil testing are the only ways of knowing precisely where they lie. It has long been understood that the lower terraces close to the river produce supple, fruity, easy-drinking wines. The medium ones produce fleshier wines. Somewhat inevitably, higher terraces have better drainage and it is here where the soil is made of limestone scree from the plateau that the richest, most ageworthy wines are made. This is T4.
Some vineyards are planted on the surrounding slopes and limestone plateaux of the quaternary era consisting of alluvial soil deposits covering the rocks and resisting erosion: these are much rarer, but they also produce very renowned wines. This is T6!
Paul Strang (in his excellent book South-West France) admits to many different terroirs but goes on to say that it is difficult to differentiate between Malbec grown on different soils here and, further, Malbecs grown on the same soils can be quite different from one another.
Indeed, often the best Cahors wines are those blended from different terroirs. In which case, the differences matter to growers certainly - they need to ensure they have the best range of soils - but for most of us, all we need to know is what is in the bottle.