The rocks of the grotto are, in fact, a magnificent representation of the red limestone that is found in the Appenines and other parts of Northern Italy. The crumbling of the water-worn stone in the foreground of the painting and the vertical splitting and cross-fracturing of the rocks behind the figures are typical characteristics and brilliantly depicted. Miss Pizzorusso is simply unfamiliar with the manner in which this rock fractures, and has interpreted the different levels of cracking as indicative of different types of rock. What Pizzorusso misinterprets as round boulders of sandstone near the top of the picture are rocks of the same variety as those in the rest of the picture with their outlines obscured by layers of dense moss, a fact that has become very clear in the London painting since the recent cleaning.
The same author, assessing the distant, dream-like landscape, also tells us that "Fjords do not exist in Italy and it is highly unlikely the glacial lakes of the Lombard region would have such steep relief surrounding them." Miss Pizzorusso had plainly not seen the glacial lakes about which she was writing. In fact, a number of the lakes of Northern Italy have precipitous cliffs and rugged formations around them.
We can presume that Leonardo, fascinated as he was by Geology, had seen not only the Apennines and the Italian Alps of the Lombardy region, but probably visited the Dolomites further east. The influences on Leonardo would have included the landscape near Milan, but also a landscape much closer to his familiar Florence. Leonardo had visited and been fascinated by the region of the upper Arno between Florence and Arezzo. The area is dominated by tall rocky crags, some a hundred metres high. Leonardo, finding fossils in the rocks, determined that the area had previously been under water.
Leonardo left many drawings of rocks and landscapes showing geological formations, including a landscape showing a flood breaking into a valley in the Royal Collection at Windsor. The critic analysing Leonardo's geology needs to bear in mind that the artist was painting in a studio, remote from the scene and relying on sketches, studies and his memory to recall the precise forms.
There is also the matter of artistic license. Leonardo was not a "landscape painter". He was not, like John Constable in the early 19th century, seeking to set down the precise features of a particular geographic location so that any viewer who knew the place could locate not only the scene but the spot from which the preparatory sketches were made. Leonardo's landscape in this painting is essentially symbolic. It is a wilderness. To create this wilderness Leonardo has drawn upon the most majestic visions that he can extract from his days spent in the mountains. It is reasonable to presume that the landscape in this painting, in company with his allegorical scenes and scientific studies of swirling waters, are based on cumulative experience and observation, like Turner's Fighting Temeraire, rather than a single fixed point of view, like Constable's Haywain.