Here, where a small stream crosses the mule track, it is possible to observe a bank of algal limestone still not collapsed. The limestone is blackish in colour, in contrast with the mostly clear one of the Pramollo succession. When struck, this limestone releases a characteristic odour of bitumen. The odour and the blackish colour indicate that the bottom of the lagoon, where the algal fragments (thalli) were deposited with mud, was devoid of oxygen: the organic particles that accumulated together with the algal limestone did not undergo putrefaction. For this reason, today these rocks contain traces of “crude oil”: this is the ancient organic matter, which had been cooked underneath the earth’s crust with the passage of geological ages.