A profile of the Lower and Middle Devonian of international significance can be found at U Nového mlýna (By the New Mill) NNM. The national nature monument lies in the parishes of Hlubočepy and Holyně on the south-western outskirts of the city of Prague. The monument was declared in 1982 on an area of 12.7ha at elevations of 255-310 metres above sea level.
Deposits of Devonian age can be found around the New Mill. The oldest part of this sequence of layers is found in the northern part of the protected area, whereas the youngest part of the layer sequence lies in the southern part of the monument. Outcrops of the oldest layers in this region can be seen in the U Nového mlýna railway cutting. These are fine-grained, grey Devonian limestones belonging to the Prague Formation, and are overlain by younger Devonian limestones of the Zlíchov Formation. The Zlíchov Formation is built of grey limestone with a slab-like structure in the higher positions and agglomerations of hornfels. A belt of firm, thinly-layered, grey-green Dalejský shales, which belong to the Dalejsko-Třebotovský Formation, runs through the U Nového mlýna NNM from east to west. These shales form the floor of a shallow valley and are less resistant to erosion than the surrounding limestones, which led to the widening of the Dalejské údolí Valley around the present-day Holyně railway station. The boundaries between the Dalejský Shales and the strata which overlie and underlie it are not visible in the protected area as they are covered by superficial deposits.
The most notable parts of the Devonian sequence of strata here are the limestones of the Dalejsko-Třebotovský Formation and Choteč Formation. The boundary between the Lower and Middle Devonian lies between these strata. The oldest and the lowest, exposed positions of the Dalejsko-Třebotovský Formation are built of the Třebotov limestone, which is reddish, tuberous and micritic at first, but transforms into light-grey, tuberous limestones with a bench-like structure as we move upwards. The Třebotov limestones have a total thickness of around 25 metres. These limestones were formed in a deeper and calm marine environment and form the highest position of the Lower Devonian. The Třebotov limestones are overlain by limestones of the Choteč Formation, which form the lowest position of the Middle Devonian. The formation of the Choteč limestones was influenced by marine currents, which carried organic debris, which explains why these limestones contain fine-grained and coarse-grained position. The border position between the Třebotov and Choteč limestones, which is also the boundary between the Lower and Middle Devonian, can be seen most easily in the Prastav quarry. This boundary is defined by the first occurrence of the conodont Polygnuthus costatus patitus. However, this fossil type can also be found in the upper part of the Třebotov limestone sequence, which lies 280cm below the lower boundary of the Choteč limestones. The majority of other fossilized fauna types are also found above this boundary and distinct changes can be seen at the base of the Choteč limestones. The biostratigraphical division here is not sharply defined. The Choteč Formation is overlain by the Srbsko Formation, but this stratum is not exposed on the territory of the national nature monument.
U Nového mlýna NNM is of the greatest significance because the international stratotype of the boundary between the Middle and Lower Devonian which was officially recognized by the 27th International Geological Congress in Moscow in 1984 lies within the monument. For this reason the “By the New Mill” NNM is one of the most important geological localities in the world.
The limestone outcrops have been re-forested with non-indigenous false acacias (Robinia pseudoacacia) and Austrian pines (Pinus nigra) so they do not form natural forest communities. On the contrary the forest-free communities with elements of rocky steppe are extremely valuable. Valuable invertebrate fauna can also be found on these remnants of rocky steppe.