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The National Hunting Grounds of Mafra stretches out over an 833 hectare expanse that contains various rocky substrata associated with its rich and diversified geological history.

The majority of the rocks visible date to the Lower Cretaceous era and correspondingly around 130 to 140 million years in age. At this time, the planet was undergoing a period of heightened greenhouse effect with an average temperature about 4ºC higher than that currently prevailing. The estimated level of CO2 would have been around six times greater than the pre-industrial reality.

The record of the paleo-environmental conditions stems from the rocks displaying fossilised rudist molluscs found in the Southeast of the Hunting Grounds, in the Pero Pinheiro region, the site of stone extraction for the building of the National Palace of Mafra. They bear witness to this site being under a tropical sea, with fairly shallow, clear and warm waters throughout the Upper Cretaceous period and thus around 90 million years ago.

The other remains of this distant period are marked into the sandstone formations, sedimentary rocks resulting from the erosion of granite in the mountain systems that would have existed around the region now home to the Hunting Grounds.

In the fluvial formations, we may note a complex network of deltas in observing various facets to the sedimentary layers, clayish beds with lesser energy alternating with beds of conglomerates and high energy fissures. In these, we encounter ballast sized from 5 to 6cm in length and for transport and movement requiring water speeds in excess of 20 cm/s, thus around half the average speed of an adult walking.

The morphology and profile of the Hunting Grounds also display the consequences of volcanic activity; seams and three volcanic chimneys that also date to the Upper Cretaceous period and more specifically between 83 and 66 million years ago.
These volcanic episodes formed part of what has become known as the Lisbon volcanic complex that left behind a range of volcanic cones across Loures, Lisbon and Mafra along with the sub-volcanic rock ring of Sintra.

The seam structures in the Hunting Grounds endow a distinctive profile due to the resistance of the basalt to any erosion and certainly in contrast to the surrounding sedimentary rocks.

Taking into account the abundance of the soils and the lack of any major visible rocky outcrops, we find the soil resulting from this volcanic period very favourable to fostering the abundant growth of forests in contrast with the soils resulting from sedimentary rock erosion.

The highest point in the National Hunting Grounds of Mafra stands at 357m in height and corresponds to one of the volcanic chimneys and differentiated from its surrounding rocks by centuries and millennia of differential erosion rates.

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