Before us: the long time of stones

A stone clock

If Earth’s whole history fitted into a day, Alto Tajo’s oldest rocks would appear at around twenty to ten at night. The first humans would arrive in the last six seconds. Everything Memory recounts — hillforts, the Lordship, timber floaters — occupies less than a final blink.

This story concerns what came before. Reading it in the landscape needs only the understanding that each rock remembers this place when it formed. Some were born on seabeds, others in rivers, lagoons or volcanoes. Their order, bottom to top, is time’s order.

A cold sea near the pole

Over 400 million years ago this land lay far away, part of Gondwana, a great southern continent near the pole. It was seabed. Graptolites, small colonial animals, lived in the mud; today they appear as grey, saw-toothed marks on black slate. In Checa’s slates they are easily found, around 430 million years old, with no surviving descendants.

Icebergs floated in that sea. Melting, they dropped trapped stones onto the bottom. One remains embedded in slate: Checa’s dropstone, from a glaciation between 444 and 428 million years ago. It is among the Geopark’s most valuable sites, and hardest to imagine: an iceberg right here.

A stone forest and a volcano

Around 300 million years ago, conifers grew at Rillo de Gallo. A volcanic eruption buried them beneath silica-rich, oxygen-poor ash, turning wood to stone without losing its structure. Like Pompeii, but with trees.

Later, around 270 million years ago, magma reached the surface near Orea in two eruptions. Its dacite rocks still stand out beside the road. Few expect a volcano in Alto Tajo.

Red rivers

Around 250 million years ago, in the Triassic, great rivers crossed a dry plain, depositing sand and pebbles stained red by iron oxide. This is the rodeno forming Barranco de la Hoz’s walls, El Huso’s monolith, Los Milagros’s towers, Chequilla’s Las Quebradas and Orea’s Peñas Rubias passages. Along the Rillo de Gallo–Pardos road, sandstone preserves ripples and cross-bedding left by currents. Fractures and erosion later sculpted these red enchanted cities.

A first sea

Between rivers and lagoons a shallow sea briefly advanced. Near Padilla del Ducado in Anguita, waves rippled its floor, preserved in stone as Padilla del Ducado’s fossil waves, around 240 million years old, Spain’s finest place to see these giant ripples.

Salt lagoons

At the Triassic’s end, coastal lagoons repeatedly dried out, leaving salt, clay and gypsum. Millions of years later, buried salt emerged dissolved in springs feeding Armallá’s and Saelices’s saltworks, as told in The salt of the earth. These clays contain aragonite, first described from Molina de Aragón specimens and now the Geopark’s symbol. The same layers underlie La Salobreja’s sinkhole and brackish lagoon in Orea.

The tropical sea

Then the sea came, warm and shallow. For over fifty million years it flooded the land; Geological Trail 5 describes a journey towards the sea, from river sandstones at Barranco de la Hoz to marine limestones at Cuevas Labradas.

It left many memories:

• In Esplegares, around 190 million years ago, rays disturbed the seabed while feeding, leaving pits: Spain’s oldest site of this trace type.
• At Cuevas Labradas brachiopod and bivalve fossils, distant relatives of scallops, are easily seen.
• At Fuentelsaz, layers around 175 million years old define a geological transition worldwide: a stratotype chosen in 2000. Spain has only five.

Later, in the Cretaceous, white, yellow and red Utrillas sands were deposited, supplying Peñalén’s ochre and kaolin, described in Alto Tajo’s kaolin, with fresh marine limestones above.

The land wrinkles

This land folded twice. First around 300 million years ago, when continents collided into one supercontinent: the Variscan folds visible along the Checa–Orea road in the oldest slates and quartzites date from then.

Much later the newer layer cake also folded. During the Tertiary, plate collisions raising the Pyrenees and Alps pushed these lands too, bending layers like a wrinkled carpet. Cuevas Labradas’s folds compressed like an accordion in the Gallo gorge; Peñalén’s bent beside Poveda’s Arroyo de la Hoz. At Hundido de Armallones a great recumbent fold appears in the canyon wall. Near Taravilla, at Hoya de la Parra, the river exposes an anticline’s cross-section.

The river cuts

Once the land rose, water began its work. Over the last few million years, the Tagus and tributaries incised the plateau, opening canyons hundreds of metres deep. Flat mesas and plateaux remained above, the river below. Any Park viewpoint makes it clear.

Water works inside too. Dissolving limestone, it opens shafts such as Alcorón’s, 63 metres deep; lowers ground into poljes such as El Cubillo in Checa; and sculpts enchanted cities such as Hoya del Espino beside Peñalén. Sometimes a river abandons a bend, leaving it dry as at Hoya de la Parra, or isolates a rock pillar such as Tormo de Mochales.

Ice

During cold periods over the last few hundred thousand years, ice did its part. In Orea’s quartzite ridges, water freezing in cracks split rock, and blocks slid downhill over frozen ground. Thus formed Orea’s stone river: thousands of angular blocks beneath which a stream can still be heard.

Only yesterday

Some rocks are as old as human history. Around 11,000 years ago, a tufa barrier 30 metres high dammed a valley to form Taravilla Lagoon, whose sediments preserve the last two thousand years’ climate. Aguaspeña’s tufa in Checa grows around four centimetres annually: two thousand years ago, when hillforts were built, it did not yet exist.

In 1578, Ocentejo’s residents sought Philip II’s help after a great canyon collapse, giving Hundido de Armallones its name. In the 20th century the Tagus began coating a failed dam’s broken concrete with tufa, creating Salto de Poveda. La Escaleruela, beside Puente de San Pedro, keeps growing each winter that brings water.

Reading the landscape

Almost everything here can be seen along Park and Geopark geological trails, reading rocks stop by stop: find them in official routes, and protection categories in protected areas. Remember one simple rule: fossils and rocks stay where they are. They are pages in a book that took 450 million years to write.