The Cretaceous Period (70 to 135 m.y. ago) was one of great contrast in New Mexico. During Early Cretaceous time, most of the central and northern parts of the state were low lands torn by erosion, while thick piles of conglomerate, sandstone, and shale accumulated in depressions in the southwestern corner. Huge volcanoes near Lordsburg added their hot ashes, bombs, and flows to the sedimentary detritus, and thick fossiliferous limestones were laid down in muddy and sandy waters of the sea—the shoreline fluctuated over tens of miles in areas south of Lordsburg, Deming, and El Paso. These Early Cretaceous beds total 20,000 feet in thickness in some areas. In eastern and northeastern New Mexico, in contrast, thin sheets of quartz sand were deposited by streams on the edge of a shallow sea, and black muds in local lagoons. Rock beds thousands of feet thick were laid down in northern and central New Mexico during Late Cretaceous time, whereas most of the southern part of the state was above sea level and was being eroded by tireless winds and streams. The shorelines made parallel northwest-trending bands across the state. These are now marked by beach sands, some of which are speckled by black minerals, high in rare elements titanium, niobium, and zirconium. Northwestern and central New Mexico was a battleground of the land and sea, with the beaches advancing and retreating fifty or a hundred miles during an instant of geologic time. Stream sands and coal beds lie landward from the beach sands which, in turn, mingle seaward with black limy shales that were flushed into the seas. The lowest of these rocks is the Dakota Sandstone—famous as an artesian aquifer in the High Plains areas of states to the northeast—Colorado, the Dakotas, Nebraska, Kansas, Wyoming, and Montana. Above is the black Mancos Shale, which in turn is overlain by the Mesaverde Group. The transitions from coal swamps and stream sands to beach deposits and then into marine black shales is characteristic of the Mesaverde in northwestern and north-central New Mexico. To the northeast, beds of the same age were laid down in an extensive muddy sea that stretched far to the east; the Pierre Shale and Niobrara chalky limestone that underlie the plains northeast of Las Vegas are typical. The cliff-forming sandstones and coal beds near Gallup are part of the Mesaverde Group and rim the entire San Juan Basin. Above are similar rocks such as the Kirtland Shale, Pictured Cliffs Sandstone, and Fruitland Formation that underlie valleys cut in the shales and cliffs carved from the sandstones in the northwest corner of New Mexico near Farmington.