XII
ON SEDIMENTARY ROCK IN SCENERY

THE national parks which are wrought in sedimentary rocks are Glacier, Mesa Verde, Hot Springs, Platt, Wind Cave, Sully's Hill, and Grand Canyon. Zion National Monument is carved from sedimentary rock; also several distinguished reservations in our southwest which conserve natural bridges and petrified forests.

Sedimentary rocks have highly attractive scenic quality. Lying in strata usually horizontal but often inclined by earth movements, sometimes even standing on end, they form marked and pleasing contrasts with the heavy massing of the igneous rocks and the graceful undulations and occasional sharp-pointed summits of the lavas.

As distinguished from igneous rocks, which form under pressure in the earth's hot interior, and from lava, which results from volcanic eruption when fluid igneous rocks are released from pressure, sedimentary rocks are formed by the solidification of precipitations in water, like limestone; or from material resulting from rock disintegrations washed down by streams, like sandstone and shale. The beds in which they lie one above another exhibit a wide range of tint and texture, often forming spectacles of surpassing beauty and grandeur.

These strata tend to cleave vertically, sometimes producing an appearance suggestive of masonry, frequently forming impressive cliffs; but often they lie in unbroken beds of great area. When a number of well-defined strata cleave vertically, and one end of the series sags below the other, or lifts above it, the process which geologists call faulting, the scenic effect is varied and striking; sometimes, as in Glacier National Park, it is puzzling and amazing.

Many granitic and volcanic landscapes are variegated in places by accidental beds of sedimentary rock; and conversely occasional sedimentary landscapes are set off by intrusions of igneous rocks.

Besides variety of form, sedimentary rocks furnish a wide range of color derived from mineral dyes dissolved out of rocks by erosion. The gorgeous tint of the Vermilion Cliff in Utah and Arizona, the reds and greens of the Grand Canyon and Glacier National Park, the glowing cliffs of the Canyon de Chelly, and the variegated hues of the Painted Desert are examples which have become celebrated.

Geologists distinguish many kinds of sedimentary rocks. Scenically, we need consider only four: limestone, conglomerate, sandstone, and shale.

Limestone is calcium carbonate derived principally from sea-water, sometimes from fresh water, either by the action of microscopic organisms which absorb it for their shells, or occasionally by direct precipitation from saturated solutions. The sediment from organisms, which is the principal source of American scenic limestones, collects as ooze in shallow lakes or seas, and slowly hardens when lifted above the water-level. Limestone is a common and prominent scenic rock; generally it is gray or blue and weathers pale yellow. Moisture seeping in from above often reduces soluble minerals which drain away, leaving caves which sometimes have enormous size.

The other sedimentary rocks which figure prominently in landscape are products of land erosion which rivers sweep into seas or lakes, where they are promptly deposited. The coarse gravels which naturally fall first become conglomerate when cemented by the action of chemicals in water. The finer sandy particles become sandstone. The fine mud, which deposits last, eventually hardens into shale.

Shale has many varieties, but is principally hardened clay; it tends to split into slate-like plates each the thickness of its original deposit. It is usually dull brown or slate color, but sometimes, as in Glacier National Park and the Grand Canyon, shows a variety of more or less brilliant colors and, by weathering, a wide variety of kindred tints.

Sandstone, which forms wherever moving water or wind has collected sands, and pressure or chemical action has cemented them, is usually buff, but sometimes is brilliantly colored.

The processes of Nature have mixed the earth's scenic elements in seemingly inextricable confusion, and the task of the geologist has been colossal. Fortunately for us, the elements of scenery are few, and their larger combinations broad and simple. Once the mind has grasped the outline and the processes, and the eye has learned to distinguish elements and recognize forms, the world is recreated for us.