Common Minerals and Rocks

by William O. Crosby

The most important features to be noted in observing and describing inclined or folded strata are the strike and dip. The strike is the compass bearing or horizontal direction of the strata. It is the direction of the outcrop of the strata where the ground is level. It may also be defined as the direction of a level line on the surface of a stratum, and is usually parallel with the axis of the fold.

Fig. 26.—Dip and strike.

The dip is the inclination of the beds to the plane of the horizon, and embraces two elements: (a) the direction of the dip, which is always at right angles to the strike, being the line of steepest descent on the surface of the stratum, and (b) the amount of the dip, which is the value of the angle between the line of steepest descent and the horizon.

In [Fig. 26], s t is the direction of the strike, and d p that of the dip. The strike and direction of the dip are determined with the compass, and the amount of the dip with the clinometer, an instrument for measuring vertical angles.

The strike is much less variable than the dip, being often essentially constant over extensive districts; while the dip, except in very large or closed folds, is constantly changing in direction and amount.

When the dip and surface breadth of a series of strata have been measured, it is a simple problem in trigonometry to determine the true thickness, and the depth below the surface of any particular stratum at any given distance from its outcrop. When the strata are vertical, the surface breadth or traverse measure is equal to the thickness.

By the outcrop of a stratum or formation we ordinarily understand its actual exposure on the surface, where it projects through the soil in ledges or quarries. But the term is also more broadly defined to mean the exposure of the stratum as it would appear if the soil were entirely removed. It is instructive to observe the relations of the outcrop to the form of the surface. Its breadth varies with its inclination to the surface, appearing narrow and showing its true thickness where it is perpendicular to the surface, and broadening out rapidly where the surface cuts it obliquely. The outcrops of horizontal strata form level lines or bands along the sides of hills and valleys, essentially contour lines in the topography; and appear as irregular, sinuous bands bordering the streams and valleys in the map-view of the country. The outcrops of vertical strata, dikes, or veins, on the other hand, are represented by straight lines and bands on the map. While the outcrops of inclined strata are deflected to the right or left in crossing ridges and valleys, according to the direction and amount of their inclination.

A geological map shows the surface distribution of the rocks, i.e., gives in one view the forms and arrangement of the outcrops of all the rocks in the district mapped, including the trend or strike of the folded strata. The map may be lithological, each kind of rock, as granite, sandstone, limestone, etc., being represented by a different color; or, it may be historical, each color representing one geological formation, i.e., the rocks formed during one period of geological time, without reference to their lithological character. But in the best maps these two methods are combined. The geological section shows the arrangement of the rocks below the surface, revealing the dip of the strata and supplementing the map, both modes of representation, the horizontal and vertical, being required to give a complete idea of the geological structure of a country. For a detailed and satisfactory explanation of the construction and use of geological maps and sections, students are referred to Prof. Geikie’s “Outlines of Field Geology.”

Cleavage Structure.—This important structure is now known to be, like rock-folds, a direct result of the great horizontal pressure in the earth’s crust. It is entirely distinct in its nature and origin from crystalline cleavage, and may properly be called lithologic cleavage. It is also essentially unlike stratification and joint-structure. It agrees with stratification in dividing the rocks into thin parallel layers, but the cleavage planes are normally vertical instead of horizontal. And the cleavage planes differ from joints in running in only one direction, dividing the rock into layers; while joints, as we shall see, traverse the same mass of rock in various directions, dividing it into blocks.