It not infrequently happens in studying the geology of stratified rockg in the field that a form of structure similar to that indicated in Fig. 21 is found. This type, in which one series of strata is seen to lie unconformably on a lower series is known as an unconformity, and has its origin in Conditions which were essentially that after the strata a had been deposited they were elevated, eroded, then sub merged, and became the sea-bottom on which were deposited the strata b. Subsequently the entire mass has been elevated and tilted. It is evident that such forms indicate the elapse of long time intervals between the deposition of the two series and the determination of unconformities are important features in establishing the time relations of different strata. Sometimes the strata may be parallel (Fig. 22) and the only indication of the unconformity will be the uneven nature of the top of the older and lower series. More often, however, the dips take different directions.
The detection of the necessary evidence by which the structure may be learned is not always easily accomplished. The forces of ero sion have been cutting and wearing away the surface, exposing outcrops at some points and obliterating the 'bed-rock' with detrital material at others, so that one learns to take advantage of every possible piece of evidence to be found. All dips are measured, faulting is closely studied and the distance of throw measured wherever possible, and all the data entered on as complete a topographic map as may be obtained.
Topographic maps show the relief or surface of the ground as it is today by means of contour lines, which are the lines drawn through all points having a common altitude. If one were to walk along the ground following the course indicated by a contour line on the map he would go neither up nor down but would remain constantly at the same elevation. Contour lines are arbitrarily spaced so as to represent equal successive vertical distances. Thus the 50-ft. contour along the coast would be the line made by the edge of the sea if it were to raise 50 ft.; the 100-ft. contour is 50 ft. above this, and so on. Many do not know the value of such maps, and the ease with which the topographic maps of the United States may be obtained for a small sum from the United States Geological Survey at Washington. An inquiry to the director thereof will bring an index map showing which portions of any state have been mapped and where these sheets may be purchased locally. In geological maps the underground position of oil-bearing measures is also shown by contour lines referred to sea level as a base, and designated with a minus sign prefixed when they signify depths below sea level, Fig. 23.
While it is of course unsafe to predicate the geological structure from map contours without field examination, yet these maps are a valuable help in the field and the topography frequently reflects the nature of the geology. Faults may be indicated by steep sharp scarps,
and folding from hills and irregularities conforming in a general way to the underground structure, although as often as not the axis of an anticline will not be found at the summit of a hill but on one of the sides.
As a simple example of the determination of structure it will be seen (Fig. 24) that in going over the hill from north to south the dip at a would be found to be 21° N. and the measure noted as a brown shale ; going further up the hill one passes over a body of light sandstone with a steeper dip, say 48° N. at b, and beyond this at c a measure of brown sandstone with dips increasing from 60° N. up to 80° and more. When the crest of the hill has been passed the same measures are traversed again in reverse order and with approxi mately the same dips at d, e and f, except that now they point south. Such evidence indicates clearly that the structure is a simple fold and that as far as the section represented by the line of the walk is concerned, the fold is symmetrical.
Suppose, however, that faulting has taken place along the lines indicated in Fig. 25. Casual observation might ascribe a greater thickness to the measure than it really has and often it is only by the most painstaking care in differentiating between minor charac teristics in exposures that one is able to detect such repetitions and establish the presence of faults. Or it may be that the structure is that shown in Fig. 20 and the dips all appear to have a single general direction. In this case the relative positions of the measures supply the key to the situation.
From the sketches shown of typical folds it is apparent that in nearly all cases where rolling hills represent anticlinal structure the dip of the strata is greater than the grade of the land surface, and that any single stratum approaches the surface as it rises, reaching the nearest point to the surface at the anticlinal axis. This rule obtains generally for monoclinal structure as well, and explains the well-known fact that holes sunk on the crest of hills are usually the shallowest, with the depths to the productive measure increasing in those further down on the slopes (Fig. 26). It should not be accepted as a rule that the anticlinal axis or summit conforms to the crest of a hill, as differential weathering and erosion may wear away the softer strata under some conditions so that the highest point topographically lies off to one side and over one flank of the anticline (Fig. 27).