GEOLOGICAL FIELD WORK object of the field examination of a prospect is to collect data from which structural and other maps may be constructed. The methods employed are, the reconnaissance survey or pre liminary examination, during which the probable key-horizons may be found, the various wells that have been drilled are looked up, and whatever information regarding them is obtainable. Any geological information that may be had on the territory in question is also to be examined to enable the laying out of a proper scheme for the final or precise survey, by which the field work is completed.
The first step for the prospector before beginning his work in the field is to obtain all the possible information he can from maps and previously published reports. Many clues may be found that may save time or enable the discovery of structure.
In many regions the topography may bear some relation to folding, and the topographic maps may be used to show such folding. In a country of large and complex folding, the topo graphic contours may show the structures very plainly (Fig. 45), the danger is in mistaking the proper correlation of the outcrop ping formations. It is possible that a suitable dome may be found, but due to strong folding the oil- or gas-bearing horizons may have been brought to the surface and eroded. This applies only where the absence of other sands below the eroded ones is known.
Massive sandstones and limestones (hard strata) may form prominent escarpments or hog-backs, running parallel to the strike of the rocks, which generally are due to regularly dipping beds, and might have been eroded by streams running in the direction of the strike. Such a topography has a "stair-step" appearance and clearly indicates the regional dip of the strata. When the dips are to the west, the steepest escarpments are on the cast side of the hills, having the west slope or dip slope, rather gentle. Wherever the master dip, or main dip, is inter rupted by a dip in the opposite direction, known as a "reversal," a fold is present. Thus if the direction of the main dip over a large territory be known, it is possible to find the reversals by working in the direction of the master dip. Reversals or dip
slopes are sometimes to one side (the reverse side) of the anti cline, the axis showing weathered knobs, flat-topped mesas and buttes, or other characteristic forms along small dips. Where the dips affect the topography of a country, they are of the larger kind, and in a territory where the dips are small (15 to 3o feet per mile) less dependence should be placed on topographic features, as the arrangement and slopes of existing hills may or may not have been caused by the dip of the beds. In such cases the actual instrumental survey is the only safe method by which attitudes may be observed. (Fig. 46a-b.) Areal geology maps may give some clue as to existing condi tions. Where older rocks are surrounded by younger ones (Fig. 47) it may indicate the presence of a fold (and sometimes a fault). Should a stream originating in the younger rocks cut across the older ones then to the younger ones, the presence of an anticline is plainly indicated. If the conditions are reversed, i.e., if a stream originates in older strata, then flows through younger ones, then back to older ones, a synclinal structure is present. (Fig. 48.) It is also possible that when younger rocks are surrounded by older ones, the presence of such a condition is due to erosion which has left the younger rocks as outliers. (Fig. 49.) Similarly inliers may be formed by erosion, causing older rocks to be surrounded by younger ones. A rapid succes sion of strata in regular order from older to younger or vice versa indicates a plane-dipping homocline, and will show the direction of the master dip of the territory.
A plunging anticline or syncline running parallel to the master dip will have a V-shaped appearance on an areal geology map. (Fig. so.) Folds and faults indicated by such methods are the larger ones, and it is possible that any of these structures may be present without a corresponding clue in areal geology maps. It is only in exceptional cases where small structures may be determined from such maps. • A valley may be running along an anticlinal axis, or the reverse may be true, where the anticlinal axis may give rise to a ridge.