Rule of "Vs"
When contacts between
formation cross valleys, the contacts are bent, or deflected, as
illustrated in Figure
1. The direction and amount of bending depends on the slope
of the valley and the dip of the beds. The following rule (the Rule of "Vs")
os helpful for determining the general dip of a bed on a geological map
or block diagram if it is not shown by a symbol:
"Where a contact crosses a valley, it forms a "V", the apex of which
points in the direction of dip of the contact (Fig. 1 A,B)."
Short, wide, open Vs indicate steep dips; longer, more narrow Vs
indicate gentle dips.
Three
exceptions to the rule are:
1) If
beds are vertical, the contact is NOT deflected on crossing a valley,
and no V is formed (Fig.
1C);
2) If
beds are horizontal, the V points up-valley and is parallel to the
contour lines (Fig.
1D);
3)
If the bed dips down the valley, but at an angle less that the amount
of the slope of the valley, the V points up-valley (Fig. 1E).
Look at Figure 2.
This geologic map has dip-and-strike symbols on it and also a river
system that has cut down into the strata. Make sure you
understand the "Rule of Vs".