Geologic Map Symbols

Common symbols to interpret geologic maps:

A: dip and strike sedimentary bedding.  The long line is correctly drawn to show the strike direction, and the thin line points in the direction of the dip.  The dip angle is indicated.  In this example, the bedding is about N20W 40 NE.

B.  Normal fault.  The ball is on the downdropped block, called the hanging wall.

C.  Contact between sedimentary rock formations.  This line is thinner than the faults, and the dotting means that the position is approximate.  In this case it is approximate the thin unit between the two contacts is a weak shale that is usually covered making the contact hard to trace.

D.  Low angle normal fault.  The two ticks are on the hanging wall.

E,F,G: dip and strike of a fault.  These were actually measured on an exposed fault plane, and thus could be a very local feature and not represent the local trend.  The arrow points down the fault plane in the dip direction, and will be on the handing wall.  The strike will be perpendicular to the dip direction

 

The letters indicate the rock formations, with the first letter being the geologic period.  The remaining lower case letters abbreviate the formation name, which includes a geographic name where you could go to see the rocks.  The formations on this map include:

  • Qal, Quaternary alluvium.  This occurs in the valleys and stream beds.
  • Mis, Mississippian Indian Springs Formation.  It was named for a small town; he military facility at Indian Springs is now Creech Air Force Base.
  • Mj, Mississippian Joana Limestone, named for the Joana mine near Ely, Nevada..
  • Ddg, Devonian Devil's Gate Limestone, named for a pass in the Eureka mining district.

Last revision 8/22/2013