A geologic map
or geological map
is a special-purpose map
made to show geological
The stratigraphic contour lines are drawn on the surface of a selected deep stratum, so that they can show the topographic trends of the strata under the ground. It is not always possible to properly show this when the strata are extremely fractured, mixed, in some discontinuities, or where they are otherwise disturbed.
Strike and dip symbols consist of (at minimum) a long line, a number, and a short line which are used to indicate tilted beds. The long line is the strike line, which shows the true horizontal direction along the bed, the number is the dip or number of degrees of tilt above horizontal, and the short line is the dip line, which shows the direction of tilt.
The oldest preserved geologic map is the Turin papyrus
, made around 1150 BCE for gold
deposits in Egypt.
A fascinating story of the first modern geologic map is told in The Map that Changed the World, by Simon Winchester. It's the story of William Smith, a canal digger who created the first geologic map of Great Britain in 1819, but ended up in debtor's prison and lived homeless for 10 years until he was recognized for his work by King William IV in 1831. (Harper-Collins publishers, 2202. ISBN 0-06-093180-9)
Maps and Mapping across the globe
In the United States
, geologic maps are usually superimposed over a topographic map
(and at times over other base maps) with the addition of a color mask with letter symbols to represent the kind of geologic unit
. The color mask denotes the exposure of the immediate bedrock
, even if obscured by soil or other cover. Each area of color denotes a geologic unit or particular rock
formation (as more information is gathered new geologic units may be defined). However, in areas where the bedrock is overlain by a significantly thick unconsolidated
burden of till
, terrace deposits
deposits, or other important feature, these are shown instead. Stratigraphic
contour lines, fault
lines, strike and dip symbols, are represented with various symbols as indicated by the map key. Whereas topographic maps are produced by the United States Geological Survey
in conjunction with the states, geologic maps are usually produced by the individual states
. There are almost no geologic map resources for some states, while a few states, such as Kentucky
, are extensively mapped geologically.
In the United Kingdom
the term geological map
is used. The UK and Isle of Man
have been extensively mapped by the British Geological Survey
since 1835; a separate Geological Survey of Northern Ireland (drawing on BGS staff) has operated since 1947.
Two 1:625,000 scale maps cover the basic geology for the UK. More detailed sheets are available at scales of 1:250,000, 1:50,000 and 1:10,000. The 1:625,000 and 1:250,000 scales show both onshore and offshore geology (the 1:250,000 series covers the entire UK continental shelf), whilst other scales generally cover exposures on land only.
Sheets of all scales (though not for all areas) fall into two categories:
- Superficial deposit maps (previously known as solid and drift maps) show both bedrock and the deposits on top of it.
- Bedrock maps (previously known as solid maps) show the underlying rock, without superficial deposits.
The maps are superimposed over a topographic map base produced by Ordnance Survey, and use symbols to represent fault lines, strike and dip or geological units, boreholes etc. Colors are used to represent different geological units. Explanatory booklets (memoirs) are produced for many sheets at the 1:50,000 scale.
Small scale thematic maps (1:1,000,000 to 1:100,000) are also produced covering geochemistry, gravity anomaly, magnetic anomaly, groundwater, etc.