Page 292 - Laboratory Manual in Physical Geology

Formation contacts
GEOLOGIC MAP:
a 2-dimensional model
of formations (colors) N
and formation
contacts.

Subparallel vertical
fractures (joints)
Blue Fm Red Fm Yellow Fm
Bedding
Plane Yellow Formation
Y
Yellow Formationellow Formation
contact
EAST CROSS SECTION Bedding
FORMATION
CONTACT

Plane
contacts
Red Formation
SOUTH CROSS SECTION

OUTCROP:
BLOCK DIAGRAM: a place where bedrock crops out (sticks out of the
a 3-dimensional oblique sketch of a block (outlined in ground). This outcrop is a roadcut (a vertical cliff
red) with a geologic map on top and geologic cross made when people cut through bedrock to make
sections (vertical) on its two visible sides. room for a highway).
FIGURE 10.2 Formations, geologic map, and block diagram.

Individual beds/strata are rarely mapped because they
1

are not wide enough to show up on a typical 7 -minute ACTIVITY

2
quadrangle map (where a pencil line equals about 6
meters, 20 ft). Geologists assign each formation a formal 10.4 Determine Attitude of Rock
name, which is capitalized (e.g., Yellow Formation

or Yellow Fm in FIGURE 10.2 ) and published with a Layers and a Formation

description of its distinguishing features and a “type Contact
locality” upon which the name and distinguishing
|
features are based. The formal name can include the THINK How do geologists map geologic
word “formation” or the name of the rock type that About It structures on and beneath Earth’s
makes up the formation. For example, the Dakota surface?
Formation is also formally called the Dakota Sandstone.
OBJECTIVE Determine the attitude of rock layers,

Geologic Maps draw geologic cross sections, and infer a formation
Geologic maps are flat (two-dimensional, like a sheet contact.
of paper or computer display) models of Earth’s surface, PROCEDURES
viewed from directly above, that use different colors 1. Before you begin , read Attitude—Strike and Dip,
and symbols to represent the locations of formations Constructing Geologic Cross Sections, Fractures
( FIGURE 10.2 ). You can search for U.S. geologic maps and and Faults, Folded Structures, Unconformities, and

formation descriptions with the National Geologic Map
Database and Geologic Names Lexicon ( http://ngmdb. Cardboard Models next. Also, this is what you will
usgs.gov ). This site also has links to state geologic maps. need :
___ dark pen (red or blue)
Block Diagrams ___ Activity 10.4 Worksheet (p. 276 ) and pencil
The widths of formations vary in outcrops and on maps 2. Then follow your instructor’s directions for
because of variations in formation thickness, angle of completing the worksheets.
tilting, and the angle of the land surface at which they

262 ■ L ABOR ATORY 10