Transcript HOW TO IDENTIFY AN ALLUVIAL FAN resources compliance.

HOW TO IDENTIFY AN
ALLUVIAL FAN
A landform that is important to cultural
resources compliance.
Field office personnel need to know
how to recognize alluvial fans
 Because cultural
resources are so
common on alluvial
fans, field office
personnel need to be
able to recognize this
land form in the field.
 An example of a fan is
illustrated on the right.
Because of the high probability of
encountering cultural resources on an
alluvial fan:
 No ground disturbing activity is to be
conducted on alluvial fans unless it is
confined to either post settlement alluvium
(PSA), or hydric deposits.
 Otherwise, assistance will be terminated, or
an exception will be made by the State
Conservationist (e.g. PL-566).
How Alluvial Fans Form
 Alluvial fans occur
where a tributary
empties into a larger
valley. As the grade
becomes less, sediment
drops out.
 This causes a fan
shaped deposit to form
at the mouth of the
tributary.
How to Identify an Alluvial Fan
 Check the mouths of
tributaries in larger
valleys while in the field.
 Check topographic
maps, and look for fan
shaped elevation lines
at the mouths of
tributaries.
 Check soils maps for
soils designated as
“local alluvium.”
We are not interested in small fans that
resulted from modern cultivation
 Modern agriculture has resulted in massive
erosion. This has led to the creation of
numerous small fans. These obviously are
not going to include prehistoric camp sites.
 These are composed of Post Settlement
Alluvium (PSA)
 Caution: Ancient alluvial fans are often
covered by a veneer of PSA resulting from
the same erosion events. They are usually
larger. Take great care in dealing with them.
PSA ALLUVIAL FANS
The 1 Acre Rule
 Often, alluvial fans that are less than 1 acre
have formed from historic erosion resulting
from modern agriculture. If they had been
forming for thousands of years, they would
likely be larger.
 Confirm for PSA by examining channels in the
fan for stratification, by using the core from a
soil probe and looking for banding, or by
seeing if the bases of trees are buried.
 If still in doubt, contact a soil scientist.
Low Angle Alluvial Fans
 Low angle alluvial fans are
the single most likely
topographic feature to
contain significant cultural
resources.
 They commonly contain
buried sites.
 Large low angle fans on
major streams may have
subtle enough topographic
changes that they are hard
to spot in the field, such as
the one pictured here. One
should also use topographic
and soils maps in
identification.
Why Do Fans Have Such a Concentration of Significant
Cultural Resources?
 Factors may include the
proximity to water, to food
resources in wetlands, to
streams that could be canoed,
and to flood protection from the
stream in the main valley.
 Spring run-off from the tributary
would annually bury with
sediment the campsites
occupied during the other
seasons, thus preserving a
stratified record valuable to
scientists.
 The large fan shown to the left
has 4 known prehistoric sites, 4
farmsteads (over 50 years of
age), and 1 cemetery.
Lidar sometimes will reveal fans
A village site is present on this
low angle fan, and is
overlooked by burial mounds
Multiple fans shown on Lidar
Mistakes can be costly
 The most expensive
archeological dig in
Iowa history, was on an
alluvial fan. The cost
was approximately $1.5
million.
 Accidental damage to a
site on a fan is not only
damage to our cultural
heritage, but also very
embarrassing to the
agency.
Some ground disturbing activities that
might be contemplated on a fan
 Diversion ditches
 Grassed waterways
 Ponds
 Wetland developments
 Tile outlets for terraces
Soils that may indicate alluvial fans
 Alluvial fans are often found in these soil map
units: Ankeny, Cantril, Castana, Coppock,
Creal, Deloit, Ely, Gravity, Judson,
Martinsburg, Moingona, Napier, Olmitz,
Spillville, Terril, Vesser, Volney, and Worthen.
 If these soils are present, you should be
suspicious that you may be dealing with an
alluvial fan. Check the topographic position.
These soils can also be associated with
colluvial slopes, upland drainages, and
stream terraces.
Facts about fans
 Many alluvial fans had a period of major
formation between approximately 2,500 and
9,000 years ago. Geologically, these fans are
known as the Corrington Member of the
DeForest Formation.
 These are usually low angle fans found in
medium to large size valleys.
 Sometimes there may be varying amounts of
young overwash known as Post Settlement
Alluvium (PSA) covering the Corrington
Member.
REMEMBER!
 Significant cultural resources are often found on alluvial fans.
 If you are uncertain about the identification of a fan, contact a
soil scientist.
 Even if you don’t see cultural resources on the surface, they
may well be lurking just below the surface.
 Always contact the State Office if you plan any ground
disturbance on an alluvial fan and you are not on hydric deposits
or PSA. You should use a soil scientist to determine whether or
not the project will be confined to hydric deposits or PSA. If the
project is not confined to such deposits and you can not avoid
the fan or substitute a non ground disturbing practice, then
terminate assistance (or in exceptional situations ask for
guidance from the State Office).