The Technique of Crossdating
Crossdating
Building a composite chronology with samples is possible if the
crossdating between all samples is significantly strong.
Crossdating
Preparing samples for Crossdating and Measuring
With the aid of a stereo microscope…
•
Mark every tenth ring with a fine point pencil using a single dot (.).
•
Every 50 years, use two dots (..).
•
Every 100 years, use three (…). Every 1000 years gets ?
•
The location of false rings, micro rings, and locally absent rings
that may be easy to miss while measuring should also be noted on
the sample.
•
These problematic rings should be annotated using locator marks
on the core mount itself and written instructions to the
measurement technician.
•
For example: “1585a” for an absent 1585 ring; “1748m” for a micro
1748 ring.
•
On the core itself, a micro ring is marked using dots on either side
of the ring. A locally absent ring is marked using offset dots.
Crossdating
Crossdating
Direct-visual inspection
•
When inspecting your cores, select the best 10 or more representative
samples, free of exaggerated periods of suppressed growth or severe
injury.
•
These are called “type cores.”
•
Visually compare the common period of growth under a microscope
looking for similar patterns of small and large rings, intra-annual
features, latewood variation, or any other consistently found annual
markers.
•
Once the first two are dated against each other, bring in another and
continue until all the initial samples have been dated.
•
By now, significant pointer years or features should be all accounted
for and the remaining samples from the site can be dated.
Crossdating
Skeleton plotting
•
This is a graphical technique that begins with the visual
examination of a tree-ring sequence under the microscope.
•
The dendrochronologist then selects the narrower rings out of a
sequence (or “window”) of rings.
•
Different dendrochronologists have different windows. A popular
number of rings to inspect is a decade or 10 rings. Simply choose
the narrower rings from that sequence.
•
The dendrochronologist then assigns a weight between 1 and 10
for the narrow rings. The narrower the ring, the higher the weight.
Crossdating
Skeleton plotting
•
Use this scheme:
• slightly narrow rings get weights of 1 to 3
• narrow rings get weights of 4 to 6
• very narrow rings get values of 7 to 9
• a weight of 10 is reserved for extremely narrow rings
• a missing ring (the “ultimate” narrow ring) is also given a
weight of 10.
•
On graph paper, plot only the narrow rings by creating bold lines
equal in length to the weight you assigned to them.
•
A missing ring will get a line weight of 10 but the line should be
dashed.
•
In addition, the skeleton plot may be enhanced with the addition of
any relevant intra-annual information (e.g., latewood width,
latewood color, trauma cells, barrier zones, etc.).
Crossdating
•
The narrowest rings are connected (blue lines) with the longest
weights in the skeleton plot (“8”).
•
The widest ring is connected (green line) with a "b" mark.
•
Note the red line which points to a ring of average width, but it seems
narrow compared to the preceding ring.
•
Perhaps it merits a small mark because of the large year-to-year
difference
[
The weights are
measured
vertically here.
Crossdating
•
One advantage of using skeleton plots for crossdating is that the rate
of growth of individual trees is essentially standardized.
•
In the example above, the three trees have different growth rates.
The same 20 years are shown for each.
•
But, note that their skeleton plots all look the same. In other words,
differences in growth rates of trees do not matter.
Crossdating
Crossdating
•
Once a series is plotted, it can be compared to a skeleton plot
created for a master or reference chronology for the area.
•
The only difference in these master plots is that their lines go down
below the horizontal zero line.
•
Notice below that the same weight scheme is used. For example,
the extremely narrow ring for 1919 got a weight of “10.”
•
Here, we have three skeleton plots for three cores. Let’s create a
master by compositing the weights from these three cores…
-
•
Now, how does this master composite plot compare to the middle
series?
-
Pretty good match!
•
Now, how does this master composite plot compare to the top
series?
-
Another pretty good match!
•
Crossdating can also be used to extend a chronology back in time
by overlapping older and older samples.
A very important application of crossdating is the absolute dating of
wood taken from archaeological sites or remnant pieces of wood that
can extend a chronology back in time.
1.5
Cabin rings
1.0
1.5
0.5
1.0
0.0
1700
0.5
1700
1725
1750
1750
1800
Reference chronology
1775
1800
1825
1850
1900
1850
1950
A very important application of crossdating is the absolute dating of
wood taken from archaeological sites or remnant pieces of wood that
can extend a chronology back in time.
1.5
Cabin rings
1.0
1.5
0.5
1.0
0.0
1700
0.5
1700
Cutting date of 1820
1725
1750
1750
1800
Reference chronology
1775
1800
1825
1850
1900
1850
1950
One of the best educational resources for learning how to create and
use skeleton plots is Paul Sheppard’s Java-based crossdating web site.
In the plot below, the top graph does not match the bottom graph.
Key in on triple narrow pairs!
Ideally, you will move your test skeleton plot one ring at a time (either to
the left or to the right) until you find a match, but key in on those obvious
ring patterns (like the three pairs of narrow back-to-back rings)!
And there’s
your match!
Your 61 rings
date from
1853 to 1913!
Check your crossdating skills! What’s wrong in the plot being tested
below?
You can see that these
three narrow rings appear
offset by a year.
Why? Easy. The ring for the
year 1689 is locally absent
on your core!