The following passage is adapted from an essay on the history of dendrochronology, the scientific method of dating tree rings.
Paragraph 1 (lines 1–11)
In the early twentieth century, astronomer Andrew Ellicott Douglass was not seeking a new way to date archaeological sites; rather, he was looking for a terrestrial record of solar activity. Speculating that sunspot cycles influenced Earth's climate, Douglass reasoned that variations in precipitation would be recorded in the growth rings of trees. He began collecting samples of ponderosa pine in the American Southwest, believing that the width of each annual ring reflected the amount of rainfall in a given year. Through this work, Douglass realized that trees of the same species in a region shared distinct patterns of wide and narrow rings, creating a shared environmental timeline.
Paragraph 2 (lines 12–23)
As Douglass expanded his database, he realized that he could build a continuous chronology by overlapping patterns from living trees with those found in older wood samples. If a living tree’s inner rings matched the outer rings of a log from a historic cabin, the timeline could be pushed back in time. This technique of cross-dating allowed Douglass to construct a master sequence that spanned centuries. The application of this method soon caught the attention of archaeologists, who had struggled for decades to establish absolute dates for the ancestral Puebloan ruins of the American Southwest.
Paragraph 3 (lines 24–36)
The true breakthrough occurred in 1929 with the discovery of a charred beam at a site in Show Low, Arizona. Using his cross-dating technique, Douglass linked this specimen to both his existing floating chronology from prehistoric ruins and his historical timeline from living trees. This single discovery bridged the gap in his data, instantly providing absolute dates for major archaeological sites across the Southwest. Beyond dating, Douglass's method pioneered the field of paleoclimatology, demonstrating that tree rings could serve as biological archives of past climates long before human record-keeping.
Match each paragraph from the passage to its primary main idea.
- Paragraph 1 (lines 1–11)The transition of tree-ring research from studying solar cycles to identifying shared environmental patterns.
- Paragraph 2 (lines 12–23)The establishment of a master timeline through cross-dating and its relevance to historical research.
- Paragraph 3 (lines 24–36)The resolution of a chronological gap and the validation of tree rings as indicators of ancient climates.