Through the growth of tree rings on pine logs, it is possible to date the time at which they were cut. If a tree is cut today, the outermost ring constitutes its growth for the year in which it is cut. Counting toward the center of the tree ring by ring, you will arrive at the date at which the tree was a young sapling. Climatic factors, dry and wet spells, are reflected in the width of the rings. Dry years usually show small, odd-shaped or stunted rings; normal years show regular well-shaped rings, and extremely wet years may result in excessively large rings. These various rings, which are arranged into patterns, can be matched with similar tree-ring patterns from still older trees, and a chart of patterns can be prepared which will extend as far back in time as you can find specimens with overlapping patterns. Against this master chart the ring pattern of any particular tree can be compared and the specimen dated. Today archeologists have such a tree-ring master chart which extends back to the time of Christ for the San Juan area. At Aztec, samples of tree rings were secured from some of the beams that still existed at the time this dating process was discovered. Such samples fall into two groups of dates. One group (with numerous samples) was placed between A.D. 1110 and 1124; the second group (with only six samples) between 1225 and 1252. The tree-ring dates indicate that the great pueblo at Aztec had undergone at least two major periods of construction. Since a large number of dates range from 1111 and 1115, this would appear to have been the first peak of building activity. It is possible that earlier samples have rotted away or have been destroyed by later Indians or by the early white settlers. Moreover, all building activity probably did not suddenly cease in A.D. 1124; it may well have continued for another 10 years, but the beams representative of this later period have since been destroyed. We can safely say that the first construction period at Aztec pueblo occurred sometime between 1110 and 1130, with most of the development occurring around 1111 and 1115. Likewise, a second major construction period at Aztec occurred sometime between 1220 and 1260, with major development in the 25-year span between 1225 and 1250.