Percival2 was the first geologist to recognize that the trap ridges of the central part of the Connecticut Valley were divisible into three groups. The higher ridges, with a steep cliff to the west and a gentle slope to the east, he called the Main ranges. At the foot of the steep western slopes there are usually subordinate shelves, which he called the Anterior ranges. On the gentle back slopes toward the east are other subordinate ridges, which he called the Posterior ranges. Later investigations have shown that the terms have a chronological as well as a spatial significance. The Anterior ranges are now known to be lava-flows erupted before the eruption of the flows forming the Main ranges, and the Posterior ranges represent lava-flows poured forth at a considerable interval of time after the Main sheets were erupted.