throughout the Central Valley would not exist had the tilt not occurred. Instead, horizontal basalt layers would have produced wide, flat-topped features like the mesas of the Southwest.

The tilting is also responsible for the characteristic profile of traprock ridges in Connecticut - a steep cliff face on the west and a gentle slope to the east. To help visualize how the tilt would produce such a profile, close this book. Hold it horizontally with the binding to the left and then tip it down to the right about three inches. Left is west and right is east. The binding represents the steep western cliffs and the cover the gentle eastern slope, all arising from the tilt of a slab of hard rock, etched out by erosion.

Despite the presence of three flows, in most places only a single large ridge formed from the tilt of the Central Valley's traprock. There are a couple of reasons for this. First, the middle flow is much thicker than the other two, overwhelming their presence and leading to just a single prominent ridge. Second, only a small amount of sedimentary material intervenes between the basalt layers, so that the three flows sometimes act topographically as one. Still, one smaller ridge usually lies in front of the main high ridge and another behind. Percival called these "anterior" and "posterior" ridges.

Because traprock ridges stand high above the surrounding countryside, the depressions between the main and posterior or anterior ridges have often been dammed to provide water supply with good water pressure. Examples of traprock reservoirs nestled between ridge crests include Lake Saltonstall (just east of New Haven), Lake Gaillard (in North Branford), Shuttle Meadow Reservoir (in Southington), and the Hartford Reservoirs (in West Hartford and Bloomfield).

The main high ridge stands out as the most prominent feature of the Central Valley: the Metacomet Ridge. This natural wall of rock divides the Valley in two. But in spite of the apparent permanence of its rocky face, the Metacomet Ridge is constantly moving eastward. The ridge arises from the intersection of the tilted traprock slabs with the ground surface; as erosion wears downward, the tilt forces the intersection to move further and further east. Could we take a look at

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