Connecticut, say, ten million years ago, we would see the Metacomet Ridge a good bit west of where it now lies.

Not all of the Central Valley's traprock ridges result from tipped lava flows. Some of the underground plumbing that brought the lava to the surface also juts out as ridges. These lava conduits were plugged by cooled molten rock that did not quite make it into the flow. It is only because of erosion that the traprock in the conduits has finally broken ground. Rock that plugs the plumbing is called "intrusive" rock, because it intrudes through the crust.

Intrusive rocks take two general forms: sills, which run parallel to the surrounding strata, and dikes, which cut across them. Actually, the same rock body may run as a sill for a while and then curve up into a dike, like a bend in a water pipe as it turns up from the cellar and into the kitchen. A good example is West Rock and East Rock in New Haven. West Rock breaks ground as a dike, but extends underground as a sill over to East Rock. The Barndoor Hills, which run through Canton, Avon, Simsbury, and Granby, also have the features of both a dike and a sill, depending on where one looks along the line of hills. Hamden's Sleeping Giant is an especially massive version of a dike called a "stock," the main line of a lava flow's plumbing system.

It is often difficult to determine whether a traprock ridge in the Central Valley is a dike, a sill, or a flow. One rule of thumb is, because dikes and sills formed differently from flows, they are usually the odd-ball ridges in their shapes and profiles. Instead of taking the profile common to flows - a north-south ridge with a steep cliff to the west and a gentler slope to the east - dikes and sills take off on their own. East Rock has a south-facing cliff, the Barndoor Hills are often steep on both sides, and Sleeping Giant runs more east-to-west than north-to-south. (Actually, even the flows don't always follow the rules: in addition to a west-facing cliff, the Hanging Hills of Meriden also have one cliff that faces south.)

All the sills and dikes in the Central Valley lie west of the Metacomet Ridge. This as well results from the tilt of the Central Valley's rocks. Because of the tilt, all rock formations to the west originally lay underneath the formations to the east. If the intrusives really are the

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