When Henry David Thoreau stood atop the “hard matter” of Maine’s Mount Katahdin and exalted in the glory of “the solid earth, the actual world,” he was thinking in human, not geologic, time. In geologic time even the most solid forms erode, even mountains disappear—or, in some cases, rise up out of what once was ﬂatter plain. The Catoctin Mountains in Maryland and Virginia, at the northeast tip of the Blue Ridge chain— where the presidential retreat Camp David is located—are examples of such forms. Most of the rock in the region was originally formed by lava ﬂow, which cooled and was covered by seas and sediments. This lava, under pressure of heat and weight, changed, or metamorphosed, into Catoctin metabasalt and greenstone, a dark greenish-gray rock highly resistant to weathering. As the softer strata on top of and surrounding this rock wore away, the mountains emerged. Today Catoctin remains a proper name, rather than a geographic term more generally. The last part of the name Catoctin contains tin, a generic Algonquian indicator for “mountains”; the ﬁrst part of the word is probably a form of Algonquian ketagi, which means “spottled or speckled,” a name that comes in part from Potomac marble, a speckled marble that occurs in the region. This residual landform is also called an inselberg and, chieﬂy in New England, a monadnock.