Black Log mountain (Oneida sandstone) on the north side of Black Log valley makes the northern township line for about 5½ miles, west of which point the Juniata-Mifflin county line crosses to the Medina sandstone crest and continues 2 miles further to the Huntingdon county line.
The Tuscarora mountain of white Medina sandstone makes the southern border line of the township for a distance of about 7 miles, with one break in the straight line of the crest, about three miles west of the gap leading to Horse valley in Perry county. The eastern line of the township is a common one with Tuscarora, 8½ miles long from the Black Log mountain on the north across Tuscarora valley to the crest of the Tuscarora mountain on the south. The western line, a common one with Huntingdon county, is of about equal length, 8½ miles, between the same mountains.
Tuscarora creek is the main stream of the township, draining with its several branches all the Tuscarora valley, as well as the Salina valley, lying along the base of the Tuscarora mountain. It enters from Huntingdon county near the southwest corner of the township and flows east-northeast generally along the south base of the South Limestone ridge to within a short distance of the Tuscarora line, where it cuts through the ridge, making a loop in the slate valley to the north before entering Tuscarora township. Willow run, its principal tributary, rises along the Shade mountain north of Peru Mills, two of its branches making gaps there in the North Limestone ridge before entering the slate valley further south.
Black Log valley, on the contrary, is drained westward through Black creek, which, taking its rise in the northwest corner of Tuscarora township, flows generally through the center of the Black Log valley over Hudson river slates No. III and Trenton limestone No. II into Huntingdon county, where, after pursuing its course for about 10 miles, it turns northward and breaks through the Black Log mountain at the Orbisonia gap.
Peru Mills and Waterloo are the only villages in Lack township, neither of them large or of much importance. This township is sparsely settled.
