Reports
Piedmont and Coastal Plain geology along the Susquehanna Aqueduct: Baltimore to Aberdeen, Maryland
1968, Cleaves, E.T.
Report of Investigations 8
Abstract
The geology of a twenty-two mile section of the Susquehanna Aqueduct was mapped from Baltimore to Aberdeen, Maryland. The trench for this water supply project crosses sediments of Lower Cretaceous (Potomac Group) and Quaternary Age and crystalline rocks upon which residual deposits (saprolite) have formed by weathering.
The crystalline rock is mantled by saprolite, a residual deposit formed by chemical weathering. Saprolite is divided into two zones, structured and massive. Structured saprolite retains structures and textures present in the parent rock, but massive saprolite, by definition, does not. Bank failures occurred only in structured saprolite and along planar features such as joints. Saturation and vibration were important contributing factors.
The alteration of rock to saprolite varies with mineralogy, texture, and structure in the parent rock. In equigranular rocks, like gabbro and quartz monzonite, joints are the focal point of weathering. In gneisses and schists, textural and mineral layering are the primary focal areas and joints secondary. Residual boulders and rock pinnacles are common in gabbro saprolite (the most common type along the Aqueduct) and excavation rates vary with frequency of bedrock boulders or pinnacles.
Alteration of rock to saprolite has taken place in early Cretaceous time and probably continues into recent time. Where sediments overlie saprolite, at least the upper portion of the saprolite was formed prior to sediment deposition. Four kinds of stratigraphic and sedimentologic evidence support this: (1) extension of the saprolite on the Piedmont beneath the Coastal Plain sediments; (2) truncation of massive and structured saprolite zones by sedimentary deposits; (3) incorporation of saprolite particles into basal sands and gravels; and (4) occurrence of lag gravels at saprolite-sediment contacts. The lower part of the saprolite may have developed after burial and thus be younger than the overlying sediment, but the extent of post-burial alteration of rock to saprolite is uncertain.
The Potomac Group consists of gravel, sand, silt, and clay which are characterized by abrupt horizontal and vertical changes. These sediments, deposited in a fluvio-deltaic environment, were derived from the adjacent Piedmont. A local source area is suggested by: (1) lithology of clasts in the gravel, mainly quartz and quartzite; (2) a non-opaque heavy mineral suite which consists primarily of staurolite and tourmaline; (3) presence of amphibole and epidote-clinozoisite in sand beds at the base of Potomac Group sediments; and (4) angularity of staurolite and tourmaline grains.
As a consequence of abrupt horizontal and vertical changes in lithology, excavation and wall stability characteristics of the sediments change abruptly. Average rates for excavating the Potomac sediments are: sand, 296 cu. yd./hr.; mixed sediments (gravel, sand, silt, and clay), 209 cu. yd./hr.; and clay, 162 cu. yd./hr. Bank failures occurred mainly in sand and were caused by a high water table and/or vibration.
Quaternary deposits in stream valleys consist of gravel overlain by a finer grained unit. In the major valleys the upper unit is a sand-silt-clay; in smaller valleys, the upper unit commonly is a gravelly clay. The gravel unit retards excavation; layers two to five feet thick were excavated at about 220 cu. yd./hr. Locally, deposits of loam occupy hollows on Potomac Group sediments. The loam is very sensitive to vibration and readily collapsed into the trench. As a consequence, excavation rates in loam were less than 170 cu. yd./hr.
The Quaternary sediments are derived from reworking of the Potomac Group sediments and from the adjacent saprolite and rock of the Piedmont, as indicated by lithology of the gravel clasts (quartz, quartzite, gabbro, amphibolite, schist, and gneiss) and by heavy minerals (amphibole and epidote-clinozoisite dominate the suites).
