The conglomerate is made up of pebbles of limestone of varying sizes, reaching in some cases a foot in diameter, but, as a rule, averaging about 2 or 3 inches.  The pebbles are usually well rounded, but sometimes show considerable angles.  The pebbles of limestone range in color from gray to blue and dark blue, and occasionally pebbles of a fine white marble are seen; with rare exceptions also pebbles of Catoctin schist and quartz occur.  They are embedded in a red calcareous matrix, sometimes with a slight admixture of sand.  As a rule the entire mass is calcareous.

The conglomerate occurs, as has been said, in lenses or wedges in the sandstone ranging from 1 foot to 500 feet in thickness, or possibly even greater.  They disappear through complete replacement by sandstone at the same horizon.  The wedge may thin out to a feather edge or may be bodily replaced upon its strike by sandstone; one method is perhaps as common as the other.  The arrangement of the wedges is very instructive indeed.  The general strike of the Newark rocks is a little to the west of north, while the strike of the Catoctin Belt is a little to the east of north.  The two series, therefore, if extended, would cross each other at an angle of 20 to 30 degrees.  The conglomerate wedges are collected along the west side of the Newark Belt and in contact usually with the Weverton sandstone.  The thick ends of the wedges along the line of contact usually touch each other.  Going south by east the proportion of the sandstone increases with rapid extermination of the conglomerate.  The thin ends of the wedges, therefore, resemble a series of spines projecting outward from the Catoctin Belt.

The result of weathering upon the conglomerate is a very uneven and rugged series of outcrops projecting above the rolling surface of the soil.

The ledges show little definite stratification and very little dip.  The topography of the conglomerate is inconspicuous and consists of a slightly rolling valley without particular features.  It approaches nearer to the level of the present drainage than any other formation, and decay by solution has gone on to a very considerable extent.  Where the draining streams have approached their baselevel, scarcely an outcrop of conglomerate is seen.  Where the areas of conglomerate lie near faster falling streams, the irregular masses of unweathered rocks appear.

When but slightly weathered the conglomerate forms an effective decorative stone and has been extensively used as a marble with the name “Potomac marble,” from the quarries on the Potomac east of Point of Rocks, Maryland.  While it is in no sense a marble, yet the different reds and browns produced by unequal weathering of the limestone pebbles have a very beautiful effect.

The thickness of the Newark formation is most uncertain.  The rocks dip at a light angle to the west with hardly an exception, and the sections all appear to be continuous.  Even with liberal deductions for frequent faults, nothing less than 3,000 feet will account for the observed areas and dips.