190 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES
THE JURASSIC PERIOD.

The deposits in Maryland which are here doubtfully referred to the
Jurassic period include the lower portions of what has been commonly
denominated the Potomac formation, but which is now known to rep-
resent several quite distinct geological horizons. This thick series of
strata which extends as a continuous belt from New Jersey south-
ward across Maryland into Virginia and which occurs also in the
South Atlantic and Gulf states can be separated upon both physical
and paleontological grounds into four formations, the two lower
known as the Patuxent and Arundel formations being provisionally
referred to the Jurassic period.

THE PATUXENT FORMATION. —The Patuxent formation, so called
from its typical development in the upper valleys of the Little and
Big Patuxent rivers, is the basal formation of the Coastal Plain series,
and is found lying directly upon the crystalline rocks of the Piedmont
Plateau. It appears near the landward margin of the Coastal Plain
and has been traced as a narrow and broken belt from Cecil county
across Harford, Baltimore, Anne Arundel and Prince George's count-
ies to the borders of the District of Columbia.

The deposits consist mainly of sand, sometimes quite pure and
gritty, but generally containing a considerable amount of kaolinized
feldspar, producing a clearly defined arkose. Clay lumps are at
times scattered in considerable numbers through the arenaceous beds.
Frequently the sands pass over gradually into sandy clays, and these
in turn into argillaceous materials, which are commonly of light color,
but often become highly colored and are locally not unlike the varie-
gated clays of the Patapsco formation. The more arenaceous deposits
are cross-bedded, and the whole formation gives evidence of shallow
water origin. The Patuxent formation is estimated to attain a thick-
ness of about 150 feet, but it may be considerably thicker at some
points. No distinctive fossils have as yet been found in this formation
in Maryland.

THE ARUNDEL -FORMATION. —The Arundel formation, so called
from Anne Arundel county, where the strata are well developed, con-
sists of a series of large and small lenses of iron-ore bearing clays