Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/359

 2. Post-Tertiary. — Neglecting the recent formations, which are of local occurrence only, the surface of the county is for the most part covered by an extensive accumulation of sand, gravel, and clay, with boulders of northern rocks, forming a portion of that general covering of drift which overlies the greater portion of the province, and extends southwards into the United States. The upper part of these deposits consists of a series of sands, gravels, and clays, more or less distinctly stratified, and usually resting upon a tenacious unstratified Boulder- clay. The boulders are derived partly from the syenitic, gneissoid, schistose, and limestone rocks of the northern Laurentian area, and partly from the wreck of the thick-bedded limestone, which will be subsequently mentioned as forming a part of the Trenton group, and which, previously to its denudation, overspread a great portion of the Laurentian rocks of the northern townships of Hastings. Many of the blocks have a volume of several cubic yards each, and are often broken up for road-metal. A single boulder or ice-borne mass of Laurentian rock, at the Shannonville Station on the Grand Trunk Railway, covers a superficial area of about 5 acres, and has a thickness of 100 feet. Isolated boulders are not unfrequently found on the tops of hills, where they have probably been left by the denudation of the deposit in which they were originally imbedded.

The accumulations of drift are sometimes heaped up in isolated hillocks, or in ranges of hills, and sometimes spread out over the valleys. A cutting in the Court House Hill, in Belleville, exposes a good section of the drift. Upon a base of Trenton limestone, the surface of which is highly polished and grooved, there is an accumulation of deposits attaining an aggregate thickness of about 60 feet, and consisting below of a tenacious Boulder-clay, overlain by a thick bed of blue clay and a series of finely stratified sands and gravels. In the blue clay there frequently occurs the cast of a peculiar organism, supposed to be a plant, which presents either a ramified or a lenticular form and attains a size of from 2 to 3 inches in diameter, and from 1 to 1-1/2 inch in thickness.

A succession of deposits, similar to that exposed at the Court House Hill, may be seen in the Oak Ridge — a range of drift-hills running across the country from East to West, having a width of from 3 to 6 miles, and varying in height from 100 to 500 feet.

On removing the superficial accumulations, the subjacent rock, whether gneiss, schist, or limestone, usually exhibits distinct traces of having been subjected to glaciation. Many of the rocks are highly polished, whilst others are distinctly striated and grooved, the general directions of the markings being from N. E. by N. to S. W. by S. Some remarkably distinct ice-scratches were exhibited in the town of Belleville in the autumn of 1864, when a cutting was made in Pinnacle Street. The section exposed about 30 feet of " hard pan," or gravel, with boulders of calcareous and syenitic rocks, resting on the Trenton limestone. The surface of this limestone, when freshly- exposed, was most distinctly polished and striated, the general bearing of the marks being N. 35° E. and