DESCRIPTIONS OF CRITICAL AREAS 203 



nearly 30 miles east of the Nipigon and for 12 miles west. The base 

 usually rests on sediments throughout this distance of over 40 miles, the 

 sediments in turn resting on a very uneven surface of Archean rocks. 



The position and attitude of the Archean island in the diabase near 

 Island portage suggest a pre-sedimentary monadnock, partly denuded 

 before the advent of the diabase, buried by it, and subsequently uncov- 

 ered by various erosion processes, the latest event in the geological history 

 being the formation of the Mpigon gorge. 



TCHIATANG BLUFF 



At the southwest corner of lake Nipigon is a deep bay, lying between 

 diabase headlands, called Grand bay. An inner bay at the bottom of 

 Grand bay, on the route leading south to Black Sturgeon lake, is called 

 Black Sturgeon bay. The south shore of this latter bay is bounded by a 

 high bluff, the relief being over 700 feet. In the narrows between Grand 

 bay and Black Sturgeon bay, a little more than half a mile from the base 

 of the bluff, is a small island, known as Gneiss island, the bedrock ex- 

 posed being gneiss. Between this island and the bluff soundings gave a 

 depth of 66 feet. The entire bluff, to the summit, is diabase, and the 

 highest point lies back from the cliffed front of the bluff (325 feet) about 

 half a mile and stands about 650 feet above the lake. At three points on 

 the face of this bluff masses of sandstone, now almost a quartzite, are 

 found in the diabase. At least one of these is about 25 feet in thickness 

 and stands nearly vertical. About 2% miles west of Gneiss island, at 

 the foot of the bluff, another mass of gneiss outcrops, rising at least 150 

 feet above the lake, and capped by trap. Four miles south of here is a 

 ridge of granite-gneiss which can be traced in a southeast direction, with 

 varied expression and at times partly covered either by sediments or by 

 diabase or by both for a distance of 20 miles, where it joins the large area 

 of Archean rocks lying east of the Black Sturgeon river. The two small 

 exposures of gneiss seen at Tchiatang bluff are topographically at a lower 

 level than the main area to the south, and the second one noted probably 

 represents the tip of the north end of a long ridge of Archean rocks. 



Following the face of the bluff south to the narrows leading to the 

 portage on the way to Black Sturgeon lake, there is a large mass of sand- 

 stone in the diabase just east of the narrows. This sandstone has a dip 

 of about 80 degrees and is probably an inclusion in the diabase. 



Four miles south of the bluff and about 1 mile east of Black Stur- 

 geon lake, there is a large area of sediments resting directly upon the 

 Archean, the actual contact and the lower basal beds being exposed. 

 About 2 miles inland the diabase overlies these beds. It is not known 



