NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 28/ 



before reaching (north of railway tracks) the red Spokane shales, which 

 underlie the Cambrian in the Spokane Hills. 



The relations of the Cambrian and the subjacent Belt terrane on the line of 

 the section from Helena eastward across the Spokane Hills to the Big Belt 

 Mountains are indicated in the diagrammatic section, figure 10, (p. 263). 



Northwest of Helena the contact between the Cambrian and the Belt terrane 

 is followed to the crossing of Little Prickly Pear Creek, 6 miles west of Marys- 

 ville. The Helena limestone outcrops all along the hills and gulches, and at 

 Marysville the subjacent Empire shales occur beneath the limestone. West of 

 the Marysville Canyon area the siliceous beds dip from 10 to 15 degrees to the 

 northwest and pass above into the Helena limestone series, on which rest the 

 Marsh shales. Crossing east-northeast, to the Gates of the Mountain, on the 

 Missouri, 18 miles north of Helena, one finds the Cambrian sandstones rest- 

 ing on the red Spokane shales. This contact is again well shown on the eastern 

 side of the Missouri River, on the road to Beaver Creek. On Beaver Creek 

 the Cambrian rests directly on the Spokane shales, which, with the Grayson 

 shales, constitute a thickness of several thousand feet between the Newland 

 limestone and the base of the Cambrian. The contact at the crossing of Soap 

 and Trout Creeks, to the northeast, is essentially the same as at Beaver Creek 

 and the Spokane Hills, although there is a variation in the beds of the Grayson, 

 which come in contact with the Cambrian. 



At most of the outcrops where the lower beds of the Flathead (Cambrian) 

 sandstones come in contact with the Belt rocks the dip and strike of the two 

 are usually conformable, so far as can be determined by measurement. This 

 holds good all around the great Big Belt Mountain uplift. It is only when the 

 contacts are examined in detail, as near Helena, that the minor unconformities 

 are discovered (figures 12 and 13), and only when comparisons are made be- 

 tween sections at some distance from each other that the extent of the 

 unconformity becomes apparent. 



Explanation of apparent conformity. — The reason for the apparent con- 

 formity in strike and dip between the two groups appears to be as follows : 

 In pre-Cambrian time the Belt rocks were elevated a little above the sea, and at 

 the same time were slightly folded, so as to form low ridges. One of these 

 ridges is now the base of the Spokane Hills, where the Helena limestone (and 

 Empire shales) and the upper portion of the Spokane shales were removed 

 by erosion in pre-Cambrian or early Cambrian time. Usually there is very 

 little, if any, trace of this pre-Cambrian erosion contained in the basal sand- 

 stones of the Cambrian. On Indian Creek, however, west of Townsend, 

 which is on the strike of the Spokane Hills uphft, the basal bed of the Cam- 

 brian is made up almost entirely of fragments of the subjacent Spokane shales. 

 Fragments of these shales were also observed in the sandstones of the Cam- 

 brian in the Little Belt Mountains near Wolsey postoffice. These illustrations 

 are exceptional, the base of the Cambrian sandstone being formed usually of 

 a clean sand, such as might be deposited where the sea was transgressing on 

 the land. 



The gentle quaquaversal uplift of the Belt rocks (of the Big Belt Moun- 

 tains) gave them a slight outward dip toward the advancing Cambrian sea, so 

 that the sediments laid down on the Belt rocks were almost concentrically 

 conformable to them. Subsequent orographic movements have elevated the 

 Belt rocks into mountain ridges and have tipped back and in many instances 

 folded the superjacent Cambrian rocks, but the original concentric conformity 



