Posed against a scenic background of giant sequoias in California’s Sierra Nevada, President Clinton on Saturday designated the nation’s newest national monument. Commercial logging, mining, and some recreational activities will be banned in the 328,000-acre Grand Sequoia National Monument, which encompasses 34 groves of the ancient, giant trees. Acting under the 1906 Antiquities Act, which allows presidents to create national monuments without congressional approval, Clinton has now protected more than 3 million acres of federal land. Green groups are pressing him to create additional national monuments in New Mexico and Idaho.